Complete Guide to Letter Boxes

Letter boxes, sometimes called letterplates or door letter boxes, are a small part of a door set but they have a big impact on security, weather performance, energy efficiency and the overall look of the entrance. Whether you are a trade installer working to PAS 24 and Secured by Design requirements, or a homeowner replacing a tired upvc letterbox, choosing the correct product and fitting it properly is essential.

This guide explains standard UK sizes, material options, security features, and provides detailed measuring and fitting instructions for timber, composite and uPVC doors. It is written for both trade professionals and DIY customers using products from the Window Hardware Store range of letter boxes and associated accessories.

1. Standard sizes

1.1 Typical UK letter box sizes

In the UK, letterplates are generally sized to accept common Royal Mail envelope formats (DL, C5, C4). For domestic front doors, the opening (aperture) is usually at least 230 mm wide to allow A4 letters to be posted without excessive folding. However, there is no single “one size fits all” standard; different manufacturers and door types use slightly different dimensions.

When talking about size there are three measurements to keep in mind:

Overall external plate size – the outside dimensions of the visible letterplate

Aperture (letter opening) size – the flap opening the mail goes through

Cut-out size – the hole you cut in the door or panel to accept the letter box

In addition, most modern letter boxes have bolt fixings or screws, so the fixing centres (distance between fixings) also matter when you are replacing an existing unit.

1.2 Common domestic letterplate sizes

The following sizes are commonly used on modern domestic doors in the UK. Actual dimensions can vary by manufacturer (Yale, ERA, Mila, Avocet, UAP, etc.), so always check the manufacturer’s data sheet, but these are useful reference points.

Typical overall external plate sizes:

250 mm x 50 mm – used on some older timber doors and narrow profiles

275 mm x 50–60 mm – common on older uPVC and aluminium doors

300 mm x 60–70 mm – a very common size on timber and composite doors

305 mm x 70 mm – a popular “12 inch” style letter box for many door slabs

330 mm x 70–80 mm – often used on contemporary composite and wide-stile doors

345 mm x 75 mm – larger format allowing generous apertures and thicker insulation

Typical aperture (letter opening) sizes:

210 mm x 40–45 mm – slim openings, older timber doors

230–250 mm x 40–50 mm – common domestic openings suitable for C5/DL mail

260–270 mm x 50–60 mm – common on “12 inch” and larger format letterplates

280–290 mm x 60–65 mm – used for high-security or thicker insulated letter boxes

Typical cut-out sizes in the door or panel:

Approx. 215–220 mm x 45–50 mm – for compact 250 mm letterplates

Approx. 230–240 mm x 50 mm – smaller domestic uPVC doors

Approx. 250–260 mm x 55–60 mm – common on uPVC and composite door panels

Approx. 270–280 mm x 60–65 mm – modern PAS 24 letter boxes with cowls and brush sets

It is not unusual to find slight differences of 2–5 mm between brands, so when swapping like-for-like, always measure the existing cut-out carefully before ordering a replacement from the letter box range.

1.3 Screw and bolt fixing centres

Many modern door letter boxes use through-fixing bolts or self-tapping screws to clamp the internal and external plates together. Fixing centres can differ significantly between manufacturers.

Common fixing centre spacings include:

200–210 mm centres – often on older timber doors

212–216 mm centres – common on some Yale, ERA and Mila products

230–240 mm centres – frequently used on uPVC mid-rail letter boxes

250–270 mm centres – larger format and security letterplates

If you are replacing an existing letter box and want to reuse the existing fixing holes, measuring the centre-to-centre distance precisely (to the nearest millimetre) is crucial. Trade installers working on volume replacement jobs will often carry a selection of sizes and use an internal cowl or inner flap tidy to cover any minor discrepancies.

1.4 Door thickness and telescopic letter boxes

Modern composite and uPVC doors are significantly thicker than older timber doors. A traditional timber front door is often 44 mm thick, whereas composite door slabs can be 44–70 mm, and some insulated uPVC panels are even deeper.

To accommodate this, many upvc letterbox designs are “telescopic” – the inner and outer parts slide together and can be adjusted to suit a range of door thicknesses, for example:

20–40 mm – thin panels and internal doors

40–60 mm – standard timber and basic uPVC doors

60–80 mm – modern composite door slabs and insulated panels

When ordering, check both the minimum and maximum door thickness for the product. If you fit a telescopic letter box into a door that is thinner than its stated minimum, the unit may rattle and leak air. If the door is thicker than the stated maximum, the inner and outer halves will not clamp together securely.

1.5 Panel and mid-rail positions

On uPVC doors the letter box is usually located in the mid-rail or in an infill panel. Standard mid-rail heights and panel sizes will often dictate which letterplate size you can use.

Mid-rail letter boxes – the opening is cut directly into the reinforced uPVC mid-rail; check reinforcement depth and avoid cutting into steel reinforcements

Infill panel letter boxes – the letter box sits in a flat uPVC panel; always confirm panel thickness before ordering a telescopic unit

On timber doors, letter boxes are typically fitted centrally, 700–1000 mm above the bottom of the door, avoiding mortice locks and existing panelling. On fire doors (FD30/FD60), the location of the aperture is often dictated by the fire test data and must comply with the manufacturer’s certification.

1.6 Standards affecting size and performance

While there is no single mandatory size for door letter boxes, several UK and European standards influence their design:

BS EN 13724 – covers requirements and testing for letter boxes and mailboxes, including aperture size, resistance to corrosion and weather

TS 008 – technical specification for enhanced security letterplates, often required on PAS 24 doorsets and Secured by Design projects

BS 476 / BS EN 1634 – fire performance standards; fire-rated letterplates for FD30 / FD60 assemblies must be tested as part of a complete doorset

For most domestic properties, choosing a good quality TS 008-compliant letter box from recognised brands (Yale, ERA, Mila, Avocet, UAP, etc.) will ensure the aperture size and performance are suitable, while providing security and weather resistance.

2. Material choices

2.1 Overview of common materials

The material of a letter box affects its durability, corrosion resistance, appearance and cost. The main choices in the UK are:

Aluminium (including anodised and powder-coated finishes)

Stainless steel (usually 304 or 316 grades)

Brass (polished or lacquered)

Zinc alloy (often die-cast, then plated)

uPVC or ABS plastics (standalone or as part of a composite design)

Many modern letterplates are actually multi-material: for example, a stainless steel outer flap on a zinc alloy body, with uPVC internal frames and nylon brush seals.

2.2 Aluminium letter boxes

Aluminium is lightweight, corrosion-resistant and easy to machine, making it a popular choice for both domestic and commercial letterplates.

Typical features:

Available in anodised finishes (silver, gold, bronze) or powder-coated colours to match door hardware

Good resistance to rust, suitable for most UK environments

Can be produced in slim, contemporary profiles for modern doors

Often used by brands such as Mila, Avocet and ERA for their mid-range ranges

Trade advice: Aluminium letter boxes are a strong all-round option for uPVC and composite doors. For coastal installations, choose products with a high-quality marine-grade powder coating or consider stainless steel to minimise staining and pitting.

2.3 Stainless steel letterplates

Stainless steel letter boxes are chosen where maximum corrosion resistance is required, or where a high-end modern look is desired.

Grade 304 stainless steel is standard for most residential hardware

Grade 316 (marine grade) is preferred in coastal or industrial environments

Brushed or satin finishes help hide minor scratches and fingerprints

Often combined with security cowls and anti-fishing plates in TS 008 designs

DIY note: Stainless steel is very durable, but surface “tea staining” can occur in harsh environments. Regular cleaning with mild soapy water helps maintain the finish. Avoid abrasive pads which can scratch the grain.

2.4 Brass and traditional finishes

Polished brass letter boxes are a classic choice for traditional timber front doors, especially in period properties.

Solid brass offers an authentic, weighty feel

Many products are lacquered to slow down tarnishing

Unlacquered “living” brass will darken and patinate over time

Available in polished, satin, antique brass and bronze tones

Trade advice: For listed buildings or conservation areas, check any local requirements on appearance. A solid brass letterplate, paired with matching knocker and door furniture, can significantly enhance kerb appeal. However, be clear with customers about maintenance – lacquer will eventually wear, and polishing may be required to restore a high shine.

2.5 Zinc alloy and plated finishes

Many mid-range letterplates use a zinc alloy body which is then plated or coated.

Can be plated in chrome, gold, black nickel, or coloured finishes

Offers good detail for decorative styles

Usually more cost-effective than solid brass or stainless steel

These are common on uPVC and composite doors supplied through volume housebuilders. Quality varies: look for products from established brands like ERA, Yale, Mila and Avocet for reliable plating and good warranty support.

2.6 uPVC and ABS plastics

Plastic (uPVC or ABS) components are widely used in upvc letterbox designs, particularly for internal frames, end caps and draught excluders. Some budget letter boxes use almost entirely plastic construction.

Lightweight and non-corroding

Available in white, brown, and foiled finishes to match uPVC frames

Lower cost but may be less rigid than metal-bodied alternatives

DIY note: For white uPVC front doors, a white or colour-matched upvc letterbox can be a neat, unobtrusive choice. However, in very exposed locations, a metal flap may close more securely in strong winds and offer better long-term durability.

2.7 Matching other door hardware

From a design perspective, it is best practice to match your letter box finish to the rest of your door hardware:

Match to handles, hinges, door knockers, spyholes and numerals

For composite doors, coordinate with the colour of the door slab and glazing cassettes

On uPVC doors, choose finishes that match existing handles (commonly white, gold, chrome, black)

The Window Hardware Store range of letterplates includes finishes to coordinate with popular handle ranges from Yale, ERA, Mila, Avocet and other leading brands, allowing a cohesive look across the entire door set.

3. Security features

3.1 Why letter box security matters

A poorly designed or badly located letter box can be a weak point in an otherwise secure door. Common risks include:

Fishing – thieves reaching through the aperture to retrieve keys or operate internal handles

Lock manipulation – using tools through the opening to attack thumbturn cylinders

Arson or vandalism – items pushed through and set alight inside

Privacy risks – being able to see directly into the property through the flap

Modern security letterplates are designed to counter these threats while still allowing easy posting of mail.

3.2 TS 008 and PAS 24

For new-build and replacement doors that need to meet PAS 24 or Secured by Design requirements, letter boxes must comply with TS 008. TS 008 specifies enhanced performance for security, draught proofing, rain and corrosion resistance.

Key TS 008 security features include:

Restricted inward opening and flap angle to reduce fishing risk

Internal cowls or shields to prevent direct access to locks and handles

Controlled aperture size and design to prevent removal of keys through the opening

Robust fixings and resistance to forced attack

Many TS 008 letterplates are used on PAS 24 accredited doorsets from major UK door manufacturers. When replacing a letter box on a PAS 24 door, it is important to maintain the same security specification; changing to a non-compliant product may invalidate the doorset’s certification.

3.3 Anti-fishing features

Fishing attacks are particularly common where keys are left near the door. Modern security letter boxes incorporate several anti-fishing measures:

Internal cowls – plastic or metal shrouds fitted on the inside, narrowing the opening

Restricted flap opening – spring tension and hinge design limit how far the flap opens inward

Brush seals and baffles – help obstruct direct access and prevent tools being pushed through

Divided apertures – some designs have split flaps or offset openings to make fishing tools difficult to manoeuvre

Trade advice: For properties with thumbturn cylinders or internal lever handles, fitting a TS 008 letterplate with a full-height cowl is strongly recommended. You can also combine this with high-security locks (TS 007 3-star cylinders) for a comprehensive upgrade.

3.4 Fire-safe and anti-arson solutions

There is no single mandatory UK standard specifically for “anti-arson” letterboxes, but best practice in higher-risk properties (HMOs, shared entrances, businesses) often includes:

Fire-rated letterplates tested to BS 476 or BS EN 1634 as part of a fire door assembly

Internal anti-arson cowls with intumescent liners that expand to seal the opening under heat

Letterboxes positioned lower on the door and away from escape routes to reduce risk

On certified fire doors, it is essential that any letter box fitted is part of the tested and certified doorset. You must not cut a generic letterplate into a fire door blank unless the door manufacturer specifically permits that model and location. For compliant replacements, always follow the door manufacturer’s instructions and use the specified hardware.

3.5 Privacy and acoustic protection

Some letterplates incorporate features that improve privacy and acoustic performance:

Internal flaps with full coverage and no gaps

Acoustic brush or foam seals around the aperture

Deep internal cowls that block line-of-sight through the door

Insulated internal frames for improved sound and thermal performance

For properties on busy roads or with draught problems, combining a quality external letter box with an internal inner flap or cowl can significantly reduce noise and air leakage.

3.6 Common security upgrades

If replacing the entire letter box is not practical, there are still simple retrofit upgrades that can improve security:

Fit an internal letter box cowl – restricts access through the opening, easy DIY fit

Install a separate external letter basket – catches post and blocks fishing tools

Relocate keys – always keep keys out of reach of the letter box and not visible from outside

Use a double-cylinder or key-operated internal lock instead of a thumbturn in vulnerable locations

These measures are relatively low-cost but provide a significant improvement against opportunistic attacks.

4. Measuring guide

4.1 Tools you will need

Before measuring for a new letter box, gather the following:

Steel tape measure (preferably with millimetre markings)

Pencil or masking tape for marking

Notepad or phone for recording dimensions

For trade use, a simple letterbox template or gauge can speed repeat measurements

4.2 Measuring to replace an existing letter box

If you are replacing an existing door letter box, you will usually want a new unit that covers the existing opening and fixing holes without additional cutting.

Step-by-step:

Measure the external plate width and height

Measure the internal plate width and height

Measure the aperture (letter opening) width and height, flap edge to edge

Measure fixing centres – from the centre of one fixing hole to the centre of the opposite one

Measure door thickness at the location of the letter box

If possible, remove the letter box and measure the cut-out size directly

For accuracy, measure in millimetres and double-check each dimension. For example:

Example note for ordering:

“Existing letterplate: external 305 mm x 70 mm, internal 280 mm x 60 mm, aperture 250 mm x 45 mm, fixing centres 260 mm, door thickness 44 mm, cut-out approx. 265 mm x 55 mm.”

With these details, you can match a suitable replacement from the letter box range, even if the original brand is unknown.

4.3 Measuring to fit a new letter box in a blank door

Where no letter box currently exists (for example, on a new timber door blank), you can choose from a wider range of sizes. The key considerations are:

Door construction and thickness

Position of existing locks, panels and glazing

Any fire or security certification requirements

Follow these steps:

Confirm door thickness – measure at the proposed letter box location

Check for obstructions – internal lock cases, panels, mouldings, and any structural rails

Decide height from bottom of door – typically between 700–1000 mm, but ensure compliance with any project specification

Select a letter box suited to the thickness and style (e.g. TS 008 security letterplate for PAS 24 projects)

Use the manufacturer’s cut-out template or instructions to mark the aperture

DIY tip: On a panelled timber door, avoid cutting across structural joints or through raised panels. It is usually best to locate the letter box in a flat, central rail where the timber is solid and provides a good fixing base.

4.4 Measuring for uPVC and composite doors

uPVC and composite doors require particular care because of their multi-layer construction and internal reinforcement.

For uPVC doors:

Identify whether the letter box will sit in the mid-rail or in an infill panel

Measure the overall thickness of the door at that point (typically 28–70 mm including panel)

If replacing, measure the existing aperture and fixing centres as described above

Check for steel reinforcement – use a magnet or light tapping to locate metal sections; avoid cutting into structural reinforcement without manufacturer guidance

For composite doors:

Measure slab thickness (commonly 44–70 mm)

Confirm with the door manufacturer whether additional reinforcing or a specific letter box type is required

Use the supplied template if the door came pre-routed for a particular brand of letterplate

Ensure the telescopic range of the new letter box suits the slab thickness

Trade advice: Many door manufacturers specify compatible letter boxes by brand and model to maintain warranty and PAS 24 certification. When working with composite or PAS 24 doors, check the door maker’s technical literature and use the same or equivalent TS 008 product where required.

4.5 Measuring door thickness and panel depth

Accurate door thickness measurement is essential for telescopic letter boxes. To measure:

Open the door and measure from the external face to the internal face using a tape or vernier

Where there is an infill panel, measure the panel thickness separately if possible (for example at the edge of a glazing cassette)

Record both total door thickness and panel thickness, as some letter boxes sit only in the panel, not the full door

Example:

A uPVC door with a 28 mm infill panel and 70 mm total frame thickness may use a letter box that only passes through the 28 mm panel. In this case, choose a product that suits 20–40 mm thickness. For a composite slab of 44 mm, select a telescopic unit covering 40–60 mm.

4.6 Allowance for internal flaps and tidies

If you plan to add an internal flap tidy or cowl (for example, from the inner flaps collection), consider the space available around the aperture on the inside face of the door.

Measure from the proposed cut-out edges to any adjacent mouldings, handles, locks or glazing

Ensure there is enough flat surface area for the internal fitting plate or cowl

Check that the internal flap can open fully without hitting handles or internal walls

On very narrow stiles or heavily panelled doors, you may need to choose a more compact internal tidy or adjust the letter box position slightly to ensure proper operation.

4.7 Common measuring mistakes to avoid

Based on trade experience, the most common errors when ordering replacement letterplates are:

Measuring only the external plate and ignoring fixing centres or cut-out size

Confusing aperture size with cut-out size

Measuring to the nearest centimetre instead of millimetres, leading to poor fit

Ignoring door thickness and buying a non-telescopic letter box for a thick composite door

Forgetting to check whether the door is a fire door or PAS 24 doorset with specific hardware requirements

Taking the time to measure accurately once will save significant time and cost compared with returning incorrectly sized products or having to adjust the door on site.

5. Fitting instructions

5.1 General safety and preparation

Fitting a letter box is a straightforward job for many DIYers, but it still requires care, especially when cutting into doors. Always:

Wear suitable eye protection when drilling or cutting

Use dust sheets or covers to protect flooring and furniture

Check for wires, glazing and reinforcement before cutting

Follow tool manufacturer safety instructions

Typical tools required:

Drill and drill bits (wood, HSS or masonry as appropriate)

Jigsaw or padsaw

Flat and crosshead screwdrivers

Masking tape and pencil

Square and tape measure

Silicone or low-modulus sealant (for exposed, draughty locations)

5.2 Fitting a new letter box into a timber door

Step 1 – Marking out:

Choose the position, usually central and approx. 700–1000 mm from the bottom of the door

Use the letter box template or manufacturer’s instructions to mark the cut-out on the door

Use a square to ensure the aperture is level and aligned with the door edges

Step 2 – Drilling pilot holes:

Drill a hole at each corner of the marked aperture, just inside the cut line

Use a drill bit large enough to accept your jigsaw blade (typically 10–12 mm)

Step 3 – Cutting the aperture:

Insert the jigsaw blade into one of the pilot holes and carefully cut along the marked lines

Support the cut-out section to prevent splintering as it drops away

Use a sharp handsaw or padsaw where a jigsaw cannot reach

Step 4 – Cleaning and trial fitting:

Clean up the edges of the cut with sandpaper or a sharp chisel

Test fit the external and internal letter box halves to ensure they slide through cleanly

If the fit is tight, trim carefully; do not oversize the aperture

Step 5 – Fixing the letter box:

Position the external part of the letter box from outside the door

Hold the internal part in place from inside, aligning the fixing holes

Insert the fixing screws or bolts and tighten evenly, avoiding overtightening which may warp the door or letter box

Check that the flap opens and closes smoothly without binding

Step 6 – Sealing (optional):

In very exposed locations, apply a thin bead of sealant around the external edges before tightening fully

Wipe away any excess sealant immediately for a neat finish

5.3 Fitting a letter box to a uPVC door

Fitting to uPVC requires more care to avoid damaging the profile and any steel reinforcement.

Step 1 – Check the door construction:

Identify whether the letter box will be in a mid-rail or in a removable infill panel

If in a panel, consider removing the panel for easier cutting on a flat surface

Step 2 – Marking out:

Use the manufacturer’s template, centred on the mid-rail or panel

Align with any existing mouldings so the letter box appears straight and level

Use masking tape to protect the uPVC surface and mark over the tape

Step 3 – Drilling and cutting:

Drill pilot holes at each corner of the aperture

Use a fine-tooth jigsaw blade suitable for plastics to cut the opening

Work slowly to avoid chipping or melting the uPVC

Step 4 – Trial fitting and trimming:

Insert the external and internal sections to check fit

Trim gently with a sharp knife or file if needed, taking care not to overcut

Step 5 – Assembly:

Insert the external half of the letter box from the outside

Slide the telescopic section to suit the panel thickness

Attach the internal half and secure with the supplied screws or bolts

Ensure that any built-in draught brushes or cowls align correctly

Trade tip: Do not overtighten fixings on uPVC as this can distort the profile and impair operation. Tighten until snug, then test the flap; if it binds, back off the fixings very slightly.

5.4 Fitting to composite doors

Composite doors use GRP or similar skins around a core. Incorrect cutting can void the manufacturer’s guarantee, so check any supplied guidelines before starting.

Use the exact template supplied by the door or letter box manufacturer

Drill pilot holes and use a fine-tooth jigsaw blade suitable for GRP/composite materials

Support the cut-out at all times to prevent cracking or delamination

Seal any exposed core material if recommended by the door manufacturer

Fit the letter box as per the manufacturer’s instructions, checking telescopic range and fixings

On PAS 24 / Secured by Design doors, always fit a TS 008-compliant letterplate in the location and orientation specified by the door maker. Swapping to a non-tested model can invalidate certification.

5.5 Installing internal flaps, cowls and tidies

Once the main letter box is fitted, you may wish to add an internal accessory from the inner flaps and tidies range, particularly on older doors without integrated draught proofing.

Typical fitting process:

Hold the inner flap tidy or cowl over the aperture from inside

Ensure it covers the opening fully and sits square to the door edges

Mark the fixing holes and drill small pilot holes if required

Fix using the screws provided, taking care not to overtighten

Check that the internal flap opens freely and does not foul the external letterplate

Internal cowls should be fitted with the open side facing downwards or as directed by the manufacturer, to minimise draughts and improve anti-fishing performance.

5.6 Adjusting spring tension and flap operation

Some higher-quality letter boxes allow minor adjustments to the spring tension or hinge position:

If the flap slams loudly, check for adjustment screws on the hinge; a small tweak can soften the action

If the flap does not close fully, check for debris, misalignment or over-tightened fixings distorting the body

On telescopic letter boxes, ensure the internal and external halves are perfectly aligned; misalignment can cause the flap to bind

Trade advice: Always test the flap several times with different sizes of mail after fitting. An incorrectly adjusted letter box can cause jams, leading to bent post and customer complaints.

5.7 Common problems and solutions

Draughts through the letter box:

Check that integral brush seals or foam strips are present and intact

Fit an additional internal flap tidy or draught excluder

Consider upgrading to a modern insulated letterplate with brush and gasket systems

Water ingress:

Ensure the external flap closes fully and is not obstructed

Check that the letter box is level; a tilted unit can trap water

Apply a thin bead of sealant around the external edge where appropriate

In very exposed coastal locations, consider a hooded or canopy-style letter box

Flap not closing or sticking:

Check for paint or sealant build-up on timber doors; trim back as needed

Inspect for twisted or overtightened fixings; loosen slightly and retest

Ensure the letter box aperture is not pinching the body; trim the opening carefully if necessary

Security concerns (fishing, visibility):

Install an internal cowl or letter basket to block access

Move keys away from the letter box area

Upgrade to a TS 008 security letterplate from the security letter box selection

5.8 Maintenance and care

Even the best quality letter box will benefit from occasional maintenance:

Clean external surfaces with mild soapy water; avoid harsh chemicals and abrasives

Inspect hinges and springs annually; a small amount of light oil can keep them operating smoothly

Check and replace worn brush seals or foam gaskets on older units to improve draught performance

On brass letterplates, periodically polish according to the manufacturer’s guidance; if lacquered, avoid abrasive cleaners that may strip the lacquer

For trade installers, offering a simple care leaflet or advice at the end of a job can reduce future call-backs. For homeowners, including letter box maintenance as part of regular door care (cleaning, checking seals, lubricating locks) will extend the life of the hardware and keep the entrance looking smart.