To find your bra size, you'll need a tape measure and your best-fitting bra (preferably an underwire, not a minimiser, sports bra or padded bra). If you don't have a flexible tape measure, try using string.
Wearing your bra, take a snug measurement around your ribcage, directly under your bust and level all around. If you get an odd number, round up to the next even number.
Take a measurement over the fullest part of your bust, keeping the tape level around your body. If needed, round up to the nearest inch.
Take your BUST measurement and subtract your BACK measurement. From the table below calculate your cup size using the difference between the two measurements.