Crib Mattress Dimensions & Size Guide (2026)
By Will Lorenz · Updated April 2026 · 6 min read
When I set up our first crib, I made the mistake of assuming any crib mattress would fit. It didn’t. There was a visible gap on one side — exactly the kind of gap the CPSC warns about. I had to return it and start over.
Whether you’re shopping for a standard crib, a mini crib, or a portable play yard, mattress size is one of those details that directly affects your baby’s safety. A mattress that’s too small can create a dangerous gap. A mattress that’s too thick can make it harder to lower the crib rail safely.
This guide covers every crib mattress size you’re likely to encounter — standard, mini, portable, bassinet, and toddler — with exact dimensions, CPSC safety specs, and tips for measuring your crib before you buy.
Standard Crib Mattress Dimensions
The U.S. Consumer Product Safety Commission (CPSC) sets federal standards for full-size crib mattresses. These aren’t suggestions — manufacturers must comply for their products to be sold legally in the United States.
| Measurement | CPSC-Regulated Range | Most Common Size |
|---|---|---|
| Width | 27⅜” – 28⅝” | 28″ |
| Length | 51⅝” – 53″ | 52″ |
| Thickness | Up to 6″ max | 4″–6″ |
The vast majority of standard crib mattresses you’ll find are 28″ × 52″. All standard cribs sold in the U.S. are built to accept a mattress in that regulated range, so brand compatibility is rarely an issue with full-size cribs.
The two-finger rule: Once the mattress is inside the crib, you should not be able to fit more than two fingers between the mattress edge and the crib side. If you can fit more than that, the mattress is too small for that crib and should not be used. This is the CPSC’s guidance and it’s worth taking seriously — gaps are a suffocation and entrapment risk.
Mini Crib Mattress Dimensions
Mini cribs are popular for smaller nurseries, apartment living, and situations where you need a secondary sleep space. They typically run about 24″ × 38″ — but this is where you have to be careful, because mini crib dimensions are not federally standardized the same way full-size cribs are.
| Mini Crib Type | Typical Mattress Size | Notes |
|---|---|---|
| Standard mini crib | 24″ × 38″ | Most common; fits most brands |
| Compact mini crib | 24″ × 36″ | Less common; always measure first |
| Folding/travel mini crib | Varies by brand | Usually requires brand-specific mattress |
Because mini crib mattress dimensions vary by manufacturer, you should always check the crib’s documentation for the exact recommended mattress size before buying. Apply the same two-finger rule: no gap larger than two fingers between the mattress and the crib frame.
If you’re looking for mini crib mattress recommendations, I’ve tested several options in our Best Mini Crib Mattress of 2026 guide.
Other Crib & Sleep Space Sizes
Toddler Bed Mattress
Good news: if you have a convertible crib, the mattress you already own will likely work when it converts to a toddler bed. Toddler bed mattresses use the same 28″ × 52″ dimensions as standard crib mattresses. No new purchase needed.
Portable Crib / Pack-n-Play Mattress
Pack-n-Play style portable cribs use a thinner, fold-flat mattress — typically around 27″ × 39″, though this varies by model. These mattresses are intentionally thin (usually 1″–1.5″) so the unit can fold compactly. Never use a thicker aftermarket mattress in a portable play yard — it raises the sleeping surface and increases fall risk.
Bassinet Mattress
Bassinets are highly variable — sizes range from roughly 13″–16″ wide to 29″–33″ long depending on the brand and model. Bassinets almost always require the mattress that came with them or a brand-specific replacement. Do not substitute a mattress from a different brand in a bassinet — the fit is too critical in such a small sleep space.
| Sleep Space | Typical Mattress Size | Standardized? |
|---|---|---|
| Standard full-size crib | 28″ × 52″ | ✅ Yes (CPSC) |
| Mini crib | 24″ × 38″ (most common) | ⚠️ Not fully standardized |
| Toddler bed | 28″ × 52″ (same as crib) | ✅ Yes |
| Portable crib / Pack-n-Play | ~27″ × 39″, 1″–1.5″ thick | ❌ Varies by brand |
| Bassinet | 13–16″ × 29–33″ (varies widely) | ❌ Brand-specific |
How to Measure Your Crib Before Buying a Mattress
Even with standardized sizes, it’s worth taking 60 seconds to measure your crib before you order. Here’s how:
- Measure the interior floor of the crib — length and width, not the exterior frame.
- Note the interior depth — the distance from the floor of the crib to the top of the lowest mattress support setting. This tells you how thick a mattress you can use without compromising the crib rail height.
- Check the manufacturer’s spec — most crib brands list a recommended mattress size range in their manual or on their website.
- Apply the two-finger rule after assembly — after the mattress is installed, check every side for gaps. Two fingers should be the maximum.
For standard full-size cribs, you almost never run into compatibility issues. The time this matters most is with mini cribs, travel cribs, and older or secondhand cribs that may not meet current CPSC standards.
Does Mattress Thickness Matter?
Yes — and it’s more important than most people realize.
The CPSC’s safety standards for cribs are based on a mattress no thicker than 6 inches. The crib rail height is calculated so that with a properly fitted mattress, there’s enough distance to prevent an infant or toddler from climbing or falling out. If you use a mattress thicker than the crib was designed for, you effectively reduce that safety margin.
In practice, most quality crib mattresses range from 4″ to 6″ thick. Some dual-sided infant/toddler mattresses (firmer side for infants, softer side for toddlers) land around 5.5″–6″. Avoid mattresses marketed as “extra thick” that push past the 6″ ceiling.
Mattress firmness is a separate concern from thickness. The American Academy of Pediatrics (AAP) recommends a firm, flat sleep surface for all infants. A mattress that’s too soft — even at an appropriate thickness — increases suffocation risk.
Crib Mattress Dimensions FAQ
What is the standard crib mattress size?
The standard crib mattress size in the United States is 28 inches wide by 52 inches long, with a maximum thickness of 6 inches. These dimensions are regulated by the CPSC and apply to all full-size cribs sold in the U.S. Most standard crib mattresses you’ll find are listed as “28 × 52” and will fit any CPSC-compliant full-size crib.
What size is a mini crib mattress?
Most mini crib mattresses measure 24 inches wide by 38 inches long. However, mini crib dimensions are not fully standardized — some compact models use a 24″ × 36″ mattress or a different size entirely. Always check your specific crib’s documentation before buying a replacement mattress.
Are all crib mattresses the same size?
Full-size crib mattresses are standardized by federal law and will fit any CPSC-compliant full-size crib (28″ × 52″). Mini crib, portable crib, and bassinet mattresses are not standardized and vary by brand and model. If you have anything other than a standard crib, measure first and verify the manufacturer’s recommendation.
How do I know if a crib mattress fits safely?
Use the two-finger test: after placing the mattress in the crib, try to slide two fingers between the mattress edge and the crib side. If two fingers fit easily or if there’s a larger gap, the mattress is too small and should not be used. A snug fit with no visible gaps on any side is what you’re looking for.
Can I use a crib mattress in a toddler bed?
Yes. Toddler bed mattresses use the exact same 28″ × 52″ dimensions as standard crib mattresses. If you have a convertible crib that transitions to a toddler bed, your existing crib mattress will fit — no replacement needed.
Is a 5-inch or 6-inch crib mattress better?
Either is fine, as long as it stays within the 6-inch maximum set by CPSC standards. Thickness alone doesn’t determine quality or safety — firmness, certifications, and a snug fit matter more. Many dual-sided infant/toddler mattresses are around 5.5″–6″ thick, which works well for both stages.
Our Verdict
For most families buying a standard crib, sizing is simple: any 28″ × 52″ mattress that meets CPSC standards will fit. The choices that require more care are mini cribs, portable cribs, and bassinets — where dimensions vary by brand and a poor fit creates real safety risks.
Before you buy, measure your crib’s interior floor, check the manufacturer’s spec, and always apply the two-finger rule after the mattress is installed.
If you’re ready to choose a mattress, our tested picks are here:
- Best Baby Mattress of 2026 — top picks for standard full-size cribs
- Best Mini Crib Mattress of 2026 — top picks for mini cribs
Disclosure: BabyMattressStore.com participates in the Amazon Associates program and other affiliate programs. If you purchase through links on this site, we may earn a small commission at no extra cost to you. Our recommendations are based on independent research and personal testing — affiliate relationships do not influence our picks.
