A: The snug fit of a crib mattress is so important for infant safety that the sizes of cribs and mattresses sold in the United States are governed by law. Before you go to purchase a mattress, measure the inside dimensions of your crib. If it’s a standard size, you should find it easy to locate a mattress that will fit in it correctly. The standard interior dimensions for cribs, dictated by ATSM International (formerly the American Society for Testing and Materials) and federal law, are a width of 28 inches plus or minus 5/8 inch and a length of 52 3/8 inches plus or minus 5/8 inch. This means that the inside of your crib should measure somewhere between 27 3/8 to 28 5/8 inches wide and between 51 3/4 and 53 inches long. Mattresses are designed to be slightly smaller than this to allow them to fit tightly within these confines. If your crib measures smaller or larger than legal specifications, you’ll need to consult the manufacturer for suggestions.