Bra Size Calculator
Find your bra size from two simple measurements. Get sister sizes, international conversions and quick fit tips — all calculated in your browser, nothing stored.
Your size
International sizes
Fit tips
- If the band rides up your back: try 30DD — snugger band, same cup volume.
- If the cups gap or wrinkle: try one cup size smaller in the same band.
- If the straps slip off your shoulders: the band is too loose, not the straps. try 30DD.
- If the cups overflow or quad-boob: try 34C or one cup size larger.
Calculations happen in your browser. Nothing is sent or stored.
How to measure
Two measurements get you to a bra size. Take both in front of a mirror, wearing a non-padded bra or no bra. A soft cloth tape measure works best — a stiff metal one bites in and gives you a smaller number.
- Find your band. Wrap the tape around your ribcage just below the bust. Keep it snug and parallel to the floor. Breathe out and record the number.
- Find your bust. Stand straight, arms at your sides. Wrap the tape around the fullest part of your bust. Keep it level — let it drape, don't pull tight.
- Enter both numbers above. Switch the toggle between inches and centimeters depending on what your tape measure shows. The result updates live.
- Try on the sister sizes too. Sizing varies by brand and cut. If the primary number doesn't feel right, your sister sizes have the same cup volume on a different band — often the better real-world fit.
How bra sizing is calculated
A bra size is a band number plus a cup letter. The band is your underbust in inches, rounded to the nearest even number (US standard — 28, 30, 32, 34, 36, 38, 40 …). The cup letter comes from the difference between your bust and band: every full inch of difference is one cup letter.
band = round(underbust_inches / 2) × 2
cup = round(bust_inches − band) → 1=A, 2=B, 3=C, 4=D, 5=DD, 6=DDD, 7=G, 8=HSister sizes share the same cup volume but ride a different band. Because the cup grows with the band, a 34C and a 36B hold the same amount — the band is just two inches bigger and the cup letter drops one step. Going the other way, 32D is also a sister of 34C.
International size chart
Band conversions across the six biggest systems. Find your US/UK band on the left and read across to your local size. Cup letters convert independently — see the cup table below.
| US | UK | EU | AU | JP | IN |
|---|---|---|---|---|---|
| 28 | 28 | 60 | 6 | 65 | 28 |
| 30 | 30 | 65 | 8 | 70 | 30 |
| 32 | 32 | 70 | 10 | 75 | 32 |
| 34 | 34 | 75 | 12 | 80 | 34 |
| 36 | 36 | 80 | 14 | 85 | 36 |
| 38 | 38 | 85 | 16 | 90 | 38 |
| 40 | 40 | 90 | 18 | 95 | 40 |
| 42 | 42 | 95 | 20 | 100 | 42 |
| 44 | 44 | 100 | 22 | 105 | 44 |
| 46 | 46 | 105 | 24 | 110 | 46 |
Cup letters by difference (bust − band, in inches):
| Diff | US | UK | EU | AU | JP | IN |
|---|---|---|---|---|---|---|
| 0" | AA | AA | AA | AA | AAA | AA |
| 1" | A | A | A | A | AA | A |
| 2" | B | B | B | B | A | B |
| 3" | C | C | C | C | B | C |
| 4" | D | D | D | D | C | D |
| 5" | DD | DD | E | DD | D | DD |
| 6" | DDD | E | F | E | E | E |
| 7" | G | F | G | F | F | F |
| 8" | H | FF | H | FF | G | FF |
| 9" | I | G | I | G | H | G |
| 10" | J | GG | J | GG | I | GG |
| 11" | K | H | K | H | J | H |
FAQ
How do I calculate my bra size?
Measure your underbust (just below the breasts) and your bust (at the fullest point) with a soft tape measure. Round the underbust to the nearest even number — that's your band. Subtract the band from the bust: every inch of difference is one cup letter (1" = A, 2" = B, 3" = C, 4" = D, 5" = DD).
Are 36C and 34C the same?
No. They share the same cup letter but different band sizes mean different cup volumes. 36C has a larger cup than 34C because the cup gets proportionally larger as the band grows. The actual sister size of 34C is 36B — same volume, different band.
What are sister sizes?
Sister sizes share the same cup volume but ride a different band. If 34C feels right in the cup but wrong in the band, try 32D (snugger band) or 36B (looser band). The cup letter shifts inversely with the band by one letter for every two inches.
Why doesn't my bra fit?
The band does about 80% of the support. If the band rides up your back, it's too loose — try a smaller band (your sister down). Cups gapping means the cup is too big; try one cup smaller. Straps digging in usually points to a too-loose band, not a strap problem.
Should I round up or down for half-inches?
For the band, round to the nearest even number — that's the US standard. For the cup, round normally (0.5 rounds up). If you're between sizes, the calculator picks the closer one; if the fit is off, try the sister size.
Is this calculator accurate?
It uses the standard band + cup formula and the most commonly published international conversion charts. Real bras vary by brand and style, so treat the result as a starting point. For a perfect fit, a professional fitting at a lingerie store is still the gold standard.
Privacy
Every calculation on this page runs entirely in your browser using JavaScript. Your measurements never leave your device — they are not transmitted to any server, not stored in cookies, and not logged. Reload the page and everything resets. The only thing this site remembers is whether you prefer inches or centimeters, kept locally in your browser so you don't have to toggle it every visit.