Take the tape measure around the torso directly under the breasts where your bra band should sit.
You might need somebody to help you do this, make sure the tape measurement is comfortable and feels right for you.
Once you have the required measurement from the tape measure ie: 33
Go to the band sizing on the bra calculator and enter the number, the calculator will then give you the band size you require.
Note the number of inches and enter it here: