Fitting a letterbox is one of those jobs that looks simple on the surface, but getting it right makes a big difference to how your door looks, feels, and performs.

A properly installed letterbox should:

• Sit flush with the door

• Open and close smoothly

• Seal well against draughts

• Align cleanly with the internal aperture

Whether you’re replacing an old letterbox or fitting one into a new door, this guide will walk you through the process step by step — including the small details that make the difference between an average job and a professional finish.

What You’ll Need

Before starting, gather everything you need so the job runs smoothly.

Tools:

• Tape measure

• Pencil

• Drill and drill bits

• Spade bit or hole saw (for new installations)

• Jigsaw or multi-tool

• Screwdriver

• Chisel (for adjustments)

• Spirit level

Materials:

• Letterbox (external and internal plates)

• Fixing bolts or screws (usually supplied)

Before You Start: Check Measurements

Before fitting anything, confirm:

• The overall size of your new letterbox

• The fixing centres match your door (if replacing)

• The door thickness compatibility

If you’re replacing an existing letterbox, always measure first to avoid alignment issues.

If you need more guidance to how, see our guide on how to measure a letterbox.

Fitting a Letterbox Into an Existing Door (Replacement)

If your door already has a letterbox cut-out, this is the simplest scenario.

Step 1: Remove the Old Letterbox

• Unscrew the fixing bolts from the inside

• Separate the internal and external plates

• Carefully remove both sides

If the old letterbox is stuck, gently ease it out without damaging the door surface.

Step 2: Clean and Inspect the Opening

Check the existing aperture:

• Remove any debris or loose material

• Check for damage around the edges

• Ensure the opening is still square and clean

If needed, lightly sand or tidy the edges to ensure the new letterbox sits flush.

Step 3: Test Fit the New Letterbox

Before fixing anything:

• Place the external plate into position

• Align the internal plate from the inside

• Check that fixing holes line up

This step avoids surprises once you start tightening screws.

Step 4: Install the Letterbox

• Insert the external plate from the outside

• Align the internal plate on the inside

• Insert fixing bolts through the door

• Tighten evenly from inside

Do not overtighten. Tighten gradually and evenly to avoid distorting the plates.

Step 5: Test Operation

Open and close the flap several times:

• It should move smoothly

• It should sit flush when closed

• No rattling or looseness

If the flap sticks, check alignment and adjust slightly before tightening fully.

Fitting a Letterbox Into a New Door

If your door does not already have a cut-out, the process requires more care and accuracy.

Step 1: Mark the Position

Decide where the letterbox will sit.

Typical positioning:

• Around waist to chest height

• Centred horizontally on the door

Mark:

• The centre line

• The width of the aperture

• The height of the slot

Use a spirit level to keep everything straight.

Step 2: Drill Pilot Holes

Drill pilot holes at the corners of your marked aperture.

This makes it easier to cut cleanly and reduces the risk of splintering.

Step 3: Cut the Opening

Using a jigsaw or multi-tool:

• Carefully cut along the marked lines

• Keep the cut straight and controlled

Take your time here — this is the most critical part of the job.

Step 4: Check the Fit

Insert the letterbox into the opening:

• Ensure it fits snugly

• Check it sits flush against the door

If needed, trim the opening slightly for a better fit.

Step 5: Drill Fixing Holes

Once the letterbox is positioned:

• Mark the fixing hole positions

• Remove the letterbox

• Drill through the door

Ensure holes are straight to allow bolts to pass through cleanly.

Step 6: Install the Letterbox

Follow the same process as a replacement:

• External plate first

• Internal plate aligned

• Insert fixing bolts

• Tighten evenly

Step 7: Final Checks

Test:

• Flap movement

• Alignment

• Tightness

Ensure everything feels solid and operates smoothly.

Tips for a Professional Finish

These are the details that make the difference.

Keep everything level

Even a slight angle will be noticeable once installed.

Avoid overtightening

Too much pressure can warp the plates or affect flap movement.

Check flap direction

Ensure the external flap opens downward and closes naturally.

Seal if needed

On exposed doors, a light seal around the back can reduce draughts.

Common Problems and How to Fix Them

Letterbox is loose

• Check door thickness compatibility

• Tighten bolts evenly

Flap doesn’t close properly

• Check alignment

• Ensure nothing is obstructing the hinge

Draughts coming through

• Consider upgrading to a letterbox with internal seals or brushes

Fixing holes don’t align

• You may need to drill new holes or choose a different size

Choosing the Right Letterbox

If you’re replacing or upgrading, it’s worth choosing a letterbox that not only fits but improves performance.

Look for:

• Correct fixing centres

• Suitable door thickness range

• Durable finish (brass, chrome, black, stainless steel)

• Draught-reducing features

You can browse a wide range of letterboxes to find options that match your measurements and style.

Final Thoughts

Fitting a letterbox is a straightforward job when approached methodically.

The key steps are:

1. Measure carefully

2. Mark accurately

3. Cut cleanly (if required)

4. Align and fix evenly

Taking your time at each stage ensures a result that not only looks good but performs well for years to come.

A properly fitted letterbox improves both the appearance and functionality of your door — making it a small upgrade that delivers a noticeable difference.