I am a little confused. What exactly is standard mattress size for baby's cribs? It doesn't seem like the crib mattresses I've seen in stores are exactly the same size.
You are right, standard crib mattress size has a little bit of wiggle room. You'll find that there is slight variation in length and width. The depth of the crib mattress can vary even more so. The important thing is that when the mattress is centered on the platform, you shouldn't be able to fit more than two fingers between the mattress and the crib rails or headboard or footbaords.
Standard Mattress Size by Width and Length
Specifically speaking, for mattresses sold in both the United States and Canada, crib mattress size may range between 27 5/8" to 28 5/8" wide and 51 3/4" to 53" in length.
Standard Mattress Size by Thickness
The thickness can really be different, specifically if you are looking at a coil crib mattress or a foam crib mattress. Coil mattress tends to be on the thicker side between 5 to 6", whereas foam mattress are more on the order of 4 to 6". A 5" depth should be fine, but keep in mind that a firm mattress is preferred over soft mattresses.
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