Bra Size Calculator
Estimate band and cup size from your underbust and bust measurements.
Results update as you type.
About this calculator
A bra size calculator estimates band and cup size from two measurements: the underbust (snug around the ribcage just under the bust) and the bust (around the fullest part). The band size is the underbust rounded to the nearest even number of inches, and the cup is set by the difference between the bust and band measurements — roughly one cup letter per inch of difference (1 in ≈ A, 2 in ≈ B, 3 in ≈ C, 4 in ≈ D, and up through DD, DDD/F and beyond).
Worked example: an underbust of 32 inches rounds to a band of 32, and a bust of 36 inches gives a difference of 36 − 32 = 4 inches, which is a D cup — an estimated 32D. Measuring in centimetres works too; the calculator converts to inches first (1 in = 2.54 cm).
Results are an estimate and a starting point only. Cup and band labelling differs between brands and countries, and “sister sizes” (like 34C and 32D, which share a cup volume) can fit similarly, so always try styles on before buying.
Frequently asked questions
How do I measure for a bra size?
Measure snugly around your ribcage directly under the bust for the underbust, then measure around the fullest part of the bust. Enter both and the calculator estimates the band from the underbust and the cup from the difference.
Why do bra sizes differ between brands?
Cup and band grading is not fully standardised, and a 34C in one label can fit like a 34D in another (sister sizing). Use this estimate as a starting point and try styles on.
How is cup size worked out from my measurements?
Subtract your band measurement from your bust measurement: each inch of difference is roughly one cup up. A 1-inch difference is about an A, 2 inches a B, 3 a C, 4 a D, then DD, DDD/F and so on. The calculator rounds the difference to the nearest inch.
What are sister sizes?
Sister sizes share the same cup volume across adjacent band sizes — a 34C, 32D and 36B hold a similar cup. If your calculated band feels too tight or loose but the cup is right, trying a sister size (down a band and up a cup, or vice versa) often fits better.
Can I measure in centimetres?
Yes — switch the units to centimetres and enter both measurements in cm. The calculator converts them to inches internally (dividing by 2.54) before working out the band and cup, so the estimate is the same either way.
Why did my band size round to an even number?
Band sizes are made in even numbers (30, 32, 34…), so the calculator rounds your underbust measurement to the nearest even inch. If you measure an odd number like 33 it rounds up to a 34 band; if that feels loose, a sister size on the next band down may fit better.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/bra-size/curl
curl "https://calculator.free/api/v1/bra-size/?units=imperial&underbust=32&bust=36"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/bra-size/?" + new URLSearchParams({
"units": "imperial",
"underbust": "32",
"bust": "36"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.