A bra size has two parts: a band number that fits around your ribcage and a cup letter that says how much fuller the bust is than the band. This calculator takes the two measurements that matter, your underbust and your bust, and gives you the size in US, UK, European and Australian systems together with the two sister sizes that hold the same cup volume. Everything is calculated in your browser from the formula shown below.
How to use the bra size calculator
- Measure your underbust. Stand straight, breathe out, and wrap a soft tape directly under the bust where the band of a bra sits. Keep it level at the back and snug enough that it does not slip, but not digging in.
- Measure your bust. Wrap the tape around the fullest part of the chest, level across the back, without squashing the tissue. Wear an unpadded bra or none.
- Enter both numbers in inches or centimetres; each box has its own unit menu.
- Read your size in the four systems and try the two sister sizes if the first fit is not quite right.
Bra size formula
cup number = round(bust − band), in inches (0 = AA, 1 = A, 2 = B …)
sister sizes = (band − 2, cup + 1) and (band + 2, cup − 1)
This is the inch method used for US and UK sizing and described on Wikipedia’s bra measurement page (retrieved 2026-10-16). Centimetre input is converted to inches first. The table lists the cup letter for each inch of difference in the four systems; it is computed from the same cup scales the UK and EU pages use.
| Bust − band (in) | US | UK | EU / FR letter | Australia |
|---|---|---|---|---|
| under 1 | AA | AA | AA | AA |
| 1 | A | A | A | A |
| 2 | B | B | B | B |
| 3 | C | C | C | C |
| 4 | D | D | D | D |
| 5 | DD/E | DD | E | DD |
| 6 | DDD/F | E | F | E |
| 7 | G | F | G | F |
| 8 | H | FF | H | G |
| 9 | I | G | I | H |
| 10 | J | GG | J | I |
The band converts as follows: the Australian band is the UK band minus 22, and the European band in centimetres is 60 + 2.5 × (band − 28), following the Brava and Fantasie charts. Band in centimetres in the last column is the band in inches × 2.54, rounded.
| US / UK band | Australia | Europe | Band (cm, approx.) |
|---|---|---|---|
| 28 | 6 | 60 | 71 |
| 30 | 8 | 65 | 76 |
| 32 | 10 | 70 | 81 |
| 34 | 12 | 75 | 86 |
| 36 | 14 | 80 | 91 |
| 38 | 16 | 85 | 97 |
| 40 | 18 | 90 | 102 |
| 42 | 20 | 95 | 107 |
| 44 | 22 | 100 | 112 |
Worked example
Underbust 32 in, bust 38 in.
- Band: 32 is already even, so the band is 32.
- Difference: 38 − 32 = 6, which is cup DDD/F in the US and E in the UK.
- Sizes: US 32DDD/F, UK 32E, Australia 10E, Europe 70F.
- Sisters: one band down and one cup up is 30G; one band up and one cup down is 34DD/E.
Five more examples from the same rule:
| Underbust | Bust | Cup number | US | UK | EU | Sister sizes (US) |
|---|---|---|---|---|---|---|
| 30 in | 34 in | 4 | 30D | 30D | 65D | 28DD/E, 32C |
| 32 in | 37 in | 5 | 32DD/E | 32DD | 70E | 30DDD/F, 34D |
| 34 in | 40 in | 6 | 34DDD/F | 34E | 75F | 32G, 36DD/E |
| 36 in | 43 in | 7 | 36G | 36F | 80G | 34H, 38DDD/F |
| 38 in | 47 in | 9 | 38I | 38G | 85I | 36J, 40H |
If your size does not feel right
- Band rides up the back: it is too loose. Go down a band and up a cup, which is the sister size below.
- Cups gap or wrinkle: the cup is too big, or the shape does not suit you; try a smaller letter or another style.
- Breast spills over the top or sides: go up a cup letter.
- Underwires sit on breast tissue: the cup is too small, or the band too tight for your shape.
The tape measure gives a starting point, not a verdict: brands cut differently and shape, firmness and spacing all matter. For the European centimetre method use the Cup Size Calculator or the European Bra Size Calculator, for a plain band use the Band Size Calculator, and for in-between sizes the Sister Size Calculator. The Bra Size Converter (US/UK/EU/AU/FR/JP) translates a size you already know. This page is general information, not a professional fitting.
Sources
The inch method, the US, UK and Australian cup scales and the sister-size rule come from the pages below; the band conversions follow brand charts that state them.
- Wikipedia — Brassiere measurement (underbust method, cup scales, sister sizes) — checked 2026-10-16
- Brava Lingerie — bra size chart (Australian and UK bands) — checked 2026-10-15
Frequently asked questions
How do I calculate my bra size?
Measure snugly under the bust and round to the nearest even inch for the band. Measure the fullest part of the bust, subtract the band, and read the cup letter from the difference: 1 inch is A, 2 is B, 3 is C and so on.
Why does my size differ between brands and countries?
US, UK, European and Australian systems use different cup letters after D, and every brand has its own fit model. Use this result as a starting point and check the brand chart, as the Bra Size Converter (US/UK/EU/AU/FR/JP) explains.
What is a sister size?
A sister size has the same cup volume on a different band: one band down and one cup up, or one band up and one cup down. 34C, 32D and 36B are sisters. The Sister Size Calculator shows the volume.
Should I measure with or without a bra?
Use no bra or a thin unpadded one, standing up with the tape level. Padding and underwires add to the number and push the result up.
What if my cup number is outside the chart?
The calculator says so rather than guessing. Very small or very large differences are covered by specialist ranges, so ask a fitter or check a specialist brand chart.