If you are unsure about your bra cup size, it is best to get fitted by a professional. But if you want to measure yourself at home, here are some instructions on how to do so: 1. Put on an unlined bra or no bra at all. 2. Use a soft tape measure to find out what size band you need. 3. Measure around the fullest part of your bust. 4. Subtract your band measurement from your bust measurement to determine cup size.