Take a measurement tape and level it around the under bust. Observe the number to the nearest whole number.
If the number is even, add 4 inches and if it's odd, add 5 inches.
The sum of calculation = Your band size
Bust/Cup Size:
Wrap the tape around the fullest part of your bust. Observe the number and round off to the nearest whole number.
Now minus the band size calculated earlier from bust size and the difference will be your correct cup size.
If the difference is 1, then cup size is A. If it's 2 then cup size is B & so on. For Ex: Bust = 44 inches, Band = 40 inches, Difference = 44-40 = 4 inches. Your Perfect Size will be = Band*Difference = 40D