This Bra-Size Calculator Is a Thing of Genius
Once you’ve done the band, then you want to measure the fullest part of your bust. Gently take the tape measure and measure it in inches as well.
Then comes the slightly tricky bit: To work out what your cup size is, you want to subtract your band size from your bust size. From the difference between the two sizes, you can work out your cup. If the difference is less than 1, then it’s AA, 1 means A, 2 means B, 3 means C, 4 means D, 5 means DD, and so on.
You’ll still need your band measurement to give you the number preceding the letter. For example, if your band size is 34 but your bust size is 37, then your bra size is 34C (difference of 3, again, means it’s a C cup). Of course, it’s worth mentioning that bras can fit differently according to different brands, but now you have the basic tools to work out your correct size.