How to know your bra size at home, you'll need a soft measuring tape. First, measure around your ribcage just under your bust to get your band size. Then, measure around the fullest part of your bust for your cup size. Subtract your band size from your bust measurement to find the difference, which determines your cup size. For example, a 1-inch difference equals an A cup, a 2-inch difference equals a B cup, and so on. Use this guide to calculate your bra size: band size + cup size (e.g., 34B).