Bra Size Calculator
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About Bra Sizing
Bra sizing looks like one number and a letter but is really two independent measurements — and misunderstanding their relationship is why most fit problems happen. The band provides ~80% of support (not the straps), and cup letters mean nothing without their band: a 32D and a 36D are entirely different volumes.
Take two tape measurements and the calculator returns your US size, the working, and your sister sizes for cross-brand adjustment. Treat the result as a fitting starting point — cut, style (plunge vs full coverage), and brand origin (UK and EU cups diverge past D) all shift the right choice by a size.
Tracking body measurements more broadly? Start with the BMI Calculator
The Band-and-Cup Method
Two measurements, one subtraction:
Band = underbust rounded to the nearest even inch Cup = bust − band: 1″=A · 2″=B · 3″=C · 4″=D · 5″=DD · 6″=DDD/F · 7″=G Sister sizes: band −2 → cup +1 · band +2 → cup −1
Worked example: underbust 31″ → band 32; bust 36″ → difference 4″ → D cup: 32D, with sister sizes 30DD and 34C. Note this is the modern measure-true convention — the old “add 4 inches to the band” method dates from stretchless fabrics and produces the loose bands behind most bad fits.
Cup Size Chart
Bust-minus-band difference to US cup letter:
| Difference | US cup | Difference | US cup |
|---|---|---|---|
| 0″ | AA | 4″ | D |
| 1″ | A | 5″ | DD |
| 2″ | B | 6″ | DDD (F) |
| 3″ | C | 7″ | G |
Past D, lettering fragments by region: US DDD = UK E territory, and EU cups run their own ladder — one more reason the tape measurements matter more than the letter on the tag.
How to Measure Properly
The two tape positions, done right:
- Underbust: directly under the bust, tape snug and level all the way around — this is where the band sits and it should feel firm, not gentle.
- Bust: at the fullest point, tape level, in an unpadded bra or none — padding inflates the cup math.
- Measure standing; exhale normally for the underbust. Re-measure every year or after weight changes — sizes drift.
- Fit check on the body: band level and snug on the LOOSEST hook (it stretches with wear), no spillover or gaping cup, straps doing almost no lifting.
- Between sizes or between brands: that's what the sister sizes are for — same cup volume, different band.
If the calculator's size and the mirror disagree, trust the fit checks — sizing is a starting point, the body is the verdict.
Frequently Asked Questions
How do I calculate my bra size?
Measure snugly under the bust (band: round to the nearest even inch) and around the fullest point (bust). Each inch of difference is one cup letter: 31″ under and 36″ bust gives 32D. Enter both above for the working and your sister sizes.
What are sister sizes?
Sizes with the same cup volume on different bands: 32D = 30DD = 34C. If a brand's band feels tight, go band up and cup down; if the band rides loose, band down and cup up. It's the single most useful sizing trick when switching brands.
Do I add 4 inches to my underbust measurement?
No — that's the legacy method from the era of non-stretch fabrics, and it's why so many bands fit loose. Modern sizing rounds your actual underbust to the nearest even number. If a true-measure band feels shockingly snug at first, that's what correct support feels like.
Why does my size differ between brands?
Cut, fabric stretch, and regional sizing ladders all differ — and past a D cup, US/UK/EU letters genuinely diverge. Keep your two tape measurements as the constant and use sister sizes to translate per brand; many brands publish their own charts against measurements.
How do I know if my band is the wrong size?
A band that rides up in back, straps that dig while lifting, or a bra that only feels supportive on the tightest hook — all say the band is too big. The band should sit level, snug on the loosest hook, and stay put when you raise your arms.
How often should I re-measure?
About once a year, and after weight change, pregnancy, or when a reliable bra starts fitting differently. Bands also stretch with wear — a bra that fit on the loosest hook when new should end its life on the tightest one.
Methodology. This calculator uses formulas and health categories recognized by the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC). It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 11, 2026
Disclaimer. This tool provides estimates for general informational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before making decisions about your health.