A standard crib mattress should measure 27.25 inches by 51.25 inches. The thickness on a standard crib mattress should not be greater than 6 inches.
These measurements are of extreme importance to prevent head entrapment and suffocation of infants in the crib. When using a standard crib mattress, there should be no more than 2 3/8 inches between the mattress and crib slats. A standard crib mattress should also be used in a toddler bed. The Consumer Product Safety Improvement Act of 2008 mandated the standard size crib and mattress to prevent death and suffocation hazards and maintain safety for infants and toddlers using cribs.
All crib mattresses, regardless of size, should be firm so that the baby's face doesn't get buried in the mattress when sleeping on his or her stomach.