How to Measure a Letterbox: 10 or 12 Inch? (UK Size Guide)

Of the 420 Yale letterboxes we sold online in the last 12 months (October 2025 to October 2026), 55% were 10 inch and 45% were 12 inch. 57% were for doors 40–80mm thick. It's close to an even split both ways, so a quick measure saves a return.

Step 1: 10 inch or 12 inch?

Measure your current letterbox on the outside of the door, from end to end. You can also measure the distance between the two fixing screws on the inside.

These are the sizes of the Yale uPVC letterbox, our best seller. Other makes are within a few millimetres. If your old letterbox is in between, measure the hole in the door: the new sleeve has to fit through it, and the outside plate has to cover it.

Step 2: How thick is the door where the letterbox sits?

Push the flap open and measure from the outside face of the door to the inside face, through the letterbox opening. Letterboxes have a telescopic sleeve that slides to fit, so you only need to know which range you're in:

• 20–40mm: usually a letterbox fitted in the door panel, the thinner infill part of a uPVC door.
• 40–80mm: usually a letterbox fitted in the midrail, the solid bar across the middle of the door, or in a composite door.

If your door measures exactly 40mm, either sleeve will fit.

Step 3: Check the fixings and finish

Most uPVC letterboxes fit with two screws from the inside, which go into the back of the outside plate. Because the screw centres for each size are standard, a new letterbox of the same size usually uses the existing holes.

Then pick a finish to match your handle and knocker: white, black, chrome, silver, polished gold or matt gold.

Got your size?

Yale uPVC letterbox in 10″ or 12″, with a 20–40mm or 40–80mm sleeve, in a choice of finishes. £11.99.

Choose your size →

How to fit a new uPVC letterbox

1. Remove the old letterbox. Undo the screws on the inside and pull both halves out.
2. Set the sleeve. Slide the telescopic sleeve to roughly the thickness of your door.
3. Fit the outside plate. Push it into the hole from outside, with the flap hinge at the top.
4. Fit the inside half and screw it to the outside plate. Tighten evenly, but don't overtighten or the plate can bow.
5. Test the flap. It should open freely and spring shut.

It takes about 10 minutes with a cross-head screwdriver.

Is your letterbox a security risk?

A letterbox can be used for "letterbox fishing", where someone pushes a hook or a wire through to reach keys or work the handle from inside. Fire, fireworks and other things pushed through the letterbox are another risk.

• Keep keys away from the door. It's the cheapest fix.
• Fit a letterbox guard on the inside. The FlapGUARD is a steel, lockable cover that still lets post through, and is Sold Secure Bronze accredited.
• Fit a fire-retardant letterbox bag. It catches all the post and can contain small fires. It's our best-selling letterbox security product.

Our guide to preventing letterbox burglary goes into more detail. If you're choosing a new letterbox for security, look for the TS 008 standard, which covers letter plates that resist fishing.

Our best-selling letterbox security

Fire-retardant letterbox bag. Catches all your post, contains small fires, and locks shut when you need it to. £99.99.

View the bag →

Still not sure?

Send a photo of your letterbox, with a tape measure across it, to info@homesecureshop.co.uk. You can also call us on 0151 521 8089 or pop into our trade counter in Aintree.

Choosing a new letterbox? Our guide to choosing a letterbox for a uPVC door compares the options.

Ready to order? See all our letterboxes for uPVC and composite doors. Order by 2pm for same-day dispatch.