Stand straight and breathe calmly. Now measure the width of the under bust with a measure tape. The measure has to be taken just below the bust, quite tighty but comfortable enough for you to breathe normally.
Read the measurement from the tape and enter it into the "Band size" box below.
For the cup size you need to measure the circumference of the bust. The measurement has to be taken on the widest part of the bust with a fairly loose measuring tape in order to not deform or squeeze the breasts. Ensuring the measuring tape is the at the same height across your bust as your back.
Read the measurement from the tape and enter it into the "Bust size" box below.