What Is The Size Of a Crib Mattress? Standard Dimensions and FAQs

If you’re searching for “What Is The Size Of a Crib Mattress”, you’re usually trying to make sure a mattress will fit safely in a crib, or wondering whether a crib mattress you already have is the right size. This guide explains standard crib mattress dimensions, how much variation is normal, and what really matters for a snug, safe fit.

What Is The Size Of a Crib Mattress in the U.S.?

In the United States, a standard full-size crib mattress is designed to fit most full-size baby cribs.

Typical standard crib mattress dimensions:

Length: about 52 inches
Width: about 28 inches
Thickness: usually 4–6 inches

There can be small variations in length and width (often up to about 5/8 inch), but full-size crib mattresses are built to fit standard full-size cribs within strict guidelines so they don’t leave large gaps.

Why Do Crib Mattress Dimensions Matter?

Crib mattress size is about more than just convenience:

Safety: A mattress that is too small can leave gaps where a baby’s arms, legs, or head may get stuck.
Fit with crib: Even standard cribs and mattresses can vary slightly, so you want to confirm they match.
Bedding fit: Sheets and mattress protectors are sized for specific mattress dimensions. A poor fit can bunch up or pop off.
Longevity: Knowing the exact size helps you plan how long your child can comfortably use the crib before moving to a toddler bed.

A snug fit—with very little space between the mattress and crib frame—is a key goal when you’re thinking about What Is The Size Of a Crib Mattress.

Are All Crib Mattresses the Same Size?

Not all crib mattresses are identical. There are two main categories:

1. Standard Full-Size Crib Mattress

As described above, these are around 28" x 52". They are made to fit full-size cribs.

2. Mini Crib Mattress

Mini cribs use a smaller mattress, commonly closer to:

Width: around 24 inches
Length: around 38 inches

Mini crib mattress sizes can vary more by model, so it’s especially important to check the crib manufacturer’s listed mattress size.

How Do I Check If a Crib Mattress Fits Properly?

A simple way to test the fit:

Place the mattress in the crib and press it to one side.
Try to fit your fingers in the gap on the other side.
General guideline: you should not be able to fit more than about two fingers between the mattress edge and the crib frame.

If the gap is larger than that, the mattress may be too small for the crib and not ideal from a safety standpoint.

How Thick Should a Crib Mattress Be?

While the length and width answer “What Is The Size Of a Crib Mattress” in the most common sense, thickness matters too.

Most crib mattresses are around 4 to 6 inches thick.
Thicker is not always better. What matters more is that the mattress is firm and supportive rather than soft and pillowy.
Very thick mattresses may sit higher in the crib, which can matter when you adjust the mattress height as your baby grows.

Always pair thickness with the crib’s mattress height settings to ensure the sides remain high enough above the sleep surface.

Do Crib Mattresses Fit Toddler Beds?

Many toddler beds are designed to use a standard full-size crib mattress. That means:

If your crib mattress is about 28" x 52", it often can be reused in a compatible toddler bed.
Mini crib mattresses usually do not fit toddler beds unless the bed is specifically designed for that size.

Checking the dimensions listed for the toddler bed will confirm whether your existing crib mattress will work.

How Do I Know Which Size Crib Mattress I Need?

Here are a few practical steps:

Measure the inside of your crib (length and width).
Check the crib’s user guide or label for “full-size” or mini crib information.
Match the mattress dimensions to the crib’s recommended size.
Remember that a small difference (a fraction of an inch) is normal as long as the fit is snug with no large gaps.

✅ Quick Mattress Takeaways

Key points about “What Is The Size Of a Crib Mattress”:

Standard U.S. crib mattress size: about 28" (W) x 52" (L)
Mini crib mattresses: smaller, commonly around 24" x 38", but sizes vary
Thickness: generally 4–6 inches
Fit test: no more than about two fingers should fit between the mattress and crib frame
Standard crib mattresses often fit many toddler beds; mini crib mattresses usually don’t
Always measure your crib and check for a snug, gap-free fit rather than relying on size labels alone

Understanding What Is The Size Of a Crib Mattress helps you choose a mattress that fits your crib correctly, works well with your bedding, and supports a stable, secure sleep surface for your child. When in doubt, measure both the crib and the mattress and look for that close, comfortable fit with minimal gaps.