You should switch your baby to size 2 diapers when your baby is between the ages of 3 months old to 8 months old or when the baby weighs between 12 lbs to 18 lbs.
Most babies will stay in size 1 diapers until 3 to 8 months of age or between 12 lbs to 18 lbs although it can also vary depending on the baby and their individual growth and build.
The best time to switch to the next size diaper for your baby or child is when the diaper no longer fits the baby or child properly, no matter the exact weight or age.
If you notice any signs such as red marks on the legs, or if the diaper is not fitting snugly around the babies belly or bottom, or the baby's diaper is leaking or having constant blowouts it's time to move up to the next diaper size.
You should then switch to size 3 diapers between the ages of 5 to 9 months or when the baby weighs 16 lbs or over.
Size 3 diapers are for babies that weigh between 16 lbs to 28 lbs, which usually is around the ages of 5 months to 24 months of age.
Size 4 diaper are for toddlers 2 years old and above including age 3 in some cases and size 5 are for 3 years and up and size 6 are also for ages 3 years and up.
Size 6 can also sometimes fit a 5 year old if they need diapers and a size 7 or size 8 diaper can fit ages 7 to 13 years old depending on the child's build and weight.