If you need to find a bra that fits right but aren't comfortable with letting a salesperson take your measurements, you can find your bra size on your own. Bra sizes consist of two parts: chest size and cup size. You only need a tape measure to find out what your body measurements are to determine the bra size you need.
Remove your shirt and bra. Take the tape measure and run it under your breasts to find out what your chest size is.
Write down the measurement and add 5 to it. For example, if your measurement is 27 inches, add 5 to it to arrive at your chest size. The chest size you would need for a bra from this example would be a 32. If you get an odd number, round it down.
Run the tape measure across the fullest part of your breasts to test for your cup size.
Write down this measurement and subtract your first measurement from this number. If this number was 30 inches, for example, you would subtract the first measurement of 27 inches from this number and arrive at 3. This number represents your cup size. If the number is a 1, your cup size is an A. If the number is 2, your cup size is a B, and so on. From this example, the bra size you should purchase would be a 32C.
Make sure not to hold the tape measure too tightly or loosely when measuring, as this could cause you to come up with an inaccurate bra size.