Bra Size Calculator: Band, Cup, Done
This bra size calculator uses the underbust + bust formula tailors and lingerie fitters trust — not the deprecated +4-inch method. Enter your underbust (band) and your full bust circumference; the tool returns band, cup, and sister sizes in US, UK, and EU. Eight out of ten women wear the wrong bra size; this fixes it. Use the calculator below to get your exact fit in 30 seconds.
How it works
Measure yourself
Take your bust, waist, and hips with a soft tape. See the full measurement guide.
Enter the numbers
Type your measurements into the calculator above. Pick inches or cm — the tool converts both ways.
Get your size
Match the result against the brand's chart and order with confidence.
Bra cup formula — bust minus band
| Difference | US Cup | UK Cup |
|---|---|---|
| 1" | A | A |
| 2" | B | B |
| 3" | C | C |
| 4" | D | D |
| 5" | DD | DD |
| 6" | DDD / E | E |
| 7" | F | F |
| 8" | G | FF |
Frequently Asked Questions
How do I measure my band size?
Around your ribcage directly under the bust, snug, with a non-padded bra or no bra. Round to the nearest whole inch — that's your band number.
How is cup size calculated?
Bust minus band. 1 inch difference = A cup, 2 inches = B, 3 = C, 4 = D, 5 = DD/E, 6 = DDD/F, and so on. UK and US cup letters diverge above DD.
What's a sister size and when do I use one?
A sister size has the same cup volume on a different band. 34C = 32D = 36B. Use it when your usual size sells out, or when the band feels tight/loose but the cup fits.
Why do my bras stretch out so fast?
If your band rides up at the back after a few weeks, the band was too big to start. Always size down to the snugger band — the new bra will fit correctly until it stretches.
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