What Size Is a Cot Mattress? | Fit Rules For US Cribs

If you’re shopping for a “cot mattress” in the United States, you’re actually looking for a crib mattress. The confusion is understandable — the word “cot” means something different in other countries, and that can lead to buying the wrong size entirely. Getting the dimensions right matters for more than just fitting the frame; it’s a basic safety requirement for your baby’s sleep.

Here’s what those numbers mean for your purchase, why they exist, and how to check the fit in your own crib.

The Standard US Cot (Crib) Mattress Dimensions

That’s the measurement of the mattress itself, not the crib. The official wording says the mattress must be at least this size to fit properly.

For context, the interior dimensions of a matching full-size crib are about 52 3/8 in. by 28 in., give or take a small fraction of an inch. Because the mattress is slightly smaller than the crib interior, you can still fit it snugly — that’s exactly the point. Those dimensions aren’t arbitrary; they’re designed so the mattress fills the sleeping surface with no dangerous gaps.

The thickness matters just as much as length and width.

Why the Fit Has to Be Snug

This isn’t about aesthetics; it’s about safety. A mattress that’s too small leaves gaps around the edges where a baby’s head or limbs could become trapped, which creates a suffocation or entrapment risk.

Two common mistakes land parents in trouble:

• Buying by marketing name: “Contour,” “deluxe,” or “breathable” tell you nothing about dimensions. Always verify the actual size specs, not the label.
• Assuming one size fits all: The term “cot mattress” varies by country, and in the US it almost always refers to a crib mattress, not a folding camp cot mattress. Don’t mix the two up when you’re shopping.

How To Check the Fit in Your Crib

Before you buy, measure your crib’s interior length and width with a tape measure, then compare those numbers to the mattress spec.

Here’s the quick check for a standard US full-size crib:

Once it’s in place, use the “two-finger test”: if you can fit more than two fingers between the mattress and the crib rail, the gap is too wide. A tight fit that leaves no visible gaps is the goal. Once you’ve got the right size sorted, a good cot mattress protector will keep spills and leaks from ruining a well-chosen mattress. If the mattress slides around or there’s visible space on any side, that’s a clear sign to exchange it for a properly sized one.

What the Federal Standard Actually Requires

The safety rules here aren’t optional or advisory. This standard backs up the size minimums and thickness cap, so any mattress sold for a US full-size crib should already conform to it. If a product doesn’t list this standard, that’s a red flag worth taking seriously.

If your crib falls outside those boundaries, these standard dimensions don’t apply, and you’ll need to measure carefully to find a matching mattress.

FAQs

Is a cot mattress the same as a crib mattress?

In the US, the term “cot mattress” is typically another name for a crib mattress, though the word “cot” is used differently in other countries. If you’re buying in the US, assume you’re looking for a crib mattress, but always verify the dimensions rather than relying on the label alone.

What happens if the mattress is too thick?

A mattress thicker than 6 inches can reduce the height of the crib rails, making it easier for an older baby to climb out. It can also interfere with the snug fit around the edges, so the thickness cap is a safety measure, not just a sizing preference.

Can I use a non-full-size mattress in a standard crib?

No, and you shouldn’t try. A smaller mattress will leave gaps that create a serious entrapment or suffocation hazard. If your crib falls within the full-size range, purchase a full-size mattress that meets the minimum dimensions and fits snugly.