How to Figure Out a Baby’s Diaper Size

Parents shouldn’t simply be guided by size guidelines on diaper packaging. Sometimes there are better signs it’s time to change diaper sizes.

Diapers are a lot like jeans: Sure, every brand of diaper comes in similar sizes, but a two in one brand seems like a four in another. Why does this matter? Not only do parents want their kids to be comfortable, they want to limit the risk of blowouts, which are more common when an undersized diaper is in play. The stakes are, as it turns out, high for parents trying to keep up with their child’s diaper situation.

“If your baby starts leaking a lot or soaking through, especially at night time, it might mean you need to move up to another size,” says Dr. Tanya Altmann, founder of Calabasas Pediatrics and author of Baby and Toddler Basics: Expert Answers to Parents’ Top 150 Question. “If you notice it’s getting too tight around the thighs or abdomen, and you’re seeing irritation from rubbing, that’s also a sign you need to move up to the next size diaper.”

Upgrading to a different size does’t just mean making space for a growing baby, it means adding more material to absorb, you know, whatever.

“If you have a dirty, explosive, leaking diaper once, you’re going to want to buy the next size,” says Altmann. “You have to change the clothes, wash everything, bathe the baby, and sometimes throw clothing away because it’s so disgusting you don’t want to wash it.  So when you have one leakage, you (should) just go up a size.”

Unfortunately, there are also consequences to going up too early: The extra material can cause the diaper to droop, while the cuffs around the thighs end up being baggy, which can also result in leakage. Still, Altmann says that many parents may want to transition to a larger size at night to maximize absorption while sticking with a smaller size during the day when diaper changes are frequent.

“Often parents will move up to another size for nighttime because you’ll have extras of a smaller size they can wear during the day when you’re changing them more,” Altmann explains. “But at night when they’re sleeping all night long they need a bigger size.”