How do I check a cot mattress fits with no gap?

What does "no gap" mean on official UK pages?

The Lullaby Trust mattresses and bedding page (reviewed 1 February 2025) says to check that the mattress fits the Moses basket or cot properly. Its second-hand mattress checklist repeats the same test in plainer words: fit the cot or Moses basket well, with no gaps. The Trust's product guide (February 2025) adds why: so there is no risk of the baby getting trapped.

That is a fit test on the cot you already own, not a verdict on the brand. Official pages do not publish a "safe gap" of their own. The number parents hunt for sits in the cot standard, not in the NHS SIDS leaflet.

BS EN 716-1:2017 (children's cots and folding cots for domestic use) requires that if a mattress is supplied with the cot, there shall be no gap more than 30 mm between the mattress and the sides and ends in any position. Instructions must also state a minimum mattress size that keeps that 30 mm limit. The Lullaby Trust lists this standard for cots and travel cots.

How do I measure the cot and the mattress?

Do this with the mattress out, then again with it in. Frames are not always a true rectangle, and a mattress that matched the box can shrink or bow after months of use.

1. 1
   Read the cot's own size line The maker's label or manual should give the internal length and width, or the mattress size the cot was built for (often 120 × 60 cm or 140 × 70 cm). Which? notes that measurements should be clearly displayed on the cot label.
2. 2
   Measure the inside of the frame Use a tape at mattress height, not along the top rail. Take length and width at both ends and in the middle. Use the smallest pair of numbers if they differ.
3. 3
   Measure the mattress itself Length and width in the centre and near each corner. A foam mattress that has been stored on its edge can read short on one side only.
4. 4
   Put the mattress in and press it home It should sit flat on the base with no corner kicked up. Then check the gap at all four sides and both ends, including the corners — that is where a short mattress shows first.
5. 5
   Run the 30 mm / two-finger check Two adult fingers side by side are a rough stand-in for 30 mm. If more than two fingers slide into the gap, treat that as over the cot-standard limit. A credit card on edge is about 1 mm; thirty of those is the standard's number if you want a ruler instead.

Do not close a gap with a rolled towel, a second mattress, a bumper or folded blankets. The Lullaby Trust lists bumpers, pods, nests and anything soft on top of the mattress as items to keep out. Extra padding is not a substitute for the right size.

Is the two-finger test the same as the 30 mm rule?

Almost, and that is why parents search for both. 30 mm is the figure written into BS EN 716-1 when a mattress is supplied with the cot. The two-finger check is the home version of that number: two adult fingers are typically around 30 mm across. It is not a Lullaby Trust test, and it is not a laboratory probe.

What else should I check besides the gap?

Fit is one of four Lullaby Trust mattress questions. A snug mattress that is soft, dipped or torn still fails the rest of the list.

• Firm. The baby's head should sink no more than a few millimetres. That line appears on the Lullaby Trust mattresses page and in the product guide.
• Flat. No raised or cushioned edges or sides; no pods or nests on top.
• Waterproof. A wipe-clean cover or layer. The Trust's second-hand checklist asks for a well-fitting 100% waterproof cover.
• Condition. No rips, tears, sagging or water damage. A fire-resistant label (the Trust cites BS 7177:2008+A1:2011 for mattresses).
• Standard mark. For cot, travel-cot and crib mattresses the Trust lists BS EN 16890:2017+A1:2021. It also notes there is no British Standard about SIDS — a construction mark is not a SIDS claim.

NHS SIDS advice matches the surface part of this: use a firm, flat mattress and keep the cot clear of anything that could cover the face.

Does a travel cot or Moses basket use the same gap check?

The no-gaps wording is the same. The Lullaby Trust says a second-hand mattress should fit the cot or Moses basket well, with no gaps. For a travel cot it says: rigid frame and base, firm, flat mattress with a waterproof cover, and do not put folded blankets or a quilt under the baby to make a thin mattress feel thicker.

The 30 mm figure is written into the cot standard (BS EN 716), which the Trust also lists for travel cots. Moses baskets sit under BS EN 1466 instead; use the maker's mattress and the Trust's "fits properly / no gaps" test rather than assuming a 30 mm cot probe applies to a woven basket.

In shortPhotograph the cot's size label, measure the inside, drop the mattress in, and check every edge. Lullaby Trust: no gaps, plus firm, flat and waterproof. Cot standard: no more than 30 mm. More than two fingers in the slot means the mattress is the wrong size — do not pad the gap.