Kids mattress size guide 2025: what type and size f mattress should you get your child?

Every child needs a good night's sleep, and the best way to ensure that happens is to get them a comfortable and supportive mattress in the right size. That's why T3 has dedicated guides to the best cot bed mattress for toddlers and the best kids' mattress, as well as a general best mattress guide that includes UK singles and US twins.

However, if you're not entirely sure what you're looking for, it can be tricky to know what size and style of mattress is suitable for what age. In this kids mattress size guide, we've gathered together all the information you need about mattress sizes for cot beds, crib beds, toddler beds and your child's first 'proper bed', plus the ages they roughly correlate to.

Bear in mind: mattress sizes are largely uniform across the industry, but inevitably there are small variations because there's no legal requirement to conform to precise sizes. So if you're planning on buying a child's mattress separately from a bed, be sure to grab a tape measure first and check that it'll fit.

What size is a crib or cot mattress?

The best mattress for a newborn is one specifically designed for babies: what's known as a cot mattress in the UK and a crib mattress in the USA. This should be the correct size for your baby's cot/crib, so it fits perfectly and there are no dangerous or uncomfortable gaps.

In the US, the standard size of a crib mattress is 27.5 x 52 inches (70 x 132cm). 

In the UK, there are two standard sizes for cot mattresses, namely, standard UK cot bed mattress size is 70cm x 140cm (27.5 x 55 inches) and continental cot size is 60cm x 120cm (24 x 48 inches).

What size is a toddler mattress in the UK and US?

Although there's no hard and fast rule, babies are usually moved from a cot bed to a toddler bed at around 18-36 months. This is a transitional bed between a cot and a full bed, although many parents decide to skip this stage and move their child straight to the latter.

In both the UK and US, mattresses for toddler beds are the same size as those for cot beds. In other words, they are 27.5 x 52 inches (70 x 132cm) in the US, and 70 x 140cm (27.5 x 55 inches) in the UK. 

In theory, then, it's fine to use the same mattress as they had in their cot, and you don't need to buy a new one. In practice, of course, there may be wear and tear on the old mattress that means it's a good time to replace it anyway. 

What size is a child's first 'proper bed' in the UK and US?

Once your child has outgrown a toddler bed, the next step is to move them to a regular bed. On the whole, the best kids' mattress is one that's at least 10cm / 4 inches longer than the height of the child sleeping on it.

Depending on their size and age, for children in the UK you'll generally want one of the following bed sizes (and mattress dimensions to match):

• UK Small Single: 75 x 120cm
• UK Standard Single: 90 x 190cm
• UK European Single: 90 x 200cm
• UK Small Double: 120 x 190cm

In the US, meanwhile, you'll be looking at the following standard sizes for children. These are all adult sizes, with the exception of the Junior category, which is specifically designed as an intermediary between a toddler bed and a full bed.

• US Junior: 27.5 x 63 inches (70 x 160cm)
• US Twin/Single: 38.5 x 74.5 inches (98 x 189cm)
• US Twin XL: 38.5 x 79.5 inches (98 x 202cm)
• US Full/Double: 53.5 x 74.5 inches (136 x 189cm)

Of course, there's no limit to how big your child's bed can or should be, beyond your budget and the size of their room. So, there's no reason in theory you can't buy them an even bigger bed than those listed. If you wish to do just that, check out our general mattress size guide for adults.