Take your measurements while wearing a suitable non-padded underwire bra or a tight fitting seamless tee-shirt. When you measure, hold the measure tape frmly around you.

To find your bra size you will need to take two measurements: around the rib cage just under your bust and around the fullest part of your bust.

Step 1: Measure Under Bust

Using a soft tape-measure, measure around your ribcage just below your bust. Pull the tape tightly around the body. Check in the mirror to make sure the tape measure is smooth across your back and parallel to the floor. Write down this measurement in inches.

Step 2: Measure Bust

Now, holding the tape measure straight, but not tight, measure around the fullest part of your breasts. Check in the mirror to make sure the tape measure is smooth across your back and parallel to the floor. Write down this measurement in inches.

Step 3: Find Your Bra Size

Using the table below find your bra size. (For example, lets say your measurement from Step 1 is 29” and your measurement form Step 2 is 35”. Look up 29” in Under Bust column and 35” in Bust row. Trace the imaginary vertical line in column 29”-30” all the way down to row 35“. There you will find your bra size, 34B, where 34 is your band size and B is your cup size).
Sourse