Crib dimensions, mattress sizes and clearances

Almost every dimension in a house is a convention. A crib is one of the few that is law. In the United States the interior of a full-size crib, the spacing of its slats, the height of its rail and the size of the mattress that goes in it are all set by federal standards, and the numbers are unusually precise: not "about 28 inches" but 28 in with a stated tolerance of 5/8 in. What follows is those figures, where each one came from, and the one place the arithmetic of the standards leaves a gap that the standards themselves do not close.

The numbers that are actually regulated#

That last row is the one that catches people out when they plan a room. The interior is the child's world and it is fixed. The exterior is the room's problem and it varies by 5 in (13 cm) of width between a slim slatted crib and a convertible one with solid ends and corner posts. Measure the crib you are actually buying, over the widest point of the top rail, and use that figure in nursery layout and clearances.

The gap: what "two fingers" is really measuring#

The field test everybody is taught is to push the mattress hard against one side and see whether more than two fingers fit in the gap on the other. Two adult fingers are about 1 1/8 in (29 mm). The European standard EN 716 is usually quoted with a 30 mm (1 1/4 in) limit for the same measurement, so the two traditions land within a couple of millimetres of each other.

The reason a gap matters is not that a baby falls into it. It is that an infant who rolls into a soft-sided slot cannot reliably lift its own head out, and a gap wide enough to admit a shoulder is wide enough to hold a face against a mattress edge. The hazard is positional, so it is the narrowest gap that must be checked, at every point along all four sides, including at the corners where a bowed mattress pulls away from the frame.

Slat spacing, and why the number is 2 3/8 in#

The 2 3/8 in (60 mm) maximum comes from anthropometry, not from tradition. The gap has to be narrower than an infant's chest, because the failure mode is a body slipping through while the head does not follow. It is the same reasoning that produces the 4 in (100 mm) baluster gap in stair and guard rules, scaled down for a child who cannot yet stand.

There is a field check that needs no tape. A standard drinks can is about 2 5/8 in (66 mm) in diameter. If a can passes between two slats, the gap is over the limit. Do the check at the middle of the span rather than at the top, because a slat that has loosened at one end bows outward under a leaning child and the gap opens where it is least visible.

EN 716 sets the same 60 mm ceiling and adds something the US standard does not: a minimum of 45 mm (1 3/4 in). A gap below that is wide enough to admit a finger or a foot and too narrow to release it. The European standard, in other words, treats the slat gap as a band rather than a maximum, and that is the better model.

Rails, mattress settings and the height that is left#

The rail height requirements are written from the mattress support, not from the sleeping surface, and this is the source of a great deal of confusion.

At the lowest setting the standard asks for 26 in (66 cm) from the support to the top of the rail. Put a 6 in (15 cm) mattress on it and 20 in (51 cm) of rail remains above the child. At the highest setting the requirement is 9 in (23 cm) from support to rail top, and the same 6 in mattress leaves 3 in (75 mm). That is not an oversight: the high setting exists for a newborn who cannot roll, and it exists so that an adult can lift the child out without bending double, an argument developed in reach zones and useful storage heights.

Placing the crib in the room#

Three of the crib's four faces can be tight to something. One long side needs 30 in (76 cm) of clear floor, and the choice of which side is permanent, because the lift only works across the long dimension.

The 36 in figure is the reach envelope of a standing toddler: about 20 in (51 cm) beyond the rail, plus a margin for a child who leans and grabs. Cord lengths, tilt devices and the safest blind types for a child's room are covered in blind and shade sizing, and the sill height that decides whether a window is reachable at all is in window sill and head heights.

US, EU and the sizes that do not swap#

The trap is the 70 cm cot bed. At 27 1/2 in it is within half an inch of the US crib mattress width, so a sheet or a mattress can appear to fit, and then the 8 in (20 cm) length difference produces exactly the gap the whole standard exists to prevent. Where sizes almost match across borders, they are worth checking twice, which is the argument made throughout bed sizes by country. Thickness bands for every other mattress in the house are in mattress sizes and thicknesses, and the wider question of which body of rules governs which figure is set out in standards, codes and sources.

Secondhand cribs and what is no longer allowed#

Drop-side cribs have been prohibited in the US since 2011. The rewritten standards assume fixed sides, and the reason is mechanical: the plastic tracks and hardware wear, the side racks slightly out of square, and a gap opens along the moving rail exactly where a mattress edge sits.

Three other features date a crib and rule it out. Corner posts standing proud of the rail by more than 1/16 in (1.5 mm) can catch clothing. Decorative cutouts in the head or foot board can trap a head. Any slat that moves when you pull it sideways has a loose joint, and one missing slat doubles the local gap to about 5 in (127 mm), well past the limit.

Convertible cribs raise a different question: what they convert into. A crib that becomes a toddler bed keeps the same mattress and the same footprint, so the room does not change. A crib that becomes a full-size bed at 54 x 75 in (137 x 191 cm) needs 24 in (61 cm) more width than the crib ever did, which should be checked against the plan now rather than in four years. The sizes at each stage are set out in kids bed sizes by age, alongside the rest of kids room and nursery dimensions. For quick conversions between the two measurement systems this page runs on, the interior unit converter is faster than arithmetic.