The height limit for a crib is 35 inches, meaning a child who reaches that height should move to a toddler or regular bed. This recommendation comes from the American Academy of Pediatrics and is echoed in federal safety warnings required on crib assembly instructions. But “height limit” also refers to the crib itself: how tall the rails need to be and how deep the interior must sit to keep your child safely contained.
The 35-Inch Rule
Once your child stands 35 inches tall, the crib no longer provides reliable containment. At that height, most crib rails sit roughly at chest level, which gives a toddler enough leverage to swing a leg over the top and tumble out. The Consumer Product Safety Commission requires crib manufacturers to include a warning in assembly instructions urging parents to move any child taller than 35 inches to a youth bed or regular bed.
Height alone isn’t the only signal. The AAP identifies three indicators that a child has outgrown their crib: they’re 35 inches tall, the top of the crib rail hits at or below the middle of their chest when standing, or they’ve already climbed out. Any one of these is enough reason to transition. Most children hit this point somewhere between 18 and 36 months, though the range varies widely depending on how fast they grow and how adventurous they are.
How Crib Dimensions Are Regulated
Full-size cribs sold in the United States must meet federal safety standards enforced by the CPSC. The interior of a full-size crib is required to measure 28 inches wide (plus or minus 5/8 of an inch) by 52 3/8 inches long (plus or minus 5/8 of an inch). These dimensions are tightly controlled so that standard crib mattresses fit snugly, leaving no dangerous gaps between the mattress edge and the crib wall.
The height of the crib sides isn’t expressed as a single number in the regulations because it depends on the mattress position. What matters is the distance between the top of the mattress surface and the top of the rail. When the mattress is set to its lowest position, most cribs place the rail roughly 26 inches above the mattress. That gap is what keeps a younger child from climbing or falling out. For bassinets and cradle-style sleep products, the minimum interior side height is 7.5 inches above an uncompressed mattress, but these are designed only for babies under five months or those who can’t yet sit up on their own.
Why Mattress Height Settings Matter
Most cribs come with two or three mattress support levels. The highest setting keeps the mattress close to the top of the rail, making it easier to lay a newborn down and pick them up without straining your back. At this level, though, the effective wall height is at its shortest. As your baby grows and starts pulling up to stand, that shallow barrier becomes a fall risk.
Lower the mattress to its middle setting once your baby can sit up unassisted, which typically happens around six months. Drop it to the lowest setting before your baby can pull themselves to standing, usually around eight or nine months. At the lowest position, the full depth of the crib rail is in play, and your child has the maximum containment the crib can offer. Skipping or delaying these adjustments is one of the most common oversights, and it directly affects how long the crib remains safe.
Fall Risks and Climbing
Falls are the leading cause of nonfatal injuries in young children, and cribs are a known source. Beds account for about a third of fall injuries in infants and 13% in toddlers, with cribs contributing a meaningful share of those numbers. The injuries typically happen when a child climbs or rolls over the rail and drops to the floor, often landing on their head.
Some toddlers figure out how to climb out well before they hit 35 inches. If your child makes it over the rail even once, the crib is no longer safe regardless of their height. Placing the mattress on its lowest setting, removing bumpers or stuffed animals that could serve as stepping platforms, and dressing your child in a sleep sack (which restricts leg movement for climbing) can buy you some time. But once a child is determined to climb, the safest response is transitioning them out of the crib rather than trying to engineer around the behavior.
When to Make the Switch
The AAP’s guidance boils down to a simple visual test: stand your child up inside the crib and look at where the top of the rail falls on their body. If it’s at or below the nipple line on their chest, the crib rail is too low to prevent them from going over. For most children, this happens between age two and three, but a tall 18-month-old can reach this threshold earlier than a smaller child who stays safely contained past their third birthday.
There’s no advantage to rushing the transition. Cribs are the safest sleep environment for children who still fit in them, because the enclosed space prevents rolling out of bed and limits nighttime wandering. If your child isn’t climbing, isn’t 35 inches tall, and the rail still sits above their chest, they can stay in the crib. When the time does come, a toddler bed that uses the same crib mattress keeps the sleeping surface familiar while removing the containment your child has outgrown.