Find your band size 1
Start by standing straight and relaxed. Make sure you’re braless or wearing a non-padded bra, then take a soft tape measure and wrap it around the bottom of the band (directly under your bust) and keep the tape measure tight and level. Now round to the nearest whole number. If it’s an even number you want to add four inches and if it’s an odd number you want to add five. The total of your calculation will be your band size. For instance if the rounded number is 34, your band size is 38.
Find your bust size 2
Now take the tape measure and wrap it around the fullest part of your chest (at nipple level). If the measurement falls between two whole numbers, round up to the nearest whole number. For instance if you land on 36.5, your bust size is 37.
Calculate your cup size 3
Subtract the band size from the bust measurement. Then refer to a bra chart to find your cup size. For example, if your bust measurement is 38 and your band size is 36, that's a 2-inch difference which means you wear a B cup. See the chart here for other measurements.
Make sure it's the right fit 5
Make sure your breasts are completely in the cups and take your fingers and slide them underneath the straps and the band to make sure the bra isn't too tight. Your breasts should sit midway between your shoulder and your elbows, otherwise you might need more support.
Put something on 6
Throw on a fitted shirt over your bra to check for bulging or underwire poking. If everything looks and feels right, you most likely found your right size!