I usually go by weight and length when buy clothes for my little guy so that be what I do when buy this sleep sack . I recommend that people look at the size chart and go more by the length than the weight when buy one of these . my baby be 25lbs when I buy this but be not 30 inch in length yet . when I receive it I be like WOW this thing be huge ! sure enough it be way too long for he at this stage and I should have go with the medium .