Finding the correct bra size is critical for comfort, support, posture, and preventing shoulder and back pain. Studies show that approximately eighty percent of women wear the incorrect bra size, usually wearing a band that is too loose and cups that are too small. By taking accurate measurements with a flexible tape measure and applying standard fitting calculations, you can determine your baseline size and navigate retail options with confidence.
A standard bra size consists of two parts: an even number representing the band size (like 34) and a letter representing the cup size (like C).
To find the band size, measure snugly around your ribcage directly under your bust. If the measurement is an even number, add 4 inches. If it is odd, add 5 inches to find the band size (this is the traditional method, though many modern brands use the direct measurement).
To find the cup size, measure around the fullest part of your bust. Subtract the calculated band size from this bust measurement. Each inch of difference represents a cup letter: 1 inch = A, 2 inches = B, 3 inches = C, 4 inches = D, and so on. For converting measurements between inches and centimeters, check out our translating measuring units tool.
Sister sizes are groups of bra sizes that share the exact same cup volume, even though the band and cup letters differ.
For example, if you wear a 34C and the band feels slightly too loose, shifting to a 32D will give you a tighter band while maintaining the same cup capacity. If you try on a 36B, the band will be looser, but the cup volume remains the same. Our online calculator helps identify sister sizes automatically.
Suppose a woman measures her underbust at 30 inches and her fullest bust at 34 inches.
To find the band size: since 30 is an even number, we add 4 inches to get a band size of 34. Next, we find the cup size by subtracting the band size (34) from the bust measurement (34): 34 - 34 = 0 inches. (Using modern direct sizing, the band is 30, and the difference is 34 - 30 = 4 inches, yielding a 30D). Under the traditional formula, a 34-inch bust and 34-inch band yields a 34AA. The calculator compares both sizing methodologies to offer the best fitting suggestions. This example shows how measurement differences define cup capacities.