Size 3 diapers are for babies between the ages of 7 months to 20 months of age.
The size of diapers by age are.
Newborn diapers: First few weeks of babies life.
Size 1 diaper for babies between 2 to 4 months of age.
Size 2 diapers for babies between the ages of 4 months to 7 months.
Size 3 diapers for babies between the ages of 7 months to 20 months.
Size 4 diapers for toddlers between the ages of 18 months to 48 months.
Size 5 diapers for toddlers between the ages of 3 years or older.
Size 6 diapers for toddlers between the ages of 4 years to 5 years and sometimes the ages of 6 to 8 years of age.
Size 7 diapers for kids between the ages of 6 to 10 years of age.
Size 8 diapers for kids betwen the ages of 8 to 13 years of age.
The biggest diaper size in kids diapers sizes is a size 8 diaper that is Pampers Swaddlers size 8, Boundless size 8 diapers and even Pampers Baby Dry size 8 diapers.
There are also other brands that are a size 8 children's diaper as well.
After the size 8 comes the Youth Size diapers that are more like adult diapers but sized for Youth and then the next largest diapers are adult diapers that come in small, medium, large, extra large and geriatric diapers.
The diaper sizes that come after size 6 diapers is a size 7 diaper and a size 8 diaper.
Most kids potty train before they even need a size 6 diaper but some kids may need a size 6 diaper or even a size 7 diaper.
And size 8 diapers can be good for kids who are disabled or have Autism or other health issues including incontinence or bed wetting.
After size 8 diapers comes Youth Size diapers.
Size 6 diapers are usually good for fitting a 3 year old or 4 or even 5 or 6 year old.
Some kids between the ages of 7 or even 8 may be able to fit into a size 7 or 8 diaper as well.
The age that kids wear size 4 diapers is between the ages of 18 months to 2 to 3 years of age depending on the size and weight of the kid.
Some kids are larger than others and may need a different size even though they are the same age.
Diapers go by weight for sizing and not age as kids are different weights and are built differently.
But most often a 2 year old would need to wear a size 4 or size 5 diaper and a 3 year old would need a size 5 or size 6 diaper.
A 4 year old may fit into a size 5 diaper or they may also need a size 6 or even size 7 diaper depending on their build.
Most often though size 6 diapers fits kids or little ones until they're potty trained between the ages of 2 and 3 years of age.
As a general rule of thumb, babies tend to stay in size 3 diapers for the longest period of time which is about six months by some estimates.
Assuming six or seven diaper changes per day, this could mean you might need anywhere between 1,000 to 12,000 size-3 diapers before moving on to size 4 diapers.
If the diaper fits too loose then the kid needs a smaller size diaper and if the diaper is leaving red marks around the legs or waist then they need a larger size diaper.