Great question! If you already have a LÍLLÉbaby® carrier, place it around your waist like you would for a back carry (no need to put baby in!) and measure the black or gray webbing that does NOT overlap the fabric waistband. Between 8 - 14 inches = small, 14 - 19 inches = medium, more than 19” = large. If you are in between sizes, size DOWN, not up, for best fit and use.