A crib mattress gets lowered because your baby is getting stronger, and the timing follows their development rather than their age. The short version: drop it the first time when they can push up onto hands and knees or sit without help, and drop it again to the lowest setting before they can pull themselves up to stand. Both moves should happen slightly earlier than you think, because babies reach these milestones without warning.
Here is how the settings work, what to watch for, and how to actually make the change without introducing a new problem.
The three mattress heights, and when to use each
Highest setting: newborn to about five months
The top position exists for your back, not for the baby. Lifting a newborn in and out of a deep crib a dozen times a day is genuinely hard on you, and at this stage a baby who cannot roll or push up is not going anywhere. Use the highest setting from day one, and enjoy it while it lasts.
Middle setting: when they can sit or push up
Lower it as soon as your baby can push up onto hands and knees, roll deliberately in both directions, or sit up unaided. For most babies that lands somewhere between five and eight months, but the milestone is what matters, not the month. A baby who can sit can lean, and a baby who can lean over a high rail is one shift of weight from going over it.
If your crib only has two positions, this is the point where you go straight to the lowest one.
Lowest setting: before they pull to stand
This is the important one, and the mistake most parents make is waiting for proof. By the time you see your baby standing in the crib, they have already been practicing. Drop the mattress to the bottom as soon as they start pulling up on furniture anywhere in the house, usually around eight to ten months. Do it that same day if you can.
How high should the rail be above the mattress?
Full-size cribs sold in the United States are built to a federal safety standard that requires at least 26 inches between the top of the mattress support at its lowest position and the top of the crib rail. That number is the design margin that keeps a standing toddler inside.
Two practical checks are worth doing yourself:
- The chest test. Stand your child up in the crib. If the top of the rail sits below their chest, the crib no longer contains them reliably.
- The mattress thickness check. A thick mattress eats into that margin. Standard full-size crib mattresses are roughly four to six inches thick for a reason, and adding a topper or a second mattress under it defeats the design entirely.
How to lower a crib mattress
Every crib is slightly different, but the process is nearly always the same. Give it fifteen minutes and a screwdriver.
- Take the baby, the sheet and everything else out of the crib, and lift the mattress out completely.
- Find the mattress support: usually a metal frame or a wooden slat platform held by brackets, bolts or hooks on the end panels.
- Loosen the hardware on one side, move the support down to the setting you want, then do the other side. Work one side at a time so the support does not drop unevenly.
- Confirm the support sits at the same height at all four corners before tightening anything. An uneven support means a tilted mattress.
- Tighten every bolt firmly, then put the mattress back in and press down on each corner to check it sits flat.
- Do the two-finger check: you should not be able to fit more than two fingers between the mattress edge and the crib frame anywhere around the perimeter.
Keep the instruction manual, or find your model's PDF online, because bracket configurations vary more than you would expect between brands. And while the crib is apart, go around and retighten all the other hardware. Crib bolts work loose over months of a baby bouncing against the rails.
What to check afterwards
Once the mattress is at the new height, look at what is now within reach. A baby standing at the lowest setting can reach several inches over the rail, so move the crib away from windows, blind cords, wall hangings, shelves and lamp cords. Cords are the specific hazard here, and the safe distance is farther than most nurseries are laid out for.
The crib itself should still be bare. No pillows, bumpers, positioners or loose blankets, and no toys a determined toddler could stack and stand on. Our guide on when a baby can sleep with a blanket covers the bedding side of this.
When lowering is no longer enough
There is a point past the lowest setting where the answer is a different bed rather than a different height. Move your child out of the crib when any of these is true:
- They have climbed out, or seriously attempted it. Once, is enough. A fall from the top of a crib rail is a longer drop than a fall from a toddler bed.
- They reach roughly 35 inches tall, the height most manufacturers give as the limit.
- The rail sits below their chest when they stand.
For most families that arrives somewhere between 18 months and three years. Some go to a toddler bed using the same crib mattress, which is the cheapest path and works well if your crib converts. Others go straight to a twin, which costs more up front and then lasts a decade. Our toddler mattress guide and our picks for the best twin mattresses for kids cover both routes, and you can browse twin mattresses if you already know which way you are going.
Frequently asked questions
When do you lower the crib mattress?
Lower it the first time when your baby can sit up unaided or push onto hands and knees, commonly around five to eight months. Lower it to the bottom setting as soon as they begin pulling to stand on any furniture, commonly around eight to ten months. Go by the milestone, not the age.
How do you lower a crib mattress?
Empty the crib and lift the mattress out, then loosen the brackets or bolts holding the mattress support and move it down one side at a time. Check that all four corners sit level, tighten every bolt, put the mattress back and confirm there is no gap wider than two fingers between the mattress and the frame.
How low should the crib mattress be?
Once your baby can stand, use the lowest setting the crib offers. Full-size cribs are designed so that the lowest position leaves at least 26 inches from the top of the mattress support to the top of the rail. If the rail sits below your standing child's chest even at the lowest setting, it is time for a different bed.
What age do you lower the crib mattress?
There is no fixed age, and going by age alone is how parents get caught out. Babies reach these milestones anywhere from four months to a year. Watch for rolling, sitting and pulling up, and act as soon as you see the earliest sign rather than waiting for the behavior to be established.
Is it safe to lower a crib mattress to the floor?
Only if your crib is designed for it. Some models place the lowest support position at or near floor level and that is fine, because the support is still doing its job. What is not safe is removing the support and resting the mattress directly on the floor inside the crib frame, which creates gaps at the sides. Follow the positions your manufacturer built in.
Does a thicker crib mattress affect the height?
Yes, and it works against you. Every extra inch of mattress reduces the rail height that keeps your child in. Stick to a standard-thickness crib mattress, roughly four to six inches, and never add a topper or a second mattress underneath to raise it.
Start with the right mattress
Lowering the support is straightforward. Choosing what sits on it matters more: crib mattresses need to be firm, flat, correctly sized and the right thickness, and those are the things you cannot correct later. Our crib mattress buying guide covers what to look for. When the crib stage ends, come see us at any of our five Los Angeles showrooms or take the sleep quiz to find the next bed. Same-day delivery is available across most of LA.
