Bra Size Calculator
Calculate your bra size from band and bust measurements in inches or cm.
Returns US, UK, EU, French, and Australian sizes with sister size suggestions.
Bra sizing uses two measurements, band size and cup size, to find a correctly fitting bra. You have probably seen the claim that 80% of women wear the wrong size. It gets repeated everywhere and it traces back to surveys run by lingerie retailers, so treat the number as marketing rather than research. What professional fitters do report consistently is the direction of the error: most people who are wearing the wrong size are in a band too loose and a cup too small, which is exactly what the old +4 rule below produces.
The Measurement Formula:
Step 1: Band Size Measure your ribcage directly under the bust, in inches, and round to the nearest even number. A 33" underbust becomes a 34 band. A 34" underbust stays a 34.
That is the whole step, and it is worth saying plainly because so many charts get it wrong. If a size guide tells you to add 4 inches to an even measurement and 5 to an odd one, that is the method US department stores used until roughly 2000. It hands you a band two sizes too loose, and since the cup is measured down from the band, a loose band also swallows the cup: a 34" underbust with a 38" bust comes out as 38AA under that rule instead of the 34D it actually is. UK fitters abandoned the +4 rule decades ago and most US brands have followed.
Step 2: Cup Size Bust measurement (fullest point) minus Band size = Cup difference
| Difference (inches) | Cup Size |
|---|---|
| under 1" | AA |
| 1" | A |
| 2" | B |
| 3" | C |
| 4" | D |
| 5" | DD / E |
| 6" | DDD / F |
| 7" | G |
Worked Example:
- Under-bust measurement: 34 inches → Band size = 34
- Bust measurement: 38 inches
- Difference: 38 − 34 = 4 inches → Cup size = D
- Result: 34D
Sister Sizing: Going up one band size and down one cup volume gives the same cup volume:
- 34D = 36C = 32DD (all hold the same volume of breast tissue)
Use sister sizing when your exact size is unavailable.
Practical Tips:
- Measure without a bra, or with a non-padded bra
- The band should be snug on the loosest hook when new, because it will stretch and you work your way to the tighter hooks
- Underwire should lie flat against the ribcage, not sit on breast tissue
- Cup spillage = go up a cup size. Empty cup = go down a cup size
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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