When shopping for a crib mattress, many parents question the size they should purchase. Actually, although crib mattresses are pretty much standard within an inch or so on either side (length or width) the best thing to do if in doubt would be to measure the inner dimensions of the crib. While a good quality crib would support a high end mattress, a less expensive crib may not do so.
Just as an example, some of the best selling mattresses manufactured by companies with a reliable reputation would include mattresses manufactured by Sealy or Serta. The Sealy Baby Posturepedic Mattress has dimensions of 51.5 x 27 x 5 inches and weighs approximately 14 pounds. The Serta Nightstar Extra Firm Crib Mattress is 52.5 x 27.3 x 5.8 inches and weighs 16 pounds, while the Sealy Baby Ultra Rest Mattress has the dimensions of 51.5 x 27 x 5 inches yet only weighs 13 pounds.
Baby Sealy Mattress
As you can see, there is only a 3 pound differential in weight, but that may make a difference depending on the supporting slats in the crib. What is most important when trying to determine size is the interior perimeter, which in most cases will accommodate any crib mattress manufactured by a reliable company. However, don't let size be your only gauge when purchasing a crib mattress. Think of the support it affords your baby for the development of strong healthy bones as well as a mattress that is non flammable and non allergenic. Even if it 'fits' it may not be safe for your child.
It's important know know the size since you'll need that to also buy a crib and cover if you'd like to protect your investment.