Crib Mattress Dimensions: Standard Size & Safe Fit
A standard full-size crib mattress measures roughly 27¼ inches wide by 51⅝ inches long, and no more than 6 inches thick. Those are the federal minimums for width and length, so most crib mattresses sold in the U.S. land within a fraction of an inch of 27.25" × 51.625". That is why almost any full-size crib mattress fits almost any full-size crib — but it is also why the small differences matter, because fit is a safety issue with infant sleep.
Below are the crib mattress dimensions you actually need, how to check the fit in your own crib, and what sizes come next when your child outgrows it.
Crib mattress dimensions at a glance
| Measurement | Inches | Centimeters |
|---|---|---|
| Width (minimum) | 27¼" | 69 cm |
| Length (minimum) | 51⅝" | 131 cm |
| Thickness (maximum) | 6" | 15 cm |
| Typical thickness sold | 4"–6" | 10–15 cm |
For comparison, the interior of a full-size crib is built to about 28 inches wide by 52⅜ inches long, with a tolerance of about ⅝ of an inch either way. The mattress is intentionally a little smaller than the crib opening — but only a little.
Why "standard" is a floor, not a fixed number
Crib mattresses are regulated differently from adult mattresses. The federal crib mattress safety standard sets a minimum width and length rather than one exact size, and the companion crib standard sets the interior dimensions of the crib itself. Manufacturers then build to the top of that range.
The practical result: a crib mattress you buy today should be at least 27¼" × 51⅝", and you may see listings at 27.5" × 52" or 28" × 52". All of those are normal. What is not normal is a mattress noticeably smaller than 27¼" × 51⅝", or one thicker than 6 inches, sold for a full-size crib.
Metric and imperial both appear on packaging
Some listings give crib mattress dimensions in centimeters, and European cot sizes do not match the U.S. standard. Convert before you buy: the U.S. minimums work out to roughly 69 cm wide and 131 cm long. A mattress sold as 70 × 140 cm — a common European cot size — is about 3½ inches too long for a standard U.S. crib.
How to check the fit in your crib
Dimensions on a listing are a starting point. The real test happens in the crib:
- Measure the crib interior, not the outside of the frame. Measure at the mattress support, corner to corner, in both directions.
- Push the mattress to one side. The widely used consumer guidance is that no more than two adult fingers should fit between the mattress and the crib side. A bigger gap than that means the mattress is too small for the crib.
- Check all four corners. A mattress can measure correctly and still leave a gap at one corner if it is rounded or compressed. Corners are where gaps are tested for a reason.
- Confirm the mattress lies flat on the support with no doming, curling, or rocking.
If the mattress does not pass those checks, the mattress is the thing to change — not the crib, and not the bedding. Do not fill a gap with blankets, towels, or padding.
Crib mattress thickness
Six inches is the ceiling for a full-size crib mattress, and plenty of good ones are 4 or 5 inches. Thinner is deliberate here for two reasons. First, a thicker mattress raises the sleep surface and shortens the effective height of the crib side. Second, infant safe-sleep guidance calls for a firm, flat surface, and thickness does not buy you firmness in a crib the way loft can in an adult bed.
This is the opposite of how adult mattresses work, where 10 to 14 inches is typical — we cover that range in our mattress thickness guide. Crib mattresses are their own category, and the numbers do not carry over.
Mini cribs, playards, and bassinets are different sizes
"Crib mattress" on its own means the full-size crib mattress above. Smaller sleep products use smaller mattresses, and they are not interchangeable:
| Product | Typical mattress size |
|---|---|
| Full-size crib | 27¼" × 51⅝" |
| Mini crib | About 24" × 38" (varies by model) |
| Playard / portable crib | About 26" × 38" (varies by model) |
| Bassinet | Varies widely; no single standard |
For any of these, the rule is the same and it is stricter than "close enough": use the mattress or pad the manufacturer made for that specific model, at the size that model specifies. A full-size crib mattress will not fit a playard, and a playard pad will leave gaps in a crib.
What comes after the crib mattress
A toddler bed uses the same mattress as a crib. That is the whole point of a convertible crib — you drop the side, add a rail, and the 27¼" × 51⅝" mattress keeps working. So the next real size change is the jump to a standard adult size:
| Size | Dimensions | Notes |
|---|---|---|
| Crib / toddler bed | 27¼" × 51⅝" | Same mattress for both |
| Twin | 38" × 75" | The usual next step |
| Twin XL | 38" × 80" | 5" longer; good for tall kids |
| Full | 54" × 75" | Room to grow, or for reading together |
Most families move straight from crib to twin, which is about 11 inches wider and two feet longer. If you are sizing a room for that step, our twin mattress dimensions guide covers clearance and layout. When you are ready to shop, our twin mattresses include options built specifically for kids, such as the BB Kids mattress from Brooklyn Bedding in twin, twin XL, and full. A mattress protector is worth adding at the same time for the same reason it was worth it in the crib.
Frequently asked questions
Are all crib mattresses the same size?
Nearly, but not exactly. Full-size crib mattresses must be at least 27¼" × 51⅝", and most cluster within about half an inch of that. Mini cribs, playards, and bassinets use different and smaller sizes that vary by model, so those are not interchangeable with a full-size crib mattress.
What size mattress does a toddler bed use?
The same one as a crib: about 27¼" × 51⅝". Toddler beds are sized around the standard crib mattress so you can reuse it. If a toddler bed calls for something different, the manufacturer will specify it.
Will a crib mattress fit in a pack and play?
No. A playard is roughly 26" × 38", which is more than a foot shorter than a crib mattress. A crib mattress will not lie flat in it, and forcing it in creates exactly the kind of gaps and uneven surface you want to avoid. Use the pad made for that playard.
How thick should a crib mattress be?
No more than 6 inches, and 4 to 6 inches is the normal range. Thinner keeps the sleep surface low relative to the crib sides and supports the firm, flat surface that infant safe-sleep guidance calls for.
Do cribs come with a mattress?
Usually not. Cribs and crib mattresses are typically sold separately, which is also why the size standards exist — so a mattress bought on its own fits the crib you already own. Check the crib's listed interior dimensions and measure before ordering.
Sizing up? Come see the next size in person
Crib sizing is mostly a measuring job. The twin, twin XL, or full mattress that follows it is a feel decision, and that is worth doing in person — especially for a kid who will sleep on it for the next decade.
Visit our Glendale showroom to compare firmness levels side by side, with help from people who do this all day. Every mattress comes with a 120-night sleep trial, so you have a full season to be sure, and we offer free delivery and setup across the Los Angeles area. Bring your room measurements and we will help you work out what actually fits.