Letterbox Sizes Guide

If you’re replacing a letterbox, one of the first questions you’ll likely ask is:

Are letterboxes a standard size?

The short answer is no — letterboxes are not one standard size.

While there are common size ranges used across the UK, different designs, door types and manufacturers mean there is no single “universal” letterbox size. That’s why measuring your existing letterbox is always the most important step before buying a replacement.

In this guide, we’ll explain how letterbox sizes work, what sizes are commonly available, and what you should look for when choosing the right one.

Once you’ve taken your measurements and understand the size you need, you can explore the full range of letterboxes to find a style and finish that suits your door. Each product includes detailed dimensions, making it easier to match your measurements and choose a replacement with confidence.

are letterboxes a standard size

Why Letterboxes Aren’t Standardised

Unlike some door hardware, letterboxes need to fit a cut-out in your door. That cut-out can vary depending on:

  • The age of the door

  • The type of door (timber, uPVC, composite)

  • The style of letterbox (traditional, telescopic, vertical)

  • Manufacturer differences

Because of this, letterboxes are produced in a range of sizes rather than one fixed standard.

 

Common Letterbox Sizes (Real UK Examples)

Although there isn’t a single standard, most letterboxes fall into a few recognisable size groups.

Looking at typical products available, you’ll commonly see sizes such as:

Smaller letterboxes (around 250mm wide)

These are often found on traditional or older doors.

Examples include:

  • Around 254mm x 79mm

  • Around 254mm x 100mm

These tend to be more compact and are often used with classic designs like postal knockers or traditional brass plates.

 

Mid-size letterboxes (around 300mm wide)

This is one of the most common size ranges for UK homes.

Examples include:

  • Around 300mm x 95mm

  • Around 305mm x 100mm

  • Around 306mm x 104mm

These sizes are widely used on timber and composite doors and are often considered the “typical” modern letterbox size — but even here, small variations exist.

 

uPVC letterbox sizes (around 310mm wide)

Modern uPVC and composite doors often use a more standardised format, particularly for telescopic letterboxes.

A very common example is:

  • 310mm x 76mm

This size appears frequently across uPVC letterbox designs, but even within this category, variations in internal components and fixing centres can still occur.

 

Larger and specialist sizes

Some letterboxes are slightly larger or designed for specific applications.

Examples include:

  • Around 315mm x 115mm

  • Around 331mm x 80mm

These are often used where:

  • A larger opening is needed

  • A decorative design is preferred

  • Internal tidies require more coverage

 

Internal letterbox tidy sizes

It’s also important to remember that internal components (letterbox tidies) have their own sizing.

Common examples include:

  • Around 260mm x 88mm

  • Around 310mm x 113mm

These don’t always match the external letter plate exactly, which is why both sides should be checked when replacing a full set.

 

What This Means in Practice

Even though you’ll see similar widths like 254mm, 305mm or 310mm repeated, the key takeaway is:

Small differences matter.

For example:

  • 305mm and 306mm may sound interchangeable — but may not align perfectly

  • Fixing centres can vary even when overall size looks the same

  • Aperture sizes may differ between designs

This is why assuming “they all look the same” often leads to fitting problems.

 

What You Should Measure Instead

Rather than focusing only on overall size, you should always check:

  • Fixing centres (most important)

  • Overall plate size (for coverage)

  • Aperture size (for functionality)

  • Door thickness compatibility

These measurements determine whether a letterbox will actually fit your door.

 

Are Any Letterboxes “Standard”?

Some sizes are more common than others — particularly:

  • Around 300mm wide for timber doors

  • Around 310mm x 76mm for uPVC doors

However, these are best described as common sizes, not true standards.

Even within these ranges, variations still exist.

 

Choosing the Right Size for Your Door

When selecting a replacement letterbox:

  • Match your existing measurements wherever possible

  • Choose a slightly larger plate if you need to cover marks from an old fitting

  • Ensure fixing holes line up or be prepared to drill new ones

  • Consider upgrading to a more common size if your current one is unusual

Taking the time to measure properly will always give you a better result than trying to match visually.

 

Final Thoughts

So, are letterboxes a standard size?

No — but they do fall into common size ranges.

Most letterboxes you’ll come across will sit within groups such as:

  • Around 250mm (smaller, traditional)

  • Around 300mm (most common modern size)

  • Around 310mm (common for uPVC doors)

  • Larger or specialist sizes for specific designs

The key is not to rely on assumptions. Even small differences in size can affect fit, alignment and appearance.

By measuring carefully and understanding these size ranges, you can confidently choose a letterbox that fits properly, looks right, and performs well for years to come.