The size of diaper that babies stay in longer is size 3 diapers up until age 1 and then after that they tend to stay in size 4 until age 2 to 3 until potty trained.
However babies and toddlers are different weights and are built different so while one baby could fit in a size 4 at age 2 or 3 another baby or toddler of the same age may need a size 5 or size 6 diaper.
The majority of babies and toddlers wear size 4 diapers up until they potty train.
Size 4 diapers are for babies and toddlers from 18 months to 3 years of age or 22 lbs to 37 lbs.
Size 5 diapers fit children 4 to 5 years old and size 6 fits children 4 to 7 years old for some kids.
Size 7 diapers fits kids between 5 years old to 8 to 10 years old and sometimes older.
After size 7 diapers is a size 8 diaper which can fit kids from 8 years old to 12 to 13 years old and sometimes older depending on how much they weigh and their build.
As babies get older they pee less and need less diapers than younger babies.
By age 2 to 3 years old most kids only need a diaper change every 3 to 4 hours depending on how much and how often they pee.
While the diaper size may appear to fit your baby, the amount of pee may have increased with their growth, so the diaper may not be able to absorb the larger amount of urine.
For disposable diapers, the bigger the size, the better the absorbency.
If you can't comfortably fit two fingers under the waistband of the fastened diaper, it's time for a bigger size.
You may need to go up a size if you notice red marks on your baby's tummy or thighs when you remove the diaper.
If the diaper isn't fully covering your baby's bottom, choose a bigger size.