Back Size:
In inches, firmly measure under the root of the bust, so the tape measure is level all the way round the body.
If it is an odd number, round up to the next even number. If it is an even number, this is your back size.
The back size is the number of your bra size, and the cup size is the letter.
Example:
If your measurement is 31", round up to make 32.
If your measurement is 32", this makes it 32.