The best way to figure out what size diaper your baby needs is to use their current weight as a guide. Typically, newborns up to 8-10 lbs will fit into newborn size diapers, Size 1 is 8-14 lbs, Size 2 is 12-18 lbs, etc. You can see all of the size to weight ratios in this helpful guide. To determine when to size up your baby's diaper - the best way to do that is to inspect the one they are currently wearing. If you notice red marks around your baby’s upper legs or tummy, then it might be time to size up.