How to Choose the Right Mattress for Your Baby's Cot

Quick answer

Choose a cot mattress that fits your cot's internal length and width within a 2 cm gap, feels firm with no soft dips, and lets air move through it. Match the mattress type to your cot, then check the cover is removable and washable. If your baby is moving to a cot bed, buy a cot bed mattress sized for that bed rather than squeezing a smaller one in.

A cot mattress is one of the few purchases where fit and firmness matter more than thread count or brand. A mattress that is too small leaves gaps where a baby can get stuck; one that is too soft can let a baby's face sink in. This guide walks through the measurements, materials, and checks that make the decision straightforward.

You do not need the most expensive option. You need one that matches your cot's internal dimensions, stays firm, and has a cover you can actually wash. Below are the decision rules we use, plus a few Nobodinoz pieces that help once the mattress is in place.

Measure the cot before you look at mattresses

Take the internal length and width of the cot, not the outside frame. Measure at the base where the mattress sits, because bars and posts can narrow the space. Write those two numbers down and take them with you. If the mattress is more than 2 cm smaller than the cot in either direction, the gap is a hazard. If it is larger, it will not sit flat.

Cot and cot bed sizes vary. A cot bed mattress is longer than a standard cot mattress, so the two are not interchangeable. If you are buying before the cot arrives, check the cot's product page for its internal dimensions rather than guessing from the name. When a listing gives no dimensions, treat that as a limitation and ask the retailer before ordering.

• Measure internal length and width at the base, not the outer frame.
• Aim for a gap of no more than 2 cm on any side.
• Write the numbers down; do not rely on memory in the store.

Firmness and breathability: what to look for

A baby mattress should feel firm when you press it with your palm, and it should spring back rather than hold a dent. Softness feels comfortable to adults but is not what a baby needs, because a very soft surface can let the head sink and block airflow. If you can easily squash the mattress flat with one hand, it is too soft for a cot.

Breathability matters too. Materials that let air pass through, and covers that are not sealed in plastic, help the mattress stay fresh. A removable, washable cover is not a luxury: nappies leak and babies spit up, and a cover you cannot wash means the mattress stays damp. Check whether the cover zips off and whether the care label allows machine washing at a sensible temperature.

• Press with your palm: it should feel firm and recover quickly.
• Choose a breathable construction over a sealed plastic feel.
• Confirm the cover is removable and machine washable.

Match the mattress type to your cot and your stage

Standard cot mattresses are usually rectangular and sized for a cot. Cot bed mattresses are longer, because cot beds convert into a toddler bed and need to cover the full sleeping platform. If you already own a cot bed, buy a cot bed mattress from the start so you are not replacing it when the side rails come off.

Some parents choose a mattress with a firmer baby side and a softer toddler side. That can work, but only if the mattress is genuinely reversible and the firmer side is used for the baby. If the listing does not state which side is which, treat that as missing information and choose a single-firmness mattress instead. A simple, firm mattress that fits well is a better buy than a complicated one that does not.

• Cot bed owners: buy a cot bed mattress, not a smaller cot mattress.
• Only choose a two-sided mattress if the firm baby side is clearly marked.
• Skip features you cannot verify from the product information.

Set up the sleep space around the mattress

Once the mattress is in, the bedding should fit it. A duvet cover that is too big bunches up; one that is too small will not stay on. For a child's bed, a kid duvet cover in washed French linen and organic cotton gives you a breathable, thermoregulating layer that works in both summer and winter, and the linen-cotton blend is hypoallergenic and antibacterial. That matters if your child has sensitive skin.

Keep the cot itself clear. A firm mattress plus a fitted sheet is the safe baseline, and soft items belong outside the sleep space. If you use a bouncer or a play area during the day, keep those separate from where the baby sleeps. A terry cotton bouncer cushion, for example, is designed for a bouncer, not for the cot, and should stay with the bouncer.

• Use a fitted sheet sized to the mattress, not a loose flat sheet.
• Keep pillows, bumpers and soft toys out of the cot.
• Keep bouncer cushions with the bouncer, not in the sleep space.

Decision rules for buying a cot mattress

• Rule 1: Fit first: Measure the cot's internal length and width at the base. Buy a mattress that fills that space with no more than a 2 cm gap on any side. If the gap is bigger, the mattress is the wrong size, no matter how good the materials are.
• Rule 2: Firm beats soft: Press the mattress with your palm. It should feel firm and spring back. If it stays dented or squashes flat easily, it is too soft for a baby. Choose a single-firmness mattress unless a reversible one clearly marks the firmer baby side.
• Rule 3: Washable cover or walk away: Check the care label for a removable cover and machine washing instructions. A mattress you cannot clean will not stay hygienic. If the cover is fixed, plan for a waterproof protector that fits the same dimensions.
• Rule 4: Match the cot, not the marketing: Cot beds need cot bed mattresses. If a listing does not give internal dimensions or state which side is firmer, treat that as missing information and ask before buying. Replace the mattress if it sags, develops a dip, or has been used by another baby for a long period.

Nobodinoz pieces that support the sleep setup

• Kid duvet cover in washed French linen and organic cotton L1N0: Two sides combine French linen with washed organic cotton; the linen is hypoallergenic, antibacterial and thermoregulating, and the cover has a check print lining. A breathable, temperature-regulating cover for a child's bed keeps bedding from feeling clammy in summer or cold in winter, and the linen-cotton blend suits sensitive skin.
• New born terry cotton bouncer cushion Growing Green: Made from extra soft organic cotton terry towelling, it fits the Growing Green bouncer and gives a newborn comfort, stability and posture support in the first weeks. Useful for daytime supervised lounging, but it belongs in the bouncer, not the cot, so it does not interfere with a firm, clear sleep surface.
• Toy bag Eco-velvet: Quilted recycled eco-velvet with a water-repellent lining, braided natural cotton handles, machine washable and made in Barcelona. Keeps toys and laundry out of the cot area so the sleep space stays clear, and the washable lining handles spills without fuss.

Who this cot mattress advice is for

Best for

• Parents buying a first cot mattress and wanting a clear fit and firmness checklist
• Families with a cot bed who need a cot bed mattress sized for the full sleeping platform
• Anyone replacing a sagging or damp mattress and wanting to avoid the same mistake

Not for

• Parents looking for a mattress for a bedside crib or Moses basket, which use different sizes
• Anyone wanting a soft, pillow-top feel for a baby, which is not a safe choice for cot sleep
• Shoppers who cannot measure their cot and will not ask the retailer for internal dimensions

Get the fit and firmness right, keep the cover washable, and the rest is detail. Measure your cot, choose a mattress that fills it with no more than a 2 cm gap, and keep soft bedding and bouncer cushions out of the sleep space. Then add the textiles and storage that make the room work day to day.