The resulting number is your child's chest measurement. If it turns out to be too big, then your child can grow into it and keep it for longer.
Again, if you don't mind the jitters and all that jazz, it may be worth your time to check out the LipoVida brand. Otherwise, Pure GCE gets my vote. Good product except the product isn't the best. I've tried a few different brands and this isn't the worst but it's not the best.
If you are selecting sizes for an older adolescent or a young teenager, be aware that youth extra large does not correlate to an adult small. A youth extra large . Gloves and mitts: For youth gloves and mitts, the hand circumference in inches equals glove size. Measure around a flat hand at the knuckles, but do not include thumb. Measure around a flat hand at the knuckles, but do not include thumb. Youth medium is the same as size 8 to 10 and youth small fits children wearing size 6 to 7. Youth X-large is converted into sizes 16 through Adult sizes are commonly used for sizes larger than youth X-large.