UK Cot & Cot Bed Mattress Sizes: Standard Dimensions & Gap Limits

Two mattress sizes cover almost every cot and cot bed sold in the UK. A standard cot mattress measures 120cm by 60cm, and a standard cot bed mattress measures 140cm by 70cm. Depth varies by product rather than by standard, and a foam mattress for this age group is typically in the region of 10cm thick.

The dimension that matters more than either is the gap. British Standard BS EN 716, which covers cots and folding cots for domestic use, sets out the permitted gap between the mattress and the cot sides and ends, and the Lullaby Trust advises that a cot mattress should fit snugly with no gaps into which a baby could roll. Getting the fit right is a safety decision, not a comfort one, and it is the reason the mattress should always be bought to the cot rather than the other way round.

Key findings at a glance

• A standard UK cot mattress is 120cm by 60cm (47in by 24in), and a standard UK cot bed mattress is 140cm by 70cm (55in by 28in).
• BS EN 716 is the British Standard governing cots and folding cots for domestic use, including the permitted gap between mattress and cot frame.
• The Lullaby Trust advises a firm, flat mattress that fits the cot snugly with no gaps, and that babies are placed on their back to sleep.
• Continental and imported cots are frequently 120cm by 60cm but not always, and a few European sizes differ enough to make a UK standard mattress an unsafe fit.
• Cot bed mattresses need to be bought against the specific frame, because 140cm by 70cm is common rather than universal across the category.
• Mattress depth is not standardised: what matters is that the distance from mattress surface to the top of the cot side remains adequate as the base is lowered.
• Second hand cots can be used safely with a new, correctly sized mattress, but the frame must still meet current standards and have all its fittings.

The two standard sizes, and why they exist

The 120cm by 60cm cot and the 140cm by 70cm cot bed are the two formats British parents encounter. The smaller size is intended for the first phase, typically from a few months old until the child outgrows a cot. The larger cot bed is designed to convert: the sides come off and the base drops, and it continues as a low toddler bed after the cot function ends.

That conversion is why the cot bed is now the default purchase in UK nurseries. It occupies more floor area from the outset, but it removes one furniture change from the first years, which matters in a small box room where the cot, a chest of drawers and a chair already fill the space. The cost is that a cot bed is considerably harder to move between rooms and rarely fits alongside a parent’s bed in the way a smaller cot can.

Neither dimension is a legal requirement in itself. They are conventions the industry has settled on, which is precisely why they cannot be assumed. A mattress described as standard cot size will fit a standard cot, but any cot bought outside those conventions, including some imported and some heritage designs, needs a mattress measured to it.

Gaps, fit and what BS EN 716 addresses

BS EN 716 sets requirements for the safety of cots and folding cots for domestic use, covering construction, strength and the dimensional relationships between the mattress and the frame. The principle behind the gap limits is straightforward: a space large enough for a baby’s limb or body to enter creates an entrapment risk, and a space between the mattress and the cot side allows a baby to roll into a position where breathing is obstructed.

The Lullaby Trust, which publishes safer sleep advice in the UK, sets out the practical version of the same point. The mattress should be firm, flat and waterproof on at least one surface, and it should fit the cot snugly. There should be no gaps between the mattress and the sides or ends of the cot. Soft, sagging or second hand mattresses of unknown history are advised against.

The check itself takes seconds. Place the mattress in the cot, push it fully into one corner and look at the gap that opens at the opposite side and end. If you can fit fingers into that gap, the mattress is too small. If the mattress has to be forced, compressed or bowed to go in, it is too large, and a bowed mattress is not flat, which is its own problem. Neither situation is solved by a mattress protector or a rolled towel.

Dimensions to check before buying

The table below sets out the sizes and clearances to verify. Every figure here is a standard dimension or a safety requirement from a named source, not a statistic.

Fitting a cot into a real UK nursery

The room is usually the third bedroom, and in a great deal of British housing stock that is a genuinely small space. A cot bed at 140cm by 70cm plus its frame will consume most of one wall, and the working requirement is not just the furniture footprint but the access: you need to be able to stand at one long side and lift a child out without leaning across the frame.

Position matters for safety as well as space. Cots should be kept clear of curtains, blind cords, radiators and anything a child can reach from inside the cot as they become mobile. That constraint frequently rules out the wall the furniture would naturally go against, so plan the room around the cot rather than fitting the cot into what is left.

Storage is the other pressure. A chest of drawers doubling as a changing surface is the most space efficient arrangement in a small nursery, and it needs a working height that suits the adult using it, generally somewhere around 85cm to 95cm at the top surface. Our childrens chest of drawers UK and the wider childrens furniture UK range are worth measuring against the room before a cot is positioned, because the cot is the piece with the least positional flexibility.

When the cot bed becomes a bed

The conversion is the point at which parents most often get caught out. A converted cot bed takes the same 140cm by 70cm mattress, which is not a standard UK single: a single mattress is 190cm by 90cm. The cot bed mattress therefore continues in use, and if it has been slept on since birth it may well be due for replacement at exactly the point it is needed for a larger child.

The move to a full single bed usually follows within a couple of years, and that is a bigger spatial change than it sounds: 190cm by 90cm plus the frame is a different room layout. It is worth measuring that future arrangement when the nursery is first set up, because it determines whether the chest of drawers, wardrobe and bed can all coexist later. Sizes across our childrens beds UK range give a useful sense of the footprint to plan for, and mattresses UK lists the standard adult and single sizes the room will eventually need to take.

What this means for UK homes

Buy the cot first and the mattress second, and measure the cot base internally rather than relying on a product description. Where the cot is imported, second hand or unusual in any way, measure twice: the 120cm by 60cm and 140cm by 70cm conventions are reliable for mainstream UK cots and not reliable for everything else.

Choose firm and flat over thick and soft, ensure at least one surface is waterproof, and buy a new mattress rather than inheriting one whose history you do not know. Check the snug fit at the point of assembly and check it again after the base height is lowered, since handling the mattress can shift how it sits.

Plan the room for access, not just for furniture. Keep roughly 60cm of clear floor on at least one long side of the cot, keep the cot away from cords, curtains and heat sources, and set out the changing surface at a height that will not cause back strain. If the cot bed is intended to convert, measure now for the single bed that follows.

Methodology

This 2026 report was compiled by the editorial team at Furniture in Fashion. It is based on standard UK cot and cot bed mattress dimensions, on our editorial assessment as a UK furniture retailer of how nursery rooms are laid out and what customers need to measure, and on named published UK guidance where it genuinely applies, specifically British Standard BS EN 716 for cots and folding cots for domestic use and the safer sleep advice published by the Lullaby Trust. It contains no proprietary sales data and no survey fieldwork. This report is general guidance on dimensions and fit and does not replace the manufacturer’s instructions supplied with a specific cot, or current NHS and Lullaby Trust safer sleep advice, which should always be followed.

Citing this report

Journalists and websites are welcome to cite this dimensional guidance with credit to Furniture in Fashion and a link back to us. Please present it as an editorial and specification led summary rather than as original safety research, and direct readers to the Lullaby Trust and NHS for safer sleep guidance itself.

Frequently asked questions

What size is a standard UK cot mattress?

120cm by 60cm, which is 47in by 24in. This is the convention for mainstream UK cots, but the cot base should still be measured internally before ordering, particularly for imported or second hand frames.

What size is a cot bed mattress in the UK?

140cm by 70cm, which is 55in by 28in. It is worth confirming against the specific frame, because the cot bed category is less uniform than the standard cot and a small difference in either dimension matters.

How big a gap is acceptable between a cot mattress and the cot?

The Lullaby Trust advises the mattress should fit snugly with no gaps into which a baby could roll, and BS EN 716 sets the dimensional requirements for cots sold for domestic use. Push the mattress into one corner and inspect the gap at the opposite side: if fingers fit into it, the mattress is the wrong size for that cot.

Can a cot bed mattress be used on a single bed later?

No. A cot bed mattress is 140cm by 70cm and a UK single mattress is 190cm by 90cm, so the two are not interchangeable. The cot bed mattress continues only while the frame is in its converted toddler bed configuration.

Is it safe to use a second hand cot?

A second hand frame can be used provided it meets current standards, has all its original fittings and instructions, and is undamaged. The mattress is the part that should be bought new and sized to that specific cot, in line with Lullaby Trust advice.