Size 1 diapers fit babies between the weights of 8 to 14 lbs and after that the baby is ready to move to size 2 diapers.
When the diaper starts to get a little too tight around the babies waist and legs and the diaper is leaving marks around your babies waist or legs then it's time to move up to the next size diaper.
Be prepared to move the baby to a new size diaper soon because babies grow out of their diaper sizes pretty quickly and they also grow out of their baby clothes quickly as well.
Be sure to have the next size diaper on hand so you can change diaper sizes for your baby when needed.
Once the baby reaches 14 lbs in weight then they're usually ready to move to size 2 diapers.