Measure the inside of the crib, the mattress, or both. You get the mattress sizes that fit the crib under the federal rules, and the gap your mattress leaves at the side.
Your crib mattress
A full-size crib takes a mattress at least 27¼″ × 51⅝″the crib 28 ± ⅝″ × 52⅜ ± ⅝″ inside · no more than a 1″ gap at the side
Quick answer: a full-size crib mattress must be at least 27¼″ × 51⅝″ (16 CFR 1241), for a crib 28 ± ⅝″ × 52⅜ ± ⅝″ inside (16 CFR 1219), and full-size crib mattresses are generally 6″ thick or less. Pushed against one side, it should leave a gap of 1″ at most, two fingers. Mini cribs have no federal size: use the mattress the crib’s maker names.
How to use the crib mattress size calculator
- Pick the crib: full-size (the standard crib) or mini.
- Measure the inside of the crib, rail to rail and end to end, at the height of the mattress support and near a corner post, if you have it.
- Add the mattress from its label or listing, edge to edge, if you have one in mind.
- You get the mattress sizes your crib takes, whether your mattress meets the federal minimum, and the gap it leaves at the side, drawn from above.
The rules behind the numbers
A full-size crib’s inside is set by federal law: “71 ±1.6 cm (28 ± 5/8 in.) wide as measured between the innermost surfaces of the crib sides and 133 ±1.6 cm (52 3/8 ± 5/8 in.) long”, measured at the mattress support and within 2″ of the corner posts (16 CFR 1219). A crib that is more than 2″ off those sizes, under 49¾″ or over 55″ long, or under 25⅜″ or over 30⅝″ wide inside, is a non-full-size crib, a mini crib among them (16 CFR 1220).
The mattress for a full-size crib “shall measure at least 27 1/4 in. (690 mm) wide and 51 5/8 in. (1310 mm) long”, and with a washed cotton fitted sheet on it, pushed into the corner of the crib, “the corner gap shall not exceed 3.15 in. (80.0 mm)” (16 CFR 1241). The CPSC chose 3.15″ as the head breadth of the smallest babies, 3.66″, less a 0.51″ margin, and calls 1″ “the already-allowed maximum gap” between the mattress and the sides of the crib. DaVinci passes on the JPMA’s version: “no more than two fingers width, one inch, between the edge of the mattress and the crib side.”
Put the two rules side by side and they leave room: a crib at its widest legal size, 28⅝″ × 53″, with a mattress at the legal minimum leaves 1⅜″ each way, more than that inch. So measure your own pair.
Crib and crib mattress sizes
| Width | Length | From | |
|---|---|---|---|
| Full-size crib, inside | 27⅜–28⅝″ | 51¾–53″ | 16 CFR 1219 |
| Full-size crib mattress | at least 27¼″ | at least 51⅝″ | 16 CFR 1241 |
| Mini crib mattress | about 24″ | about 38″ | DaVinci |
| Babyletto Origami mini crib, inside | 24″ | 37″ | Babyletto |
| Its mini crib mattress | 23½″ | 37″ | Babyletto |
| Non-full-size crib | under 25⅜″ or over 30⅝″ | under 49¾″ or over 55″ | 16 CFR 1220 |
Four worked examples
A crib 28″ × 52⅜″ inside, the rule’s middle size. It takes a mattress 27¼–28″ × 51⅝–52⅜″; one at the legal minimum leaves ¾″ across and ¾″ along.
The widest legal crib with the smallest legal mattress. 28⅝″ × 53″ and 27¼″ × 51⅝″: pushed to one side, the mattress leaves 1⅜″ across and along. Both meet the rules, and the gap is still more than an inch.
A 28″ × 52¼″ mattress in a 28¼″ × 52½″ crib. It fits, with ¼″ across and ¼″ along.
A mini crib, 24″ × 37″ inside (Babyletto’s Origami), with its 23½″ × 37″ mattress: it fits, with ½″ across and nothing along.
Pack ‘n Play and play yard mattresses
Play yards share their standard with non-full-size cribs, ASTM F406 “Non-Full-Size Baby Cribs/Play Yards”, made federal law for play yards by 16 CFR 1221. Its mattress follows a stricter rule than a crib’s: a replacement is not judged by its size alone. Under 16 CFR 1241, an after-market play yard mattress must meet the play yard standard’s tests “when tested with each brand and model of product in which it is intended to replace the mattress”: stability, cord and strap length, mattress, height of sides, floor strength and mattress vertical displacement. So the CPSC’s advice is short: “Use only the mattress provided with your play yard or a mattress that specifically identifies the brand and model number of your play yard.”
The reason is the gap. “Babies have suffocated in gaps between an undersized mattress, or extra padding, and side walls of a product” (CPSC). In 2025 the CPSC told parents to stop using two after-market play yard mattresses sold online: Hiccapop’s PlayPad, which failed “federal thickness and dimension requirements”, and Beianpp’s “Pack and Play”, which failed the thickness rule and did not “identify the specific brands and model numbers of the play yards” it fits. A mattress that measures right but names no play yard is not one the rule allows; never add a second mattress or padding on top of the one that came with it.
Common mistakes
- Measuring the outside of the crib, or at the top rail. The size that counts is the inside, at the height of the mattress support.
- Trusting “standard size”. A crib and a mattress can both meet the rules and still leave more than an inch.
- Putting a full-size mattress in a mini crib, or a mini mattress in a full-size one. A mini crib takes the mattress its maker names; a replacement must be at least that size and lie flat.
- Using an old crib without measuring it. The full-size crib rule applies to cribs sold since June 28, 2011 (16 CFR 1219).
Size the rest of the bedding with the bedding size calculator, plan the nursery in the bedroom layout planner, and draw it to scale in the free 2D planner.
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Crib mattress size questions
What is the standard crib mattress size?
A full-size crib mattress is at least 27¼″ wide and 51⅝″ long by federal law (16 CFR 1241), and generally 6″ thick or less.
What are standard crib dimensions?
Inside, a full-size crib is 28 ± ⅝″ wide and 52⅜ ± ⅝″ long: 27⅜ to 28⅝″ by 51¾ to 53″, measured at the mattress support (16 CFR 1219). The outside depends on the crib.
What is a full-size crib mattress?
The mattress for a standard crib: at least 27¼″ × 51⅝″, and it must pass the corner test, a gap of 3.15″ at most with a washed cotton fitted sheet on.
What are mini crib dimensions?
There is no federal mini size. A crib under 49¾″ long or 25⅜″ wide inside is an undersize, non-full-size crib (16 CFR 1220). Babyletto’s Origami mini crib is 37″ × 24″ inside.
What size is a mini crib mattress?
About 38″ × 24″ for DaVinci’s mini cribs; 37″ × 23½″ for Babyletto’s. A replacement must be at least the size of the mattress the crib came with and lie flat (16 CFR 1241).
How big a gap is OK between the mattress and the crib?
One inch at most, two fingers, with the mattress pushed against the other side: the JPMA’s advice, and the CPSC’s “maximum gap of 1 inch”.
What size mattress fits a Pack ‘n Play?
The one that came with it, or an after-market mattress that names your play yard’s brand and model number: the federal rule (16 CFR 1241) has it tested with each play yard it is sold for, and the CPSC says to use nothing else. A crib mattress, or any mattress chosen by its size, does not qualify.
What size crib sheet do I need?
A fitted sheet for a full-size crib mattress, 27¼″ × 51⅝″ or a little more, that wraps fully around and under it. The corner test is done with a 100% cotton fitted sheet washed hot and dried twice, so a sheet that stays snug after washing is the one to use.