Japanese bras are labelled with the cup letter before the band number: B70, C75, D80. The system, set out in the JIS bra size table, is very regular: the band moves in steps of 5 cm and the cup moves in steps of 2.5 cm of difference between top bust and underbust. This calculator takes your two measurements in centimetres and gives your Japanese size, together with the nearest UK, US and European sizes.
How to use the Japanese bra size calculator
- Measure the underbust (アンダーバスト) around the ribcage right under the breasts, level and firm.
- Measure the top bust (トップバスト) around the fullest part of the bust, level and not tight.
- Enter both in centimetres or inches.
- Read the size. The JIS size is written cup first. The conversions are approximate, because the scales differ.
Japanese bra size formula
cup step k = round((top bust − underbust) ÷ 2.5) − 2
cups: AAA = 5.0 cm, AA = 7.5, A = 10.0, B = 12.5, C = 15.0, D = 17.5, E = 20.0, F = 22.5, G = 25.0, H = 27.5, I = 30.0
The cup table is the JIS bra size table as printed by HugKum, confirmed by Oggi’s description (AA from 7.5 cm, then 2.5 cm steps, up to I at about 30 cm). The bands run 65, 70, 75 and up to 120. For conversion, each European cup is matched to the nearest JIS step by comparing the centre of its EN 13402 range with the JIS differences (retrieved 2026-10-15).
| JIS cup | Bust − underbust (cm) | Nearest EU cup (convert) |
|---|---|---|
| AAA | 5 | — |
| AA | 7.5 | — |
| A | 10 | AA |
| B | 12.5 | A |
| C | 15 | B |
| D | 17.5 | C |
| E | 20 | D |
| F | 22.5 | F |
| G | 25 | G |
| H | 27.5 | H |
| I | 30 | I |
| Band | Underbust (cm) |
|---|---|
| 65 | 62.5 – 67.5 |
| 70 | 67.5 – 72.5 |
| 75 | 72.5 – 77.5 |
| 80 | 77.5 – 82.5 |
| 85 | 82.5 – 87.5 |
| 90 | 87.5 – 92.5 |
| 95 | 92.5 – 97.5 |
| 100 | 97.5 – 102.5 |
| 105 | 102.5 – 107.5 |
| 110 | 107.5 – 112.5 |
| 115 | 112.5 – 117.5 |
| 120 | 117.5 – 122.5 |
Worked example
Underbust 70 cm, top bust 82 cm.
- Band: 70 cm is the 70 band exactly.
- Difference: 82 − 70 = 12 cm; 12 ÷ 2.5 = 4.8, rounded to 5, minus 2 = step 3, which is B.
- Japanese size: B70. Nearest UK 32A, US 32A, European 70A.
Using Japanese sizes
- Buying from Japan: check whether the shop lists JIS sizes or its own; Japanese brands often add their own size labels.
- Smaller cups: AAA and AA exist in Japan but have no equivalent in Europe or the US charts used here.
- Band tension: the Japanese bands are in centimetres but real fit depends on stretch, so try the neighbouring band.
- Cross-check against a Japanese brand’s measuring guide, because the exact steps are rounded.
To convert in the other direction use the Bra Size Converter (US/UK/EU/AU/FR/JP); for the European system see European Bra Size Calculator. This page is general information, not a professional fitting.
Sources
Every figure above comes from these pages, read directly on the dates shown. Brand names are used only to identify published charts.
- HugKum — JIS bra size table (cup by bust − underbust, bands 65–120) — checked 2026-10-15
- Oggi — how bra cup size is decided (AA at 7.5 cm, 2.5 cm steps) — checked 2026-10-15
- MobileFish — EN 13402 cup ranges (for the conversion) — checked 2026-10-15
- Fantasie — size guide (band conversion for Japan) — checked 2026-10-15
Frequently asked questions
How are Japanese bra sizes written?
The cup letter comes first, then the band: B70 is a B cup on a 70 band. The band is the underbust to the nearest 5 cm (the 70 band covers 67.5–72.5 cm) and the cup is the difference between top bust and underbust.
What are the Japanese cup sizes?
On the JIS table AAA is about 5.0 cm of difference, AA 7.5 cm, A 10.0 cm, B 12.5 cm, C 15.0 cm, D 17.5 cm, E 20.0 cm, and so on in steps of 2.5 cm to I at 30 cm.
Is a Japanese B the same as a European B?
No. The Japanese A already starts at a 10 cm difference and each letter adds 2.5 cm, whereas the European standard starts AA at 10–12 cm and adds 2 cm per letter, so a Japanese B is close to a European A. The converter shows the nearest match.
How do I convert Japanese to UK or US sizes?
The band is the same as the European band, so 70 is UK/US 32 and 75 is 34. For the cup, use the converter result and then try the neighbouring cup, because brands vary. See Bra Size Converter (US/UK/EU/AU/FR/JP).
Why do I see approximate values?
The JIS table marks cup differences as “about” (約). Brands round their own tables, so the calculator rounds to the nearest 2.5 cm step. Use the maker’s chart for a specific bra.