Cot Mattress Sizes UK: Every Standard Size Explained

These are the sizes you will actually meet in the UK, from smallest to largest. Where a size varies by brand we say so.

Across the 270 plus models in our fitment database, 140 x 70 cm covers more cots than any other size, with 120 x 60 cm second and 132 x 70 cm a meaningful third. Everything else is brand-specific.

There is no law that says a UK cot must take a 120 x 60 cm mattress. The safety standard for cots, BS EN 716-1:2017, governs things like bar spacing and rail height, not the internal size of the cot, which it only requires to be somewhere between 90 and 140 cm long. So every manufacturer picks its own dimensions, and plenty of them deviate from the two common sizes.

The repeat offenders we see in our database:

• Boori cot beds are 132 x 70 cm. They look exactly like standard cot beds, and a 140 x 70 cm mattress genuinely will not go in. This is the single most common mis-buy we know of.
• SnuzKot takes 117 x 68 cm. Neither standard size fits.
• Stokke Sleepi is oval. It needs a shaped 122 x 68 cm mattress; no rectangular mattress fits safely.
• Older IKEA models vary. Current IKEA cots are 120 x 60 or 140 x 70, but discontinued models sold second-hand are not always either.
• Cribs are the wild west. Bedside cribs differ by a centimetre or two between models, and with a crib a centimetre matters.

This is exactly why Nappo exists: pick your cot brand and model and we tell you the mattress size that fits it, so you never have to trust a guess.

If your cot is second-hand, unbranded or not in our database yet, measure it. Three rules:

• Measure inside the cot, not outside. You want the internal length and width at mattress-base level, from inside face to inside face. The external frame size is irrelevant and always bigger.
• Measure at the base, in two or three places. Some cots taper slightly or have fixings that intrude. Use the smallest figure you get.
• Buy to the internal size, not under it. The mattress should fit snugly. The Lullaby Trust is clear that a cot mattress must fit the cot with no gaps around the edge, because a gap is a trap for small limbs. If your cot measures 140 x 70 cm, buy 140 x 70 cm, not "close enough".

If you want a number, the cot safety standard BS EN 716-1 puts one on it: the gap between the mattress and the sides and ends of the cot must not exceed 3 cm. In practice a correct mattress needs a gentle push to go in and sits flat without bowing.

Most UK cot mattresses are around 8 to 10 cm thick. That figure is industry convention rather than official guidance: no safety body publishes a minimum depth, and a well-made 8 cm mattress supports a baby perfectly well. Within the normal range, thickness buys durability and comfort for a growing toddler, not safety.

What the rules do fix is a maximum. BS EN 716-1 requires at least 50 cm from the surface of the mattress to the top of the cot rail with the base in its lowest position (20 cm in the highest), which is why your cot has a maximum mattress depth mark inside it. A mattress that is too deep raises your baby too close to the top of the rail, and an older baby will climb. If your cot has an adjustable base, drop it to the lowest position as soon as your baby can sit up.

So: match the length and width exactly, keep the depth within whatever your cot's maximum marking allows, and do not assume thicker is better.

Size gets the mattress into the cot; these rules make it safe once it is in. All of this is standard guidance from The Lullaby Trust, the UK's safer sleep charity:

• Firm and flat. The Lullaby Trust's words: "The safest mattresses are firm, flat and protected by a waterproof cover." Their firmness test is simple: when your baby lies on it, their head should not sink in by more than a few millimetres. No raised or cushioned edges or sides.
• Waterproof cover. A wipeable, waterproof layer keeps the core clean and dry, and is part of the same Lullaby Trust rule above.
• No gaps. Covered above, but it bears repeating: the mattress must fit the cot without gaps around the edge.
• Be careful with second-hand mattresses. The Lullaby Trust recommends a new mattress for each baby where possible. If you do reuse one, their checklist asks for a fully waterproof cover, no rips or tears, no sagging, a fire-resistant label, a snug fit in your cot, and storage that was clean, dry and smoke-free in between.
• Look for BS EN 16890:2017+A1:2021. It is the British and European safety standard for cot and crib mattresses and includes firmness and entrapment testing. A mattress sold for UK cots should state compliance.

First, check it really is: look your cot up, because several "odd" cots have an off-the-shelf mattress size that is simply less common, like Boori's 132 x 70 cm.

If it genuinely is non-standard, you have two honest options. Several UK manufacturers make cot mattresses to measure in any size or shape, usually for a modest premium over the standard sizes, with delivery in about a week or two. That is the right answer for older cots, imported cots and heirloom furniture. The wrong answer is forcing a near-miss standard size in anyway: a mattress that has to bow to fit will leave gaps at the corners, and one that is too small leaves a gap along an edge. Both fail the no-gaps rule above.

One more case: if the cot is very old, check it before you invest in any mattress. Bars wider apart than modern standards allow, peeling paint or a drop side with worn fixings are reasons to retire the cot, not re-mattress it.