How to Choose a Cot Mattress | Fit and Safety Rules That Matter

Choosing a cot mattress comes down to a few measurable facts: the crib’s interior size, the mattress’s own dimensions, and whether the surface stays firm under your baby’s weight. The Consumer Product Safety Commission (CPSC) regulates full-size crib mattresses under 16 CFR part 1241 and requires compliance with the ASTM F2933-21 standard. Get these numbers right and the rest of the decision is straightforward.

Measure the Crib Interior Before You Shop

Your crib’s interior dimensions are the single most important number in this process. The mattress you buy must match that space with almost no room to spare.

Never buy by age label alone. Cribs and mattresses are sold as separate products, and a mattress that fits a mini crib will not safely fit a full-size crib. Pull out a tape measure and check the crib’s interior width and length yourself before ordering anything.

What the Federal Safety Rules Actually Cover

Crib mattresses are subject to their own federal rules, separate from the crib itself.

These rules apply to full-size crib mattresses sold in the United States. Non-full-size sleep products — mini cribs, travel cribs, bassinets, and portable play yards — have different dimensions and fall under separate requirements. A mattress made for one category is not automatically safe in another.

Even when you buy a mattress sold together with a crib, verify the mattress dimensions independently. The mattress has its own standard, and the combo packaging does not guarantee a compliant fit.

Firmness and Fit: The Two Tests That Matter

Firmness is not a preference — it is a safety requirement.

After you install the mattress, check the perimeter gap. A snug fit reduces entrapment risk and keeps the sleep surface flat and stable.

Common Mistakes That Create Unsafe Gaps

The most frequent error is buying by the age range printed on the box instead of measuring the crib. Age labels describe typical use, not fit. An oversized mattress bows in the middle; an undersized one leaves dangerous gaps at the sides.

Another mistake is choosing a thicker mattress for comfort. And a soft mattress that feels plush to an adult is exactly the wrong surface for an infant — the firmness test above is non-negotiable.

Before you finalize a purchase, confirm the label says “full-size crib mattress.” If the product is marketed for mini cribs, travel cribs, or bassinets, it is a different category with different rules. When you find a mattress that checks every box, our tested roundup of cot with mattress options that meet safety standards can narrow down your choices.

FAQs

Can I use a crib mattress in a bassinet or travel crib?

No. Full-size crib mattress requirements do not apply to bassinets, travel cribs, mini cribs, or portable play yards. Those products have their own dimensions and safety standards. Always match the mattress to the specific sleep product’s interior measurements and verify the label matches the product category before use.

How tight should a crib mattress fit?

A crib mattress should fit snugly with no gap wider than 1 3/8 inches between the mattress edge and the crib side. If the mattress shifts easily when you push it, it is too small for that crib. If it bows or bends to fit, it is too large.

Does a thicker crib mattress mean more comfort and safety?

No. For full-size cribs, the mattress must be no more than 6 inches thick. Thicker mattresses exceed the federal rule and position the baby higher than the crib design intends.