Crib mattress size

What US federal rules say a full-size crib mattress must measure, and what changes for mini cribs and play yards.

Two kids have a pillow fight across a pair of twin beds while a beagle puppy watches in a cheerful shared bedroom

Short answer: a full-size crib mattress must measure at least 27¼ in (690 mm) wide and 51⅝ in (1310 mm) long.^{[1 · Oct 29, 2026]} Pushed into a corner of the crib with a fitted sheet on, the gap at the corner must not exceed 3.15 in (80 mm).^{[1 · Oct 29, 2026]}

Full-size crib mattress dimensions

Requirement	Inches	Metric
Minimum width	27¼ in^{[1 · Oct 29, 2026]}	690 mm^{[1 · Oct 29, 2026]}
Minimum length	51⅝ in^{[1 · Oct 29, 2026]}	1310 mm^{[1 · Oct 29, 2026]}
Maximum corner gap	3.15 in^{[1 · Oct 29, 2026]}	80 mm^{[1 · Oct 29, 2026]}

These are minimums from 16 CFR 1241.2, which requires crib mattresses to meet the ASTM F2933-25 standard with federal changes.^{[1 · Oct 29, 2026]} Because they are minimums, a compliant mattress can measure a little more, so check the listed size of the one you're buying.

How the corner gap is tested

A 100% cotton fitted sheet is washed in hot water (at least 122 °F) and dried on high at least twice, then wrapped fully around and under the mattress.^{[1 · Oct 29, 2026]}
The mattress is placed in a test crib, against the walls and into a corner.^{[1 · Oct 29, 2026]}
The shortest gap between the mattress and the opposite crib corner is measured; the test crib corner sits 53 in from one wall and 28⅝ in from the other.^{[1 · Oct 29, 2026]}
The mattress is turned 180° and the measurement is repeated.^{[1 · Oct 29, 2026]}

This is a lab test for manufacturers, restated in our own words. It isn't a home measuring method, but it explains why the sheet and the fit matter.

Mini cribs, non-full-size cribs and play yards

A mattress sold with a non-full-size crib is tested with that crib under the non-full-size crib standard.^{[1 · Oct 29, 2026]}
A replacement mattress for one must be at least the same size as the original mattress and lie flat on the floor of the crib, touching its mattress support.^{[1 · Oct 29, 2026]}
If the original mattress has a floor support structure, the replacement needs one at least as thick.^{[1 · Oct 29, 2026]}
Replacement play yard mattresses are tested with each brand and model of play yard they're meant to fit.^{[1 · Oct 29, 2026]}

So for a mini crib or play yard, use the size and model list the product maker gives, not the full-size numbers above.^{[1 · Oct 29, 2026]}

Next size up

When a child moves to a bed, the usual next step is a twin. Compare it with a twin XL on our twin vs twin XL page, or see every size on the mattress size chart. RV and oversized beds are on the RV and oversized size page.

Sources: 1. eCFR, 16 CFR 1241.2, Requirements for crib mattresses. Checked Oct 29, 2026. Trademarks belong to their owners; figures are restated in our own words and tables, not reproduced.