Cot Mattress Sizing and Fit: Why the Right Match Matters

Quick answer

A cot mattress should fill the cot base with no more than a finger-width gap at any side, sit flat without sagging, and match your cot's stated dimensions. If the mattress is too small, bowed or soft at the edges, fix the fit first; if the mattress itself is worn or the wrong size, replace it rather than padding the gap.

Measure the cot, not the old mattress

Cot dimensions vary between brands and models, so start by measuring the inside of the cot base from side to side and front to back. Use those two measurements as your target, then check the mattress label or product page against them. A mattress sold as a standard cot size can still be a few centimetres out for your specific cot, which is where most fit problems begin.

• Measure the internal base, not the outer frame.
• Note the shape: rectangular, oval or rounded-corner cots need a matching mattress shape.
• Check the depth of the cot sides so the mattress sits at a safe height.

What a good fit actually looks like

Once the mattress is in the cot, press down on each corner and run your hand around the edge. A well-fitted mattress stays in contact with the cot sides and does not slide when you push it. The gap test is simple: if you can fit more than a finger between the mattress and the cot side, the fit is too loose. A small gap can let a baby's limb become trapped, and a loose mattress can also shift during sleep.

• No more than a finger-width gap at any side.
• The mattress should not slide or lift easily at the corners.
• The sleep surface should be firm and flat, not bowed or dented.

Why gaps and sagging happen

Gaps usually come from a size mismatch, a mattress that has compressed over time, or a cot that is not perfectly square. Sagging is more often a materials issue: lower-density foam or worn springs lose their shape, especially in the middle where your baby sleeps. If the mattress has a waterproof surface, check that it still lies flat after washing, because a shrunken or distorted cover can pull the mattress out of shape.

• Size mismatch: the mattress is smaller than the cot base.
• Compression: foam or filling has flattened with use.
• Cover distortion: a shrunken or ill-fitting cover pulls the mattress.
• Frame issues: a warped or uneven cot base creates uneven support.

Fixes that are safe, and fixes that are not

The safe fix is always to correct the mattress or the cot, not to fill the gap. Never roll towels, blankets or foam into the sides of a cot to make a mattress fit, because those added items can cover a baby's face or create a soft, unstable edge. If the cot itself is the problem, check that the base is assembled correctly and that all fixings are tight before you blame the mattress.

• Safe: replace the mattress with the correct dimensions.
• Safe: reassemble or tighten the cot base if it is uneven.
• Unsafe: adding rolled towels, blankets or extra foam to close a gap.
• Unsafe: using a mattress that is too small and hoping the sheet holds it in place.

Protecting the sleep surface between washes

A mattress that stays dry and clean keeps its shape longer. Night-time leaks and spills are common, and a mattress that gets damp repeatedly can develop odours or break down faster. A waterproof fitted sheet that bonds a waterproof layer to a soft organic cotton surface lets you protect the mattress without adding a separate noisy or crinkly protector. Look for a built-in safety strap so the sheet stays firmly in place, and check that the sheet is sized to your mattress dimensions.

• Choose a sheet that matches your mattress size and depth.
• A strap or elasticated edge keeps the sheet from riding up.
• Wash according to the care label to avoid shrinkage that affects fit.

Cot mattress fit diagnosis

• Gap wider than a finger at the sides: The mattress is smaller than the cot base or has compressed. Measure the cot base and compare it with the mattress dimensions on the label. If the mattress is the wrong size, replace it. Do not pad the gap.
• Mattress slides or lifts at the corners: The fit is too loose, or the base is uneven. Check that the cot base is assembled correctly and that the mattress sits flat on it. If the cot base is sound and the mattress still moves, the mattress is the problem.
• Dips or soft spots in the middle: Foam or filling has lost support, often after months of use. Press the centre with your hand and compare it with the edges. A mattress that stays compressed after you remove your hand should be replaced.
• Cover shrunken or distorted after washing: A cover that no longer fits can pull the mattress out of shape. Check the care label and wash according to the instructions. If the cover cannot be restored, replace the mattress rather than using it with a poor fit.
• Cot itself is warped or damaged: No mattress will fit correctly on a damaged base. Inspect the frame, fixings and base for cracks or bends. If the cot is damaged, stop using it and choose a different cot or a mattress designed for a sound frame.

Product facts that help with fit and care

• 2-in-1 Waterproof Fitted Sheet: The 2-in-1 Waterproof Fitted Sheet bonds non-toxic waterproof polyurethane with GOTS-certified organic cotton flannel, so one sheet protects the mattress and gives a soft sleep surface. You avoid buying or washing two separate sheets, and the mattress stays dry, which helps it keep its shape and fit.
• Jersey Sleep Suit Bag 0.2 TOG: The Jersey Sleep Suit Bag 0.2 TOG is made from natural fibres and is TOG-rated for warmth, designed to take the guesswork out of dressing your child safely for sleep. Once the mattress fit is correct, a TOG-rated sleep bag gives you a simple warmth layer without loose blankets in the cot.