How to Measure a Letterbox

Measuring a letterbox properly is the key to getting a replacement that fits perfectly, looks right, and works smoothly day after day.

While letterboxes may appear fairly standard, there are actually several variations in size, fixing positions and door compatibility. Even small differences can mean a new letterbox won’t line up with your existing holes or won’t fully cover the cut-out in your door.

Whether you’re replacing a worn letterbox, upgrading the finish, or fixing draught issues, this guide will walk you through exactly what to measure, what matters most, and how to avoid the common mistakes that cause fitting problems.

Are letterboxes a standard size?

No, letterboxes are not completely standardised, although many fall within common size ranges.

In the UK, you’ll often find typical measurements such as:

• Overall length around 250mm–300mm
  
• Fixing centres commonly between 230mm–260mm

However, these are not fixed standards. Different manufacturers, styles and door types can vary slightly, especially with older doors or decorative designs.

Because of this, it’s always important to measure your existing letterbox rather than relying on assumptions. Even a small difference in fixing centres or overall size can affect how the new letterbox fits.

Why Accurate Letterbox Measurement Is So Important

A letterbox is fitted through a pre-cut opening in your door. Unlike some other door hardware, there is very little flexibility when it comes to sizing.

If your measurements are incorrect:

• The new letterbox may not cover the existing hole
  
• Fixing screws won’t line up
  
• Gaps or exposed edges may be visible
  
• Installation becomes more complex than expected

Taking accurate measurements ensures a clean, secure fit and avoids unnecessary adjustments or replacements.

The 4 Key Measurements You Must Take

To correctly measure a letterbox, you need to record four essential dimensions:

• Overall length (external size)
  
• Aperture size (internal slot opening)
  
• Fixing centres (distance between screw holes)
  
• Door thickness

Each one plays a different role in ensuring compatibility.

1. Overall Length (External Plate Size)

This is the total length of the letterbox across the front plate.

How to measure:

• Measure from one end of the external plate to the other
  
• Do not include moving flaps or projections
  

Why it matters:

This measurement ensures the new letterbox will fully cover the existing cut-out in your door.

If the replacement is smaller than your current one, it may leave visible marks or exposed areas where the old letterbox sat.

2. Aperture Size (Letter Slot Opening)

The aperture is the opening where post is delivered through the door.

How to measure:

• Measure the width of the opening
  
• Measure the height of the opening
  

Why it matters:

• Ensures letters and small parcels fit through comfortably
  
• Matches the internal cut-out in your door

If the aperture is too small, post may get stuck.
If it’s too large, you may expose rough or unfinished edges inside the door.

3. Fixing Centres (Most Critical Measurement)

Fixing centres refer to the distance between the two screw holes that secure the letterbox.

How to measure:

• Measure from the centre of one fixing hole to the centre of the other
  
• Always measure in millimetres
  

Why it matters:

This determines whether your new letterbox will align with your existing fixing holes.

If this measurement is even slightly off, you may need to drill new holes or adjust the installation — something most homeowners want to avoid.

4. Door Thickness

Letterboxes are designed to fit specific door thickness ranges.

How to measure:

Measure from the outside face of the door to the inside face

Why it matters:

If the letterbox is not suitable for your door thickness:

• It may not tighten properly
  
• The internal and external plates may not connect securely
  
• The letterbox may feel loose or unstable
  
• Visual Guide: Letterbox Measurements

Use this diagram to understand exactly where each measurement is taken:

Standard Letterbox Sizes (UK Guidance)

While letterboxes vary, there are some commonly used size ranges in the UK.

Typical ranges include:

• Overall length: approximately 250mm to 300mm
  
• Fixing centres: often between 230mm and 260mm
  
• Aperture width: varies depending on style

These are not fixed standards, so always measure your existing letterbox rather than relying on estimates.

Measuring Different Door Types

The type of door you have can affect how you measure and what to consider.

uPVC and Composite Doors

• Usually have pre-cut apertures that must be matched closely
  
• Often require letterboxes with specific fixing centres
  
• Limited flexibility for adjustment

Accuracy is especially important with these doors.

Timber Doors

• Easier to adjust if needed
  
• Minor changes to fixing holes or aperture are possible
  
• Greater flexibility when replacing
  
• What If Your Measurements Don’t Match?

This is a very common situation.

If your measurements don’t match standard sizes:

• Choose a slightly larger external plate to cover the existing cut-out
  
• Ensure fixing centres are as close as possible
  
• In some cases, new fixing holes may be required

If your current letterbox is non-standard or older, upgrading to a more common size can make future replacements easier.

Common Problems That Indicate You Need a New Letterbox

While measuring, it’s worth checking whether your current letterbox needs replacing.

Signs include:

• Draughts coming through the door
  
• Loose or rattling letterbox
  
• Flap not closing properly
  
• Corrosion or worn finish
  
• Noise when the flap moves

Modern letterboxes often include draught reduction features and improved build quality, making them a worthwhile upgrade.

Choosing the Right Replacement After Measuring

Once you have your measurements, the next step is choosing a suitable replacement.

When comparing options, consider:

• Matching fixing centres wherever possible
  
• Choosing a finish that suits your door
  
• Ensuring compatibility with your door thickness
  
• Selecting a design that improves insulation or reduces draughts

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

Each product includes detailed specifications to help you confirm compatibility before purchasing.

Common Mistakes to Avoid

Even with a simple job like this, a few mistakes can cause issues:

• Measuring from the edge of screw holes instead of centre to centre
  
• Forgetting to check door thickness
  
• Assuming all letterboxes are the same size
  
• Choosing a replacement that is too small to cover the existing cut-out
  
• Ignoring aperture size

Taking a careful, methodical approach avoids all of these problems.

Final Thoughts

Measuring a letterbox correctly is a straightforward process, but it requires attention to detail.

Focus on the four key measurements:

• Overall length
  
• Aperture size
  
• Fixing centres
  
• Door thickness

With these confirmed, you can confidently choose a replacement that fits properly, looks right, and performs well.

A well-fitted letterbox not only improves the appearance of your door but also ensures smooth operation, better insulation and long-term durability.

Once you have your new letterbox, you'll need to fit it. See our How to fit a letterbox guide.