Bra Size Calculator
Our free bra size calculator finds your US band and cup size from two simple tape measurements — underbust and bust — with instant results in inches or centimeters, plus sister sizes to try when shopping.
Enter your measurements
Snug tape directly under the bust, around the ribcage.
Loose tape around the fullest part of the bust, kept level.
Your results
| Band size | — |
|---|---|
| Cup size | — |
| Bust − band difference | — |
| Sister sizes | — |
Sizing varies by brand — this is a starting point. Try sister sizes when shopping.
How to use this calculator
Choose inches or centimeters, then enter your two tape measurements. Your underbust measurement is taken with the tape snug around your ribcage directly beneath the bust, while standing relaxed and breathing normally. Your bust (overbust) measurement is taken with the tape loose around the fullest part of the bust, keeping the tape level all the way around.
Enter both numbers and your US band and cup size appear instantly, along with sister sizes — alternative band-and-cup combinations with roughly the same cup volume that are worth trying if your calculated size feels off.
How it works
If you entered centimeters, both measurements are converted to inches (÷ 2.54). Your band size is your underbust measurement rounded to the nearest even number — for example, 31.5″ becomes 32 and 32.4″ becomes 32.
Your cup size comes from the difference between your bust measurement and your band size: a 1″ difference is an A cup, 2″ a B cup, 3″ C, 4″ D, 5″ DD/E, 6″ DDD/F, 7″ G, 8″ H, 9″ I, and 10″ J. If the difference is half an inch or less, the band alone may be too loose and you may need a smaller band or a re-measure.
Sister sizes are computed by going up one band size (+2) and down one cup size, or down one band size (−2) and up one cup size — for example, 30D has sisters 32C and 28DD. This is the standard US sizing method used as a starting point; actual fit varies by brand and style.
Frequently asked questions
How do I measure for bra size?
Stand relaxed and breathe normally. With a soft measuring tape, measure snugly around your ribcage directly under your bust (underbust) — this sets your band size. Then measure loosely around the fullest part of your bust (overbust), keeping the tape level. Enter both numbers into the calculator above; it handles inches or centimeters.
What are sister sizes?
Sister sizes are different band-and-cup combinations that hold roughly the same cup volume. Going up one band size and down one cup size (or vice versa) gives an equivalent fit — for example, 30D, 32C, and 28DD are sister sizes. They are useful when your calculated size isn't available or the band feels too tight or loose.
Why does my size differ between brands?
There is no universal manufacturing standard for bra sizing. Band elasticity, cup depth, and shape vary between brands and even between styles from the same brand. Use this calculator's result as a starting point and try sister sizes when shopping; fit is what matters, not the label.
US vs UK bra sizing — what's different?
US and UK sizing are identical up to cup D (A, B, C, D). Above that, the systems diverge: US DD/E, DDD/F, G, H follow a different sequence than UK DD, E, F, FF, G, GG. This calculator uses the standard US method; check a brand's size chart when shopping internationally.
How often should I re-measure?
Re-measure whenever your weight changes noticeably, after pregnancy or breastfeeding, or about once a year. Body shape shifts over time, and many people wear the wrong size simply because they haven't re-measured in years.