Crib dimensions: full-size vs mini crib, and how much room each needs
Short answer: every full-size crib sold in the US has the same inside size by law, 28" × 52⅜" (± ⅝"), so most measure about 54–57" long and 30–31" wide outside. Mini cribs have no single standard size, but popular ones are about 37" × 24" inside and 40" × 26" outside. A mini saves about 14" of wall and 4" of depth.
Sources: CPSC business guidance for full-size and non-full-size cribs (16 CFR 1219 and 1220), the federal crib mattress rule (16 CFR 1241), and the makers' product pages. Always check your crib's label for its own weight and height limits.
Same scale, same wall: the mini is 14½" shorter and about 4" narrower. Frames add only 1–2" around the mattress on slim cribs like these; chunkier convertible cribs add more. (Makers usually round the 52⅜" standard to 52".)
What "full-size" and "mini" mean
Full-size crib: the federal standard fixes the inside at 28" × 52⅜" (with ⅝" tolerance), so any full-size crib mattress fits any full-size crib. Differences between models are all in the frame: posts, headboards and rails.
Mini crib (federal term: non-full-size): anything with an inside smaller than 49¾" long or 25⅜" wide. There's no one mini size, so buy the mattress the maker recommends. Babyletto, for example, recommends a 37" × 23.5" mattress for the Origami.
Mattress fit: the federal mattress rule limits the gap at a corner of a full-size crib to about 3". Practically, a mattress that matches the crib should sit snug with no gap you could fit a hand into.
Will each fit? An 8x9 nursery
Mini crib on the left wall, a typical 40" × 20" 3-drawer dresser on the right, and a small 30" × 34" chair below the crib, all clear of the door swing. Scale: 1 ft = 48 px.Same room with a full-size crib. It still fits, with 45½" between crib and dresser, and the chair still fits below it, just with less space around it. Scale: 1 ft = 48 px.
In an 8x9 a full-size crib works: the walkway only drops from 49½" to 45½", and either way there's room to open drawers (they need about 36"). The real difference is wall length. Below the mini there's about 65" of wall left; below the full-size, about 50". That 14" is often the difference between a small armchair and a glider that needs 60"+ to recline (see glider space).
Put a full-size or mini crib in your own room and see which one leaves space to walk.
Room-sharing in your bedroom, where every inch beside the bed counts.
Very small nurseries (around 7x8 or a closet-sized room), or a room you also need for a chair and dresser.
A second crib at grandparents' or in a living area. Some minis fold or roll.
The trade-off: babies outgrow minis sooner. Minis have lower weight limits (45 lb on the two above vs 50 lb on the full-size ones) and a shorter sleeping area, so many families move to a full-size crib or toddler bed earlier. Some minis convert to a twin or a small toddler bed; check what the conversion needs before you count on it.
Measuring tips
Use the outside dimensions from the product page for the room, and the inside dimensions for the mattress.
Add ½–1" for baseboards, and keep the crib off window walls and away from cords.
Leave 24–30" of clear floor along the open side so you can lift your baby out.
Follow the crib's assembly instructions and lower the mattress as your baby starts to sit and pull up. Safe-sleep guidance from the AAP: a firm, flat mattress that fits snugly, a fitted sheet, and nothing else in the crib.
In the US, a full-size crib's inside must be 28" × 52⅜" (± ⅝"), per the federal crib standard. Outside, most full-size cribs are about 53½–57" long and 29¾–31" wide.
How big is a mini crib?
There's no single size. Popular minis like the Babyletto Origami (39.25" × 25.75") and DaVinci Jenny Lind Mini (40" × 26") are about 37" × 24" inside.
What size mattress does a full-size crib need?
A full-size crib mattress, which the federal rule requires to be at least 27¼" × 51⅝". Any full-size crib mattress fits any full-size crib.
Can I use any mini crib mattress in a mini crib?
Not always. Mini cribs aren't standardized, so use the mattress size the crib's maker recommends, and make sure it fits snugly with no gaps.
Will a full-size crib fit in an 8x9 room?
Yes. It needs about 54" of wall and 30" out from it. In an 8x9 with a 40" dresser opposite, you still get about 45" to walk, though there's less room left for a glider.