Some diaper brands will offer measurements (along with the usual weight ranges) for their products. Checking your baby's measurements against these is a fantastic way to ensure you are buying the proper size, as weight can be quite subjective (based on body type) and measurements are objective.

Tip - don't have a tape measure in the house? Grab a piece of string to measure, then compare that against your ruler/yardstick/measuring tape.
  1. Legs - Measure along the edge of the leg, where the diaper meets the skin
  2. Waist - Measure along the edge of the diaper, around the tummy
  3. Rise - Measure from the top of the diaper at the front, through the legs, to the top of the diaper's back
Measuring for a wool cover? Put baby in the diaper you wish to use under the cover (if you'll use a variety of fitteds/contours/prefolds/flats, measure both the thinnest and thickest to get a sense of the size that will work best.