This bra size calculator finds your size from two tape measurements — snug underbust and full bust — and gives US, UK and EU sizes plus your sister sizes, the alternative fits most fitting guides never mention.
Fit checks that outrank any calculator: the band does 80% of the support (it should sit level and snug on the loosest hook when new), straps shouldn’t dig or slip, and the center gore should roughly tack. Studies and fitters alike find most people wearing a band too big and cups too small — this measurement method (band ≈ underbust rounded to even) corrects the old “+4 inches” habit that caused it.
Band from Underbust, Cup from the Difference
The modern method: band size = underbust rounded to the nearest even number; cup = full bust − band, at 1 inch per letter (1″ A, 2″ B, 3″ C, 4″ D, 5″ DD…). The antique “+4” rule (adding 4 inches to underbust) survives in some brands and is the single biggest source of wrong sizes in drawers worldwide. International labels differ only in letters beyond D and band units (EU bands run 60, 65, 70…). Part of our Everyday Life Calculators collection.
Worked Example (Verified)
- Underbust 33″, bust 37″ (defaults): band 34 · difference 3 → 34C (US = UK here), EU 75C; sister sizes 32D and 36B
- The +4-method comparison: 33 + 4 → band 38 with a negative-ish cup difference → the “38A-ish” poor fit that sends people to fitters; measurement systems differ, bodies don’t
- Sister sizes explained: 32D, 34C and 36B hold roughly the same cup volume on different bands — the swap to make when a band runs loose/tight but the cups are right
- Between sizes is the default state of humans: try the calculated size and both sisters before concluding anything
Fitting Wisdom
- Measure in a thin, unpadded bra (or none), tape snug for underbust and relaxed for bust, exhale normally for both.
- Sizes shift with weight change, hormones, pregnancy, training — remeasure yearly; a size is a snapshot, not an identity.
- Brand variance is real: the calculator gives the measurement-true size; individual labels drift by up to a size around it.
- Asymmetry is normal (most people, up to a cup) — fit the larger side and pad or adjust the strap on the smaller.
Related Tools
- Shoe Size Converter — the other measurement-to-label puzzle
- Ring Size Converter — same idea, smaller circle
- Everyday Life Calculators — the full collection
Frequently Asked Questions
How do I measure my bra size?
Two measurements: underbust (snug tape directly under the bust, level all around) and full bust (tape around the fullest point without compressing). Band = underbust rounded to an even number; cup letter = bust minus band in inches (1″ = A, 2″ = B, 3″ = C, 4″ = D, 5″ = DD). 33″/37″ gives 34C.
What are sister sizes?
Sizes with the same cup volume on different bands: 34C’s sisters are 32D (smaller band, bigger letter) and 36B. Use them when the cups fit but the band doesn’t — down a band needs up a cup letter to keep the same volume, and vice versa.
Why do different calculators give me different sizes?
Two methods circulate: the modern one (band = underbust rounded even) and the legacy “+4 inches” method, which many older brands still build to. They can differ by a full band and cup. Fit signals — band snugness, gore tack, no spillage or gaping — arbitrate better than any single system.
US vs UK vs EU sizes — what’s different?
US and UK agree up to D, then diverge in letters (US DD/DDD/G vs UK DD/E/F/FF…). EU bands are centimeters in steps of 5 (34 band = 75). This calculator shows all three so a label’s origin stops mattering.
How often should I remeasure?
Yearly, and after weight changes of ~10 lb, pregnancy, or new training regimes. Most fit problems blamed on “the bra” are measurements from three life-chapters ago.
Is this bra size calculator free?
Yes — free, instant, no sign-up. A starting point for fitting; a good fitter remains the gold standard for tricky fits.