Sizing & fit · Free tool
Bra size calculator
Two tape measurements. No “+4 inches” myth, no email gate. Get your band, cup, US/UK/EU conversions, and both sister sizes — computed entirely on your device.
By Cluzy editors · Method verified August 30, 2026
Your measurements
Directly under your bust, tape snug and level. Exhale first.
Over the fullest part, tape relaxed — not tight. Lean forward 90° if braless.
Your size appears here
Enter both measurements above. All the math happens on your device — nothing is uploaded.
How to measure yourself
You need a soft measuring tape and about ninety seconds. Stand in front of a mirror so you can see the tape stays level around your back — a dipped tape is the number one source of bad results.
Bust — the fullest point
Wrap the tape around the fullest part of your bust, usually at nipple level. Keep it relaxed against the skin, not compressing. If you are braless, lean forward to 90° so everything falls into the tape’s path — upright braless measuring understates your cup.
Underbust — where the band sits
Wrap the tape directly under your bust, where the wire or band of a bra rests. Exhale, then pull the tape snug — firm against the ribcage without digging in. This number becomes your band size.
How the math works — and why we don’t add 4
Your band size is your snug underbust measurement rounded to the nearest even number. That’s it. The old advice to add 4 or 5 inches comes from the 1930s, when bands were cut from rigid, non-stretch fabric and needed slack to be wearable at all. Modern bands are elastic and engineered to anchor at your true ribcage measurement; adding inches is why so many bands ride up and shoulder straps end up doing work they were never meant to do.
Your cup size is the difference between your bust and your band: each whole inch is one cup letter. A 3-inch difference on a 34 band is a 34C; a 5-inch difference on a 30 band is a 30DD. The cup letter means nothing on its own — a 30DD and a 36C hold similar volume — which is exactly why the sister sizes above matter when a brand runs tight or loose.
Band & cup conversion charts
Shopping a European brand, a UK full-bust label, or a secondhand listing from another country? These two charts cover the conversions the calculator applies.
Band sizes
| US / UK | EU | FR / ES | IT | AU / NZ |
|---|---|---|---|---|
| 28 | 60 | 75 | 1 | 6 |
| 30 | 65 | 80 | 2 | 8 |
| 32 | 70 | 85 | 3 | 10 |
| 34 | 75 | 90 | 4 | 12 |
| 36 | 80 | 95 | 5 | 14 |
| 38 | 85 | 100 | 6 | 16 |
| 40 | 90 | 105 | 7 | 18 |
| 42 | 95 | 110 | 8 | 20 |
| 44 | 100 | 115 | 9 | 22 |
Cup sizes, by bust − band difference
| Difference | US | UK | EU |
|---|---|---|---|
| 1″ | A | A | A |
| 2″ | B | B | B |
| 3″ | C | C | C |
| 4″ | D | D | D |
| 5″ | DD | DD | E |
| 6″ | DDD | E | F |
| 7″ | G | F | G |
| 8″ | H | FF | H |
Buying bras secondhand or vintage
Lingerie is one of the best-value categories on Poshmark, Depop, and eBay — new-with-tags designer bras routinely sell for a third of retail. Two rules keep you from bad buys. First, trust measurements over labels: ask the seller for the band measured flat and stretched, and compare the stretched number to your underbust. Second, remember that vintage sizing often follows the old add-4 convention, so a “36” from the 1960s can fit like a modern 32. Your sister sizes from the calculator double your odds of finding a match in any listing feed.
Selling instead? Bras with accurate measurements in the listing sell dramatically faster. Check what your brand actually brings with our brand resale value guides, and net out platform fees with the Poshmark or Depop fee calculator.
Bra sizing FAQ
Do I add 4 or 5 inches to my underbust measurement?
No. The add-4 rule dates to the 1930s, when bras were sewn from rigid fabric with no stretch and the band needed slack to be wearable. Modern bands are elastic and are designed to be worn at roughly your actual underbust measurement. Adding inches is the single most common reason bands ride up and cups gape. This calculator uses your snug underbust measurement, rounded to the nearest even band size.
Why is my result different from the size I've always worn?
Around 8 in 10 people wear a band that is too big and a cup that is too small — usually because of the old add-4 method and vanity-sized store fittings. A 36C and a 32DD hold a similar cup volume, so the size you are used to may simply be your sister size with a stretched-out band. Try the calculated size on the loosest hook and judge by fit, not by the label.
How tight should the band be?
Firm. The band does about 80% of the supporting work, so it should sit level all the way around, parallel to the floor, and feel snug on the loosest hook when new — you should be able to slide two fingers under it, but not pull it more than an inch or two away from your body. If it rides up your back, it is too big.
What is the difference between US, UK, and EU cup sizes?
The letters match up to D, then diverge. After D, US brands typically run DD, DDD, G, H; UK brands run DD, E, F, FF, G, GG, H — the UK system uses double letters and is the standard for full-bust brands; EU brands run a plain E, F, G, H ladder on a centimeter-based band (70, 75, 80). The calculator shows all three so you can shop any brand or secondhand listing without converting by hand.
What is a sister size?
A size with the same cup volume on a different band: go down a band and up a cup (32D → 30DD), or up a band and down a cup (32D → 34C). Sister sizes matter constantly when you buy secondhand — if a listed bra is one band off from your size, the sister size tells you whether the cups will still fit.
Should I measure with or without a bra on?
Measure the underbust either way — it barely changes. For the bust, measure braless or in a thin non-padded bra, and lean forward to about 90 degrees so the tissue falls into the tape's path. Measuring upright in a padded bra can understate your cup by a full size or more.
Do vintage and thrifted bras fit like modern sizes?
Often not. Vintage pieces frequently follow the old add-4 convention (so a vintage 36 can fit like a modern 32) and elastic loses its recovery with age. When you thrift a bra or shop one on Poshmark or Depop, ask for the flat and stretched band measurements in inches and compare them to your underbust, rather than trusting the label size.
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Free to download · iOS & AndroidMethod notes
- r/ABraThatFits community fitting guide (modern measuring method)
- Bravissimo fitting room guide (UK full-bust sizing)
Method verified August 30, 2026. Sizing varies between brands and eras; treat any calculator as a starting point and judge the final call by fit. Your measurements never leave your browser.