Bra Size Calculator
Enter your band (underbust) and bust measurements in inches to get your UK and US bra size — the two diverge above a D cup (UK DD, E, F, FF… vs US DD/E, DDD/F, G…) — plus your sister sizes and a step-by-step measuring guide.
36DDD/F US / 36E UK
Band 36 (EU 80), cup #6 of bust-band difference
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Above a D cup, US and UK letters diverge: US and UK match through D, then US uses DD/E, DDD/F, G, H, I… while UK uses DD, E, F, FF, G… — always check a brand's own size chart when buying UK lingerie brands in the US or vice versa.
Sister sizes (same cup volume, different band)
| Band | US cup | UK cup |
|---|---|---|
| 32 | H | FF |
| 34 | G | F |
| 38 | DD/E | DD |
| 40 | D | D |
How to measure (step by step)
- Wear an unpadded (or no) bra. Wrap a soft tape snug around your ribcage, directly under your bust, keeping it parallel to the floor — this is your band/underbust measurement.
- Wrap the tape around the fullest part of your bust (usually nipple level), parallel to the floor, snug but not compressing — this is your bust measurement.
- Band size: round underbust to the nearest whole inch; if it's even add 4, if odd add 5.
- Cup size: subtract the rounded underbust from the bust measurement; each inch of difference is one cup letter (1"=A, 2"=B, 3"=C, 4"=D…).
Method and US/UK letter divergence cross-checked against bra-calculator.com, TailorSizeGuide and The Lingerie Advisor. This is a fit estimate, not medical advice. How we calculate →
How to measure your bra size (band and bust)
Bra size comes from two measurements, both taken snug but not tight, with the tape parallel to the floor: band — wrapped directly under your bust, around your ribcage — and bust — wrapped around the fullest part of your bust, usually at nipple level. A 32" band with a 38" bust gives a 36DDD/F in US sizing (36E in UK sizing): the band rounds to an even number and adds 4 (32" stays even, so 32+4=36), and each inch of bust-band difference is one cup letter (38-32=6" → cup letter #6).
The calculator above runs this exact math for any two measurements, so you don't need to memorize the rounding rule (even underbust +4, odd underbust +5) or the cup-letter table yourself.
Why US and UK bra sizes match up to a D cup, then diverge
Band numbers are identical in US and UK sizing — a 34 band is a 34 band on both sides of the Atlantic. Cup letters also match through A, B, C and D. Above a D cup, though, the two systems split: US brands typically continue DD/E, DDD/F, G, H, I, while UK brands continue DD, E, F, FF, G. A US 36DDD/F is a UK 36E — same cup volume, different letter, because the two countries never standardized cup-letter progression past D.
This divergence is the single biggest source of confusion when buying lingerie across US and UK brands, and it's brand-dependent at the margins (a "G" from one UK brand and a "G" from a US brand are not the same size) — when in doubt, use the band+bust measurements above rather than converting a letter you already own.
Sister sizes: the same cup volume in a different band
A "sister size" is a bra with the same cup volume as yours but a different band, useful when your usual size is sold out or doesn't quite fit. The rule: go up one band size and down one cup letter, or down one band and up one cup letter. A 36DDD/F has the sister sizes 34G and 38DD/E — all three hold roughly the same cup volume, just distributed over a tighter or looser band.
Sister sizing works because cup volume is relative to band size, not an absolute measurement — a "D cup" on a small band is a much smaller cup in absolute terms than a "D cup" on a large band.
Frequently asked questions
How do I calculate my bra size from band and bust measurements?
Round your underbust (band) measurement to the nearest whole inch — if even, add 4; if odd, add 5 — for your band size. Then subtract that rounded underbust from your bust measurement; each inch of difference is one cup letter (1"=A, 2"=B, 3"=C, 4"=D, and so on). A 32" band with a 38" bust gives a 36DDD/F (US) / 36E (UK).
What is the difference between US and UK bra sizes?
Band numbers are identical, and cup letters match through D. Above D, they diverge: US commonly uses DD/E, DDD/F, G, H, I; UK uses DD, E, F, FF, G. A US DDD/F is a UK E for the same cup volume.
What are sister sizes in bras?
Bras with the same cup volume but a different band size. Move one band size up and one cup letter down (or the reverse) to find a sister size — useful when your size runs out or the fit is close but not quite right.
How much is a UK DD in US sizing?
A UK DD is roughly a US DD/E — US and UK match through D, and DD is one of the first letters where naming conventions start to differ between brands, so always check the specific brand's own chart at this cup range.
Do I measure with or without a bra on?
Measure wearing a well-fitted, unpadded (or no) bra for the most accurate band and bust numbers — a padded or ill-fitting bra can shift both measurements enough to change your calculated size by a full cup or band.
Why does my cup letter change depending on the brand?
Cup-letter naming is not standardized past a D cup, and even band-sizing rounding varies slightly by brand and country. Treat any calculated size (including this one) as your best starting point for trying on, not a guaranteed universal fit.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →