To calculate bra size, get the band size and bust size in inches and subtract the band size from the bust size.
cup size = bust - band
Take this value and find where it is on the cup size chart below:
|Bust/Band Difference (inches)||Cup Size|
The band size with the cup size .e.g 36C will approximately be the right bra size.
To make things easier, use this bra size calculator. For best results, provide the measurements in inches.