Skivvydoodles' manufacturer - My Boy Sam - recommends purchasing Skivvydoodles one size up from a child's age for average sizes and two sizes up for tall or huskier sizes. The fuller fit adjusts to many shapes comfortably. Please email us if you need specific measurements, such as arm or in-seam length; be sure to tell us which style you are interested in knowing about. As a general rule of thumb, the size pant that your child presently wears is a good indicator of size to order. If your child is wearing a Husky or Plus size, (for example, Size 12H) it is best to order the next size or two up. We've had so many requests for a baseline size chart, we are providing this chart which is a GENERAL guideline. THIS IS NOT the "official" sizing for Skivvydoodles. NOTE: The Skivvydoodles Too Underwear sizing is trimmer than the pajamas. We recommend ordering at least a full size up from the current size your child wears for the underwear. 3-6 month 25" - 27" 14 - 18 lbs. 6-12 month 27 1/2" - 29" 19 - 22 lbs. 12-18 month 29 1/2" - 31" 22 1/2 - 25 1/2 lbs. 24 month 31 1/2" - 33" 26 - 29 lbs. TODDLER 2T Weight 26-29 lbs. Height 32-35" 3T Weight 30-34 lbs. Height 34-37" 4T Weight 34-38 lbs. Height 36-40" CHILD Size 4 Weight 35-39 lbs. Height 39" Size 5 Weight 38-41 lbs. Height 42" Size 6 Weight 40-48 lbs. Height 46" Size 7 Weight 60 lbs. Height 49" Size 8 Weight 65 lbs. Height 52" Size 10 Weight 75 lbs. Height 55" Size 12 Weight 85 lbs. Height 58" Size 14 Weight 95 lbs. Height 60" Size 16 Weight 100 lbs. Height 62" Size 18 Weight 110 lbs. Height 64"