What Size Is Crib Mattress? Standard Dimensions and FAQs

If you’re wondering “What size is crib mattress?”, you’re usually trying to make sure a mattress will fit your baby’s crib safely and snugly. Crib mattress sizes are more regulated than adult mattress sizes, but there are still a few details parents often aren’t sure about.

This FAQ-style guide walks through standard crib mattress dimensions, how much variation is normal, and what to check before you buy or use a mattress.

What Size Is Crib Mattress in the U.S.?

In the United States, a standard crib mattress is designed to fit a full-size crib. Regulations set both length and width ranges so that the mattress fits tightly inside the crib frame.

Typical standard crib mattress size:

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

More precisely, a full-size crib mattress is usually around 27 1/4 inches wide by 51 5/8 inches long, with slight allowed variations. This means most standard crib mattresses you see on the market will be very close to this size.

The goal is a snug fit with very little space between the mattress and the crib sides.

Are All Crib Mattresses the Same Size?

Not exactly, but most are very similar.

Standard vs. Mini Crib Mattress

When people ask “What size is crib mattress?”, they usually mean standard crib mattress. However, there are also mini cribs and other specialty cribs, which use smaller mattresses.

Here’s a simple comparison:

Mini crib mattresses are not interchangeable with standard crib mattresses, and they will not safely fit in a full-size crib.

Why Does Crib Mattress Size Matter So Much?

Crib mattress size is primarily about safety and comfort.

• A mattress that is too small can leave gaps between the mattress and crib frame.
• A mattress that is too big may not lay flat or may sit too high, affecting stability and sheet fit.

A snug-fitting mattress helps prevent your baby from rolling into a gap or getting a limb caught. That’s why the question “What size is crib mattress?” is more than just a measurement issue—it’s a safety concern.

How Can I Tell If My Crib Mattress Fits Properly?

A common rule of thumb is the “two-finger test”:

• Place the mattress in the crib.
• Try to slide two fingers between the mattress edge and the crib side.
• If more than two fingers fit easily, the mattress is likely too small for that crib.

Other things to check:

• The mattress lies flat with no bowing or bending.
• Crib sheets designed for your mattress size fit smoothly and don’t pop off easily.
• Corners of the mattress are aligned with the corners of the crib.

How Thick Should a Crib Mattress Be?

Crib mattress thickness can vary, but most standard crib mattresses are:

• Around 4 to 6 inches thick for full-size cribs
• Often a bit thinner for mini cribs

A very thick mattress might affect how high your baby lies in the crib relative to the rails, while a very thin mattress might not feel as supportive. For most parents, a moderate thickness that fits snugly and lies flat works well.

Does “What Size Is Crib Mattress” Change for Toddler Beds?

Many toddler beds are designed to use the same mattress as a standard crib. In those cases, the answer to “What size is crib mattress?” is effectively the same as “What size mattress does this toddler bed use?”:

• About 27–28" wide by 51–52" long

If your crib converts to a toddler bed, it will often specify that it uses the existing crib mattress. Always check the bed’s instructions before assuming.

Are Crib Mattress Sizes Different in Other Countries?

Yes, dimensions can vary in other regions. For example:

• Some countries use slightly different standard sizes for cots and cot beds.
• Mattresses may be sold under names like “cot,” “cotbed,” or “junior bed”, each with its own typical dimensions.

If you have an imported crib or mattress, or you’ve moved between countries, measuring both crib and mattress is especially important.

✔️ Quick Mattress Takeaways

Key points consumers should understand about What Size Is Crib Mattress

• Standard U.S. crib mattress size is typically around 27 1/4" x 51 5/8", often described as 28" x 52".
• Mini cribs use smaller mattresses, so a mini crib mattress will not safely fit a full-size crib.
• A correctly sized crib mattress should fit snugly, with no more than two fingers fitting between the mattress and crib sides.
• Most standard crib mattresses are 4–6 inches thick; mini crib mattresses are often a bit thinner.
• Many toddler beds are designed to use the same size mattress as a standard crib.
• If crib and mattress are from different brands or regions, measure both to confirm a proper fit.

With a clear understanding of what size is crib mattress, you can more confidently choose a mattress that fits your crib, supports your baby, and aligns with common safety guidelines for a snug, secure sleep surface.