Onesies are typically sized by weight and height. The difficulty lies in the differences from brand to brand. Just like women's jeans, where a size 4 from The Gap might not fit the same as a size 4 from Banana Republic, baby onesie manufacturers' sizes also differ.
However, there are staple brands like Gerber and Carter's that act as good gauges for onesie sizing. Thankfully, the sizes for both brands are similar.
Gerber Onesie sizing:
Newborn 5-8 lbs/17-21 in.
0-3 Months 8-12 lbs, 21-24 in.
3-6 Months 12-16 lbs, 24-26 in.
6-9 Months 16-20 lbs, 26-28 in.
12 Months 20-24 lbs, 28-30 in.
18 Months 24-28 lbs, 30-32 in.
24 Months/2T 28-32 lbs, 32-24 in.
Carter's bodysuit sizing:
Newborn 5-8 lbs, up to 21 in.
3 Months 8-12.5 lbs, 21.5-24 in.
6 Months 12.5-16.5 lbs, 24-26.5 in.
9 Months 16.5-20.5 lbs, 26.5-28.5 in.
12 Months 20.5-24.5 lbs, 28.5-30.5 in.
18 Months 24.5-27.5 lbs, 30.5-32.5 in.
24 Months 27.5-30 lbs, 32.5-34 in.
2T 29-31 lbs, 34.5-36.5 in.
From the above, you can estimate the following when buying Kiditude cool baby clothes:
0-6 Month onesie, 8-16 lbs, 21-26 in.
6-12 Month onesie 16-24 lbs, 26-30 in.
12-18 Month onesie 20-28 lbs, 28-32 in.
When purchasing a baby onesie, take into account the shrinking factor! If your choice of onesie is 100% breathable, baby soft cotton, it may shrink up to 1 full size after the first wash and dry cycle. Considering you wash your baby's clothes prior to wear to remove dust, lint and chemicals, you'll want to remember to buy larger, especially if the item is not pre-shrunk. If your choice of onesie is a cotton blend, there will be less shrinkage.
One Step Ahead also makes onesie extenders. These are difficult to find in the store. We found them online. They work with multiple brands of onesies based of the size of the snaps and side on which the snaps are located. These are great for when your baby is outgrowing one size but is still too small for the next. The extender is a strip of cotton fabric with snaps at both ends that attach to the snaps on the onesie to provide a few extra inches of length. Since it snaps between the legs, it's not noticeable once covered by shorts, dress or pants.