Measure your frame size
First, measure your torso (in inches). Place the measuring tape below the underwire, directly under where your bust finishes, right across your back. Take that value and add 4 if it’s an even number, and 5 if it’s and odd number. For example, if your frame measurement equals 30, add 4, for a total of 34. If your frame is a 29, add 5 and your frame is 34.
Measure your bust size
Then measure your bust across the apex, which is the strongest part of your bust, where your nipples are. Make sure the tape measure is straight across your back. This will give you a larger measurement than your torso.
Find your cup size
For every inch that differs between your torso measurement and your bust measurement, you add a cup size. For example, if your torso measurement is 30 (giving you a size 34), and your bust measurement is a 34, then every additional inch from 30 to 34 equals your cup size. Therefore, the first inch is an A, the second inch is a B, the third is a C and then a D. So, a frame that has a 30 torso with a 34 bust equals a 34D bra size.