Bra Size Calculator
Two measurements, in inches or centimetres. You get your band and cup, your sister sizes, the UK, US and EU labels — and the actual maths, so you can work it out yourself next time.
Here is the whole method before you start, because it is genuinely simple: your band is your ribcage measurement, and your cup is the difference between your bust and your band — roughly one inch of difference per cup size. That is it. Nothing gets added, nothing gets padded out.
How to measure, in about five minutes
You need a soft tape measure, a mirror, and a thin unpadded bra or none at all. A thick padded bra adds inches that are not yours. Stand upright and stay relaxed — hunching or holding your breath both throw the reading off.
- Ribcage. Wrap the tape directly under your bust, where the band of a bra sits. Keep it level all the way round — use the mirror to check it is not riding up at the back — and pull it snug, because a real band is firm. Breathe out and read it.
- Bust. Wrap the tape around the fullest part of your bust, usually across the nipple line, parallel to the floor. This one rests lightly. You are capturing volume, not squeezing it.
- Enter both. The calculator does the rest and shows its working.
One warning about a rule you will still see everywhere: do not add four inches to your ribcage measurement. That habit comes from a stiff-fabric era and it puts most women in a band that is far too big — which is the single most common fit mistake there is. Measure snug, use the number. The full measuring guide covers the small mistakes that skew each reading.
What this calculator cannot tell you
Most size calculators online hand you a label and stop, as though the matter is settled. It is not, and pretending otherwise is how women end up trusting a number over their own mirror. So here is the honest limit of this tool:
- It knows two numbers and nothing else. It cannot see your breast shape, how full you are at the top or bottom, how firm or soft the tissue is, or how wide your breast root sits on your ribcage. Two women with identical tape measurements often need different cups.
- Your size is really your sizes. Cup volume is relative to the band, and brands cut differently from each other and from the UK, US and EU charts — especially above a D cup. The same body is honestly three or four different labels depending on the shelf.
- The try-on is what settles it. Band level and snug, cups smooth with no spillage and no gaping, centre gore flat against your breastbone, straps steadying rather than carrying. Those five checks beat any calculation, including this one.
Fit before size — always
If the number here and the bra on your body disagree, the body wins. A calculator gets you close so you know which sizes to pull off the rail. It is a starting point, not a verdict, and it is never a judgement on your body. Most fit problems turn out to be fit problems — it is the bra, not you.
Where to go next
- How should a bra fit?The five checks that tell you whether the size you just calculated actually fits. Start here.
- How to measure at homeBoth measurements step by step, plus the small mistakes that quietly skew your numbers.
- What is sister sizing?Why a 34C, a 32D and a 36B hold the same cup — and when to swap between them.
- Parts of a braBand, cup, gore, wings, cradle — every piece named, so the fit advice makes sense.
Bra size calculator questions
How accurate is a bra size calculator?
It is accurate at arithmetic and blind to everything else. A calculator turns two tape measurements into a label, which gets most women close. It cannot see breast shape, how firm or soft your tissue is, how wide your root is, or how the brand in front of you cuts its cups. Treat the answer as the size to try on first, then judge the bra by how it fits: band level and snug, cups smooth, centre gore flat.
Should I add 4 inches to my underbust measurement?
No. The old "+4 inches" rule dates from an era of stiff, non-stretch band fabric, and on modern bands it lands most women in a band that is too big. Measure your ribcage snugly and use that number, rounded to the nearest even size. A loose band is the single most common fit mistake, and it is what pushes the supporting work onto your straps.
What are sister sizes and why does the calculator show them?
Cup volume is relative to the band, so a 34C and a 32D hold roughly the same amount — what changes is how snug the band feels. Those are sister sizes. They matter twice over: when your size is sold out, and when the cup feels right but the band does not. Going down a band and up a cup keeps the volume and tightens the band.
Is my UK bra size the same as my US or EU size?
Up to a D cup, band numbers and cups usually match. Above a D they split: UK doubles letters (DD, E, F, FF, G), the US prints DDD or a fourth D, and EU runs one unbroken alphabet where EU E equals UK DD. Brands then vary within each system. So the same body can be three different labels on three shelves. Use the conversion as a starting point and check the brand chart.
What if my measurement falls between two band sizes?
Start with the tighter band. A new bra should fasten comfortably on its loosest hook, because band elastic relaxes over months of wear — so a band that is already loose on the loosest hook has nowhere to go. If the tighter band will not fasten on the loosest hook at all, step up and take the sister size in the cup.
How often should I re-measure?
Roughly every 6 to 12 months, and any time your body changes: weight loss or gain, pregnancy and nursing, menopause, or conditions like PCOS or thyroid that shift your size. Most women change size more often than they expect, which is one reason so many end up in the wrong one.