To determine cup size measure the underbust and add 4 if the underbust is an even number or add 5 if the underbust is an odd number to determine the band size. Add to a tight measurement because a loose measurement doesnt make sense anyway for the reasons i have written about before. The standard bra fitting advice measure under the bust and then add 4 inches is wrong not because of 4 inches but because it doesnt mention tight measurement probably for the reasons brittany writes about. Your band size should essentially be your underbust measurement more on that in the next section rounded to the nearest inch. Add 4 inches 10 centimeters. Bra band sizes go in 2 inch increments.
If its an odd number youll add one inch. And actually if you are smaller chested its not so great. The add four method was popularized by warners in the 1930s when bra design was in its infancy and it simply doesnt work with modern bras. If the measurement is even. Adding 4 5 inches wont give you the support you need if you are a large busted lady. To get the cup size subtract the band size from the overbust measurement of the fullest part of the bust.
Be aware that most brands no longer use this band measurement system. It uses your rounded underbust measurement as your band measurement. Add 5 inches 125 centimeters. If you look up how to measure your bra size on the internet most of the guides you will find tell you to measure under your breasts and then add 4 5 inches to that measurement to get your band size. If the measurement is odd. The idea of using your true measurement as a starting point rather than adding 4 inches has been around for a while but the debate really got heated when simply yours attempted to defend their bra size calculator by claiming that a 30 inch bra actually measures 21 to 22 inches unstretched or 25 to 26 inches stretched.
So 28 inches 30 inches 32 inches and so on.