No, they are different sizes. You have to measure your crib. If you look online and see what type of crib you have, it should give you the mattress size that goes with it. Some cribs are bigger than others.
Mami is right. When you purchase your crib it will tell you what size of mattress fits it.
generally its best to purchase them at the same time cause they will show you the right one for your crib, when i got mine they threw mattress in for free