What Size Mattress Does a Cot Bed Need? (UK Size Guide)

Choosing the right mattress for your cot bed sounds simple, until you start measuring it. With UK cot beds varying slightly between brands, and safety standards requiring a precise fit, knowing exactly what size mattress you need is one of the most important decisions you'll make for your baby's sleep space.

This guide walks you through standard UK cot bed mattress sizes, why accurate measurements matter, and what to do when your cot bed doesn't quite match the mattress on the shelf.

The Standard UK Cot Bed Mattress Size

In the UK, the standard cot bed mattress size is 140cm x 70cm. This has become the industry norm for full-sized cot beds - the kind designed to convert into a toddler bed once your child outgrows the cot stage.

For smaller cots (not cot beds), the standard cot mattress size in the UK is 120cm x 60cm.

These two sizes are often used interchangeably online, but they aren't the same. A "cot" and a "cot bed" are different products:

· Cot - a smaller, fixed-size baby bed that typically uses a 120 x 60cm mattress.

· Cot Bed - a larger, convertible bed that grows with your child and typically uses a 140 x 70cm mattress.

If your cot bed converts into a toddler bed, the same mattress usually carries through to that next stage – which makes getting the right size from day one even more worthwhile.

Why Accurate Cot Bed Mattress Measurements Matter

Cot bed mattress measurements aren't just a matter of fit and finish. UK safe sleep guidance, including advice from The Lullaby Trust and the British Standards makes clear that a properly fitted mattress is essential for safe sleep.

The risks of a poor fit are practical and serious:

· Gap Entrapment - a mattress that's too small leaves gaps at the sides, where small arms, legs or even a baby's head can become trapped.

· Suffocation Risk - gaps can create pockets where bedding bunches up, or where a baby could roll into a position that restricts breathing.

· Stability - an undersized mattress shifts around in the night, creating uneven surfaces and unpredictable sleep conditions.

How Should a Cot Bed Mattress Fit? (Safety Checklist)

A cot bed mattress should fit securely within the frame without movement.

Use this quick checklist:

· The gap between the mattress and cot bed frame is no more than 3cm (30mm) on any side

· The mattress sits level and even across the base

· It does not shift or slide during normal use

· The edges align neatly with the sides of the cot bed

· It sits on a solid base that supports the mattress properly

· Bedding fits closely without loosening or bunching

Guidance from The Lullaby Trust and relevant UK safety standards highlights that a well-fitted mattress helps reduce the risk of gaps forming around the sleep surface.

How to Measure Your Cot Bed Correctly

Before buying a mattress, always measure the internal dimensions of your cot bed - not the outside of the frame. Here's how:

1. Remove any existing mattress so you have a clear view of the base.

2. Measure the length from the inside of one end panel to the inside of the other.

3. Measure the width from the inside of one side rail to the inside of the other.

4. Note both measurements in centimetres.

5. If your cot bed has curved ends or rounded corners, note these too - they affect mattress shape, not just size.

Always measure twice. A few millimetres can be the difference between a snug, safe fit and a gap that doesn't meet safety guidance.

What Materials Are Best for Cot Bed Mattresses?

Cot bed mattresses are made using a range of materials, each offering different practical benefits.

Common types include:

· Foam mattresses – lightweight and widely available; should be high-density to maintain support

· Pocket sprung mattresses – contain individual springs for more structured support and durability

· Natural fibre mattresses – often use materials such as coir and wool, which can support airflow and temperature regulation

When a Custom Cot Mattress Is the Safest Option

Most parents assume any 140 x 70cm mattress will fit any UK cot bed - but that isn't always the case. Some cot beds, particularly older models, vintage finds, imported designs and handcrafted frames, have non-standard internal dimensions. Even a 1–2cm difference can take a cot bed outside the safe-fit guidance. If your cot bed falls outside standard sizing, a custom cot mattress is often the safest choice. Custom mattresses are made to your exact measurements, ensuring the fit meets safety standards regardless of how unusual your cot bed's shape may be.

A custom mattress is worth considering if:

· Your cot bed has a curved or shaped end panel.

· Your cot bed was bought second-hand and you don't know the original mattress size.

· You've measured and there are gaps present with your current mattress which requires replacing.

· Your cot bed is imported, handmade or from a smaller, independent brand.

You can browse standard sizes in the cot and cot bed mattress collection, or have one made entirely to your own measurements.

A custom mattress ensures that the fit remains within recommended safety limits, particularly where standard sizes would leave gaps exceeding guidance. For unusual cot bed shapes or dimensions, this is often the most reliable way to achieve a secure sleep surface.

Don't Forget the Mattress Protector

Once you've found the right mattress, the protector that goes over it needs to fit just as accurately. If you're going custom on your mattress, pair it with a made-to-measure mattress protector so the whole sleep surface fits as it should.

Cot Bed Mattress Fit and Bedding Compatibility

UK cot bed bedding is generally sold to fit the standard 140 x 70cm mattress. Fitted sheets in particular, so if your mattress falls outside it of the norm then we recommend custom bedding too, ensuring perfect and snug fitting bedding.

For safety, the right fit is always worth it.

How Often Should You Replace a Cot Bed Mattress?

A cot bed mattress should be replaced if it no longer provides a firm, flat and supportive surface.

You should consider replacing a cot bed mattress if:

· It shows signs of sagging, dipping or uneven wear

· The cover is damaged, torn or no longer hygienic

· It no longer feels firm or has lost its original structure

If possible, it is recommended to use a new mattress for each baby. 

Guidance from organisations such as The Lullaby Trust emphasises the importance of maintaining a firm and supportive sleep surface throughout use.

A Final Word

Getting your cot bed mattress size right is one of those decisions that quietly shapes the next two or three years of your child's sleep. The good news is that, once you've measured properly, it's a decision you only need to make once.

Measuring carefully and considering a custom-fit mattress is the safest, simplest way to make sure the fit is right from the very first night.

For the perfect mattress, browse our collection of carefully chosen sleep essentials, or contact us today to speak to our team – we're always happy to help.