📏 How to Measure a Letterbox: The Complete Homeowner’s Guide

A letterbox might seem like a simple household detail, but it plays an important role in both your home’s security and its curb appeal. Whether you’re upgrading your front door or replacing an old mail slot, the right letterbox can make a big difference — and it all starts with knowing how to measure a letterbox correctly.

In this quick and practical guide, we’ll walk you through how to measure a letterbox, what materials to look for, and the key features that make a letterbox secure, stylish, and built to last.

🧰 Why Measuring Your Letterbox Matters

Choosing the wrong size can lead to frustrating fitting issues — gaps, loose fittings, or restricted openings that make mail delivery difficult. Measuring ensures your new letterbox not only fits perfectly but also seals tightly for improved insulation and weather protection.

Before buying, take a few minutes to grab a tape measure and record three main measurements:

1. Width (side to side of the aperture)

2. Height (top to bottom of the slot)

3. Depth (door thickness the letterbox passes through)

A detailed guide from House Nameplate explains how standard letterbox sizes in the UK are often designed to accommodate A4-sized mail and why checking the width, height, and depth of the slot is so important.

🔎 Step-by-Step: How to Measure a Letterbox

Follow this simple method to make sure your new letterbox fits your door perfectly:

• Remove your existing letterbox (if you’re replacing one). This helps expose the cut-out size in your door.

• Measure the aperture width — the internal opening through which post is delivered.

  • Most standard letterboxes are around 250mm wide, with apertures around 230mm x 40mm.

• Measure the aperture height.

  • Height is often around 40–50mm, but always confirm to avoid a tight or loose fit.

• Measure the door thickness (depth).

  • This is crucial for ensuring the fixing bolts or sleeves are long enough.

• Double-check before ordering.

  • Even small differences in size can affect fitting quality and draught protection.

💡 Pro tip: Write your measurements down clearly and compare them with the specifications on the product page before purchasing.

🎨 Match Your Letterbox to Your Door Style

Once you know how to measure a letterbox, it’s time to choose a design that complements your door and overall aesthetic.

• Modern homes: Choose sleek finishes such as chrome or matte black for a minimalist look.

• Traditional properties: Gold or antique brass adds warmth and timeless charm.

• uPVC doors: Look for standard-sized, easy-fit models that sit flush and blend seamlessly with your existing door hardware.

➡️ Explore all styles here: Shop the full letterbox collection

Style Tip: Match your letterbox finish to your door handle, knocker, and hinges for a polished, cohesive entryway.

🛡️ Prioritise Quality and Security

When you know how to measure a letterbox correctly, your next priority should be finding one built to last. A well-constructed letterbox not only looks great but also protects your home from weather and security threats.

🧱 Recommended Materials

• Aluminium: Lightweight, corrosion-resistant, and ideal for coastal properties.

• Stainless Steel: Sleek, modern, and highly durable.

• ABS Plastic with coated finishes: Common in uPVC letterboxes, offering both strength and value.

🔒 Security Features to Consider

• Anti-fishing flaps to stop intruders from reaching through.

• Internal draught excluders for insulation and privacy.

• Secure fixings to prevent tampering or forced entry.

For added safety, pair your letterbox with an internal fire-retardant bag or security cage to protect against arson or mail theft.

📏 Common Letterbox Sizes

Once you understand how to measure a letterbox, you’ll notice that most follow standardised sizing — but checking remains crucial:

Door Type	Typical External Plate Size	Aperture (Opening)	Door Thickness Compatibility
uPVC	250mm x 70mm	230mm x 40mm	20–80mm
Composite	265mm x 75mm	240mm x 45mm	44–70mm
Timber	Variable	Variable	Custom-fit

🧮 Always measure your existing opening before ordering — even a few millimetres off can affect alignment and performance.

✨ Colour and Finish Options

The right finish can elevate your entryway and complement your home’s style. Popular finishes include:

• White: Clean, neutral, and timeless.

• Black: Modern and bold, ideal for contrast doors.

• Chrome: Reflective and sleek for contemporary designs.

• Gold: Traditional and luxurious, perfect for classic homes.

Many Home Secure letterboxes include UV-resistant coatings to prevent fading and weather seals to keep rain and draughts at bay.

🔧 Installation & Maintenance

Once you’ve chosen your letterbox, installation is straightforward — especially when you’ve taken accurate measurements.

• Use the manufacturer’s fitting template (if provided).

• Ensure a snug, even fit with minimal gaps.

• Tighten screws securely but avoid over-torquing, which can warp frames.

• Clean occasionally with mild soap and water — avoid abrasive cleaners.

Regular checks after extreme weather help maintain performance and appearance.

📜 Understand the TS 008 Security Standard

If you’re upgrading your letterbox, look for TS 008:2022 certification — the UK’s official security and performance standard for letter plates.

This specification, set by the Door & Hardware Federation (DHF), ensures a letterbox can resist forced entry, block “fishing” attacks, and maintain strength during fire or weather exposure.

Choosing a TS 008-compliant model gives you peace of mind that your letterbox contributes to your home’s overall security, not just its style.

💬 Final Thoughts

Knowing how to measure a letterbox accurately makes the whole replacement process easier, faster, and more reliable. Once you’ve measured correctly, choosing the perfect design is all about balancing aesthetics, materials, and security.

From brushed chrome to matte black, and from anti-fishing flaps to draught protection, the right letterbox enhances both your home’s look and its security.

So, before you order your next upgrade — measure twice, choose once, and make sure your new letterbox fits as perfectly as it looks.

Check out the featured products from this article below 👇 Including Elegance Wall Mounted Letter Box – Outdoor Mailbox