Choosing the right mattress for your baby's sleep space is one of the most important decisions you'll make when preparing your nursery. Whether you're buying a Moses basket, bedside crib, cot, cot bed or toddler bed, selecting the correct mattress size is essential for achieving the proper fit and ensuring the mattress is suitable for the furniture it is designed to be used with.

One of the biggest misconceptions is that all cots use the same mattress size, or that every cot bed requires a 140 × 70 cm mattress. In reality, nursery furniture is available in several sizes, and manufacturers may use different dimensions depending on the design of the product.

For example, many traditional cots use a 120 × 60 cm mattress, while many larger cot beds use 140 × 70 cm. However, some cot beds also use 120 × 60 cm mattresses, so the name of the furniture alone should never be used to determine which mattress to buy.

The safest and most reliable approach is always to purchase the mattress size recommended by the furniture manufacturer.

Throughout this guide we'll explain everything you need to know about nursery mattress sizes in the UK, including common dimensions, how to measure correctly, current safety standards, mattress depth, compatibility, and answers to the questions parents ask most often.

Whether you're expecting your first baby or replacing an existing mattress, this guide has been written to help you make an informed decision.

Contents

• Why Choosing the Correct Mattress Size Matters
• UK Nursery Mattress Sizes at a Glance
• Moses Basket Mattress Sizes
• Crib Mattress Sizes
• Bedside Crib Mattress Sizes
• Travel Cot Mattress Sizes
• Standard UK Cot Mattress Sizes
• Standard UK Cot Bed Mattress Sizes
• Cot vs Cot Bed – Why Mattress Size Doesn't Tell the Whole Story
• Toddler Bed Mattress Sizes
• How to Measure a Cot Mattress Correctly
• What If My Mattress Doesn't Fit?
• Mattress Thickness Explained
• Understanding UK Nursery Mattress Safety Standards
• Fire Safety & Flame Retardants Explained
• Common Cot Mattress Buying Mistakes
• Frequently Asked Questions

Why Choosing the Correct Mattress Size Matters

A nursery mattress should never be viewed as a universal product. Even if two cots appear almost identical, their internal dimensions may differ slightly, and a mattress that fits one product perfectly may not be suitable for another.

The NHS recommends that a baby's mattress should fit snugly inside the cot, with no significant gaps around the edges where a baby's head could become trapped. Likewise, The Lullaby Trust advises choosing a firm, flat mattress that fits the sleep space correctly and is intended for that particular product.

Buying a mattress that is too small may leave gaps between the mattress and the cot sides, while a mattress that is too large may not lie completely flat or could become compressed within the frame. Neither situation is desirable.

For this reason, reputable nursery furniture manufacturers always specify the mattress dimensions designed for their products.

Before purchasing a mattress, always check:

• The manufacturer's recommended mattress size
• The internal dimensions of the cot or cot bed
• The maximum permitted mattress depth (where specified)
• Whether the mattress supplied with the furniture should be replaced with an identical size
• Any compatibility guidance included within the product instructions

Choosing the correct dimensions from the outset helps ensure the mattress is appropriate for the furniture and avoids unnecessary returns or exchanges.

UK Nursery Mattress Sizes at a Glance

The table below summarises the mattress sizes most commonly encountered across UK nursery furniture.

Important: Mattress size alone does not determine whether furniture is a cot or a cot bed. A convertible cot bed may use either a 120 × 60 cm or a 140 × 70 cm mattress depending on its design.

Understanding Nursery Mattress Dimensions

Nursery mattresses are almost always listed using three measurements:

Length × Width × Depth

For example:

120 × 60 × 10 cm

means:

• Length: 120 cm
• Width: 60 cm
• Depth: 10 cm

Some manufacturers reverse the first two figures and list the mattress as 60 × 120 cm. This normally refers to exactly the same mattress, but it is always worth checking the product specification before ordering.

Depth is equally important. A mattress may have the correct length and width but still be unsuitable if it exceeds the maximum depth specified by the cot manufacturer.

Throughout this guide, measurements are given in metric units because these are the dimensions used by the vast majority of UK nursery furniture manufacturers.

Moses Basket Mattress Sizes

A Moses basket is often one of the first sleep spaces used during the newborn stage, but unlike cot mattresses, Moses basket mattresses do not follow a single standard size.

Instead, dimensions vary according to the design of the basket and the manufacturer.

You'll commonly find mattresses that are:

• Rectangular
• Oval
• Rounded at one or both ends
• Tapered to fit a specific basket

Even two baskets with similar external dimensions may require different mattresses because the internal base shapes are different.

For this reason, replacement Moses basket mattresses should always match the manufacturer's recommended dimensions.

Rather than measuring the outside of the basket, measure the internal base where the mattress sits, unless the manufacturer already provides the exact replacement size.

If a replacement mattress is available specifically for your basket model, that is usually the simplest and most reliable option.

When replacing a Moses basket mattress, always check:

• Length
• Width
• Shape
• Corner profile
• Maximum permitted depth
• Manufacturer compatibility

Avoid trying to adapt or trim a mattress to make it fit. A nursery mattress should always be designed for the product it is intended to be used with.

Why This Guide Is Different

Many mattress guides stop after saying "buy a 120 × 60 cm cot mattress."

Unfortunately, nursery furniture isn't that simple.

Over the next sections, we'll explain why cribs, bedside cribs, travel cots, cots, cot beds and toddler beds all have different sizing considerations, how to identify the correct mattress for each, and the UK safety standards every parent should understand before making a purchase.

Crib Mattress Sizes

Cribs are typically designed for babies during the early months and are generally smaller than a full-sized cot. However, unlike standard cot mattresses, there is no single universal crib mattress size in the UK.

Different manufacturers produce cribs in a range of dimensions, meaning the correct mattress size depends entirely on the model you own. Some cribs use rectangular mattresses, while others have rounded corners or unique shapes that require a model-specific replacement.

For this reason, it's always best to check the manufacturer's instructions before purchasing a replacement mattress.

When choosing a crib mattress, look for one that:

• Matches the exact dimensions recommended by the manufacturer.
• Fits snugly without leaving gaps around the edges.
• Lies completely flat without bending or buckling.
• Is suitable for the specific crib model.

Even if two cribs appear similar, they may not use the same mattress, so avoid assuming that a mattress from one brand will fit another.

Bedside Crib Mattress Sizes

Bedside cribs, sometimes called bedside sleepers or co-sleeping cribs, have become increasingly popular because they allow babies to sleep in their own separate sleep space while remaining close to a parent's bed.

Like standard cribs, bedside cribs do not have a universal mattress size.

Many manufacturers design their bedside cribs with bespoke mattresses that are made specifically for that model. Some have rounded corners, curved edges or attachment systems that make a generic replacement unsuitable.

If you need a replacement mattress, always check:

• The exact model of your bedside crib.
• The mattress dimensions specified by the manufacturer.
• Whether replacement mattresses are designed specifically for your product.
• The recommended maximum mattress depth.

A mattress that appears to fit visually may still leave small gaps or prevent the crib from functioning as intended.

As with all nursery sleep products, the mattress should fit snugly and should never require forcing into place.

Travel Cot Mattress Sizes

Travel cots are designed differently from permanent nursery furniture, so their mattresses deserve special consideration.

Unlike standard cots, travel cots are supplied as complete sleep products, and the mattress or padded base is designed to work with the folding frame. Although these mattresses are often thinner than those used in full-sized cots, this is usually intentional and forms part of the product's overall design.

Parents sometimes wonder whether adding a thicker mattress will make a travel cot more comfortable. However, manufacturers generally recommend using only the mattress supplied with the travel cot or a replacement that has been specifically approved for that model.

Using a mattress that wasn't designed for the travel cot may affect how it fits within the frame or alter the intended sleeping environment.

If you're replacing a travel cot mattress, check:

• The exact make and model of the travel cot.
• The manufacturer's recommended mattress dimensions.
• The maximum permitted mattress depth.
• Whether the replacement mattress has been designed specifically for your travel cot.

Never assume that a standard 120 × 60 cm cot mattress will fit a travel cot simply because the measurements appear similar. Travel cots vary in their internal dimensions, corner shapes, folding mechanisms and mattress specifications.

Why Nursery Mattress Sizes Aren't Universal

One of the biggest sources of confusion for parents is the assumption that nursery mattresses follow a small number of fixed sizes.

In reality, while certain dimensions are common, there is no single standard that applies to every nursery sleep product.

Manufacturers design furniture in different ways to suit their own collections, styles and conversion features. As a result, two products described as "cot beds" may use different mattress sizes, while two products requiring the same mattress size may be described differently.

Rather than relying on product names, always use the manufacturer's specified mattress dimensions when choosing a replacement.

Standard UK Cot Mattress Size

The mattress size most commonly associated with a traditional UK cot is:

120 × 60 cm

This measures approximately:

• 120 cm long
• 60 cm wide
• 47.2 inches × 23.6 inches

A 120 × 60 cm mattress is widely available throughout the UK and is used by many nursery furniture manufacturers. Because of its popularity, you'll often see it described as a standard cot mattress size.

However, it's important to remember that common does not mean universal.

Some cots use different dimensions, while some cot beds also use a 120 × 60 cm mattress. The correct choice is always the one specified by the furniture manufacturer.

You may also see the measurements written as 60 × 120 cm. In most cases, this describes exactly the same mattress, simply with the dimensions presented in a different order.

When purchasing a replacement mattress, check not only the length and width but also the mattress depth, as some manufacturers specify a maximum permitted depth for safe use.

Which Furniture Uses a 120 × 60 cm Mattress?

A 120 × 60 cm mattress is commonly found in:

• Traditional cots.
• Compact nursery furniture.
• Some convertible cot beds.
• Selected European nursery furniture.

Because this size is used across several different furniture types, it should never be assumed that a mattress labelled "120 × 60 cm" is suitable simply because your furniture is called a cot or cot bed.

Always match the mattress to the product specifications rather than the product name.

Standard UK Cot Bed Mattress Size

The second size you'll commonly encounter in the UK is:

140 × 70 cm

This measures approximately:

• 140 cm long
• 70 cm wide
• 55.1 inches × 27.6 inches

A 140 × 70 cm mattress is frequently used with larger cot beds designed to convert into toddler beds, providing additional sleeping space as children grow.

Compared with a 120 × 60 cm mattress, it offers:

• 20 cm more length
• 10 cm more width
• Approximately 36% more sleeping surface area

That additional space is one of the reasons why many manufacturers choose this size for furniture intended to last beyond the baby stage.

Like the 120 × 60 cm mattress, however, 140 × 70 cm is not a universal cot bed size.

While many cot beds require a 140 × 70 cm mattress, others are designed around a 120 × 60 cm mattress and still convert into a toddler bed.

Cot vs Cot Bed: Why Mattress Size Doesn't Tell the Whole Story

This is one of the most common misunderstandings when buying nursery furniture.

Many parents assume that:

• A 120 × 60 cm mattress means it's a cot.
• A 140 × 70 cm mattress means it's a cot bed.

In reality, that isn't how nursery furniture is defined.

The key difference between a cot and a cot bed is whether the furniture is designed to convert into a toddler bed or another larger sleeping configuration.

A traditional cot is generally intended for use during the baby stage, whereas a cot bed is designed to adapt as your child grows, usually by removing the cot sides and converting into a toddler bed.

Both types of furniture may use different mattress sizes depending on the manufacturer's design.

For example:

• A convertible cot bed may use a 120 × 60 cm mattress.
• Another cot bed may use a 140 × 70 cm mattress.
• A traditional cot may also use 120 × 60 cm.

This is why it's important not to choose a mattress based solely on the furniture's name.

Instead, always check:

• The manufacturer's recommended mattress size.
• The product specifications.
• The internal sleeping dimensions.
• Any compatibility information provided with the furniture.

Choosing by measurement rather than marketing description is the simplest way to ensure you purchase the correct mattress.

Toddler Bed Mattress Sizes

Many cot beds are designed to convert into toddler beds, allowing families to continue using the same furniture as their child grows.

The most common mattress size used for toddler bed configurations is:

140 × 70 cm

Because the mattress remains the same when the cot bed is converted, there's often no need to purchase a new mattress, provided it remains in good condition and the manufacturer advises it is suitable to continue using.

However, not every toddler bed uses a 140 × 70 cm mattress. Some compact convertible cot beds continue to use a 120 × 60 cm mattress, while other standalone toddler beds may have their own manufacturer-specific dimensions.

As with every nursery sleep product, the correct mattress is always the one specified for the furniture.

Rather than shopping by age or product description, shop by the manufacturer's recommended mattress dimensions.