Bra Size Converter & Sister Size Calculator
Convert bra sizes across 8 international standards — US, UK, EU, FR, IT, AU, JP, KR. Enter your measurements or a known size. Includes sister size calculator and accurate cup divergence for US/UK/AU from DD onwards.
Measure → Size
How to Measure Your Bra Size
Accurate measurements give you the best starting point when shopping across international brands. Use a soft measuring tape and take measurements without wearing a bra or while wearing a non-padded bralette.
- 1
Measure your underbust (band)
Wrap the tape measure snugly around your ribcage, directly below your bust — right where the underwire would sit. Keep the tape parallel to the floor and breathe normally; do not pull too tight or leave slack. Round to the nearest whole centimeter. This is your underbust measurement and determines your band size.
- 2
Measure your bust (fullest point)
Wrap the tape measure loosely around the fullest part of your bust, usually across the nipple line. Keep the tape parallel to the floor. Do not compress the breast tissue — the tape should be touching but not pressing in. Round to the nearest whole centimeter.
- 3
Calculate the difference
Subtract the underbust from the bust measurement. This difference (in cm) determines your cup size. For example, if underbust = 75 cm and bust = 90 cm, the difference is 15 cm — corresponding to approximately cup E (EU), DDD (US), or E (UK). Enter both values in the calculator above to see all 8 systems at once.
- 4
Try sister sizes if needed
If your calculated size is unavailable or doesn't feel right, try a sister size: go down one band size and up one cup size for a tighter fit with similar cup volume, or go up one band size and down one cup size for a looser fit. Sister sizes are an approximation — cup volume is conserved in principle but the fit experience differs, so try before committing to a size.
Measure snugly around your ribcage directly below your bust. Keep the tape parallel to the floor.
Measure loosely around the fullest part of your bust. Keep the tape parallel to the floor.
Results
Enter your measurements or a known size above to see all 8 international standards.
How to Measure Your Bra Size
Accurate measurements give you the best starting point when shopping across international brands. Use a soft measuring tape and take measurements without wearing a bra or while wearing a non-padded bralette.
- 1
Measure your underbust (band)
Wrap the tape measure snugly around your ribcage, directly below your bust — right where the underwire would sit. Keep the tape parallel to the floor and breathe normally; do not pull too tight or leave slack. Round to the nearest whole centimeter. This is your underbust measurement and determines your band size.
- 2
Measure your bust (fullest point)
Wrap the tape measure loosely around the fullest part of your bust, usually across the nipple line. Keep the tape parallel to the floor. Do not compress the breast tissue — the tape should be touching but not pressing in. Round to the nearest whole centimeter.
- 3
Calculate the difference
Subtract the underbust from the bust measurement. This difference (in cm) determines your cup size. For example, if underbust = 75 cm and bust = 90 cm, the difference is 15 cm — corresponding to approximately cup E (EU), DDD (US), or E (UK). Enter both values in the calculator above to see all 8 systems at once.
- 4
Try sister sizes if needed
If your calculated size is unavailable or doesn't feel right, try a sister size: go down one band size and up one cup size for a tighter fit with similar cup volume, or go up one band size and down one cup size for a looser fit. Sister sizes are an approximation — cup volume is conserved in principle but the fit experience differs, so try before committing to a size.
- 5
A note on the old +4 method
Older US measurement guides (and some current ones) add 4 inches to the underbust to calculate band size. This method was developed for less elastic fabrics and is now considered outdated by most fitting experts. Modern bras use more elastic fabrics, and the current standard is to use the underbust measurement directly (rounding to the nearest even number for US/UK sizes). This calculator uses the modern direct method.
Bra Size Chart — All 8 Standards
Scroll horizontally on mobile. Data from Wikipedia 'Bra size' article, cross-validated with calcipedia.org, unittables.com, and bra-calculator.com.
Band Size Chart (Underbust → Band Numbers)
▾
| Underbust (cm) | EU | FR/BE/ES | IT | US / UK | AU/NZ | JP / KR |
|---|---|---|---|---|---|---|
| 60 | 60 | 75 | 0 | 28 | 6 | 60 |
| 65 | 65 | 80 | 1 | 30 | 8 | 65 |
| 70 | 70 | 85 | 2 | 32 | 10 | 70 |
| 75 | 75 | 90 | 3 | 34 | 12 | 75 |
| 80 | 80 | 95 | 4 | 36 | 14 | 80 |
| 85 | 85 | 100 | 5 | 38 | 16 | 85 |
| 90 | 90 | 105 | 6 | 40 | 18 | 90 |
| 95 | 95 | 110 | 7 | 42 | 20 | 95 |
| 100 | 100 | 115 | 8 | 44 | 22 | 100 |
Cup Size Chart (Bust Difference → Cup Letters)
▾
| Diff (cm) | US cup | UK cup | AU cup | EU cup | JP cup | KR cup |
|---|---|---|---|---|---|---|
| 2.5 | A | A | A | A | A | A |
| 5.1 | B | B | B | B | B | B |
| 7.6 | C | C | C | C | C | C |
| 10.2 | D | D | D | D | D | D |
| 12.7 | DD | DD | DD | E | E | E |
| 15.2 | DDD | E | E | F | F | F |
| 17.8 | DDDD | F | F | G | G | G |
| 20.3 | H | FF | G | H | H | H |
| 22.9 | I | G | H | I | — | — |
| 25.4 | J | GG | I | J | — | — |
| 27.9 | K | H | J | K | — | — |
| 30.5 | L | HH | K | — | — | — |
⚠ US, UK, and AU cup letters diverge from DD (≈12.7 cm difference) onwards. Do not assume US DD = UK DD beyond this point.
Data source: Wikipedia 'Bra size' article — the only publicly available source that mathematically aligns all 8 systems with citations. Cross-validated against 4 competitor sites.
Brand Size Variation
Bra sizing varies more between brands than almost any other garment category. Victoria's Secret, Aerie, Skims, and Uniqlo all use slightly different internal measurements for the same labeled size — sometimes by a full cup size. This converter provides international standard conversions based on Wikipedia's mathematical formulas. Brand-specific sizing should always be verified against the brand's official size guide before purchasing, especially for non-returnable or custom items.
The most common sources of brand variation are: (1) different underbust measurement rounding conventions (some brands round to nearest 2 inches, others to nearest even inch); (2) different cup progression systems — some brands use DD→DDD where others use DD→E; (3) variation in how much ease is built into the cup. When in doubt: try on, or refer to the brand's own size chart rather than a conversion table.
Understanding International Bra Sizing
Bra sizing is a two-dimensional system. Unlike shoe sizes (which map a single foot length to a number), a bra size is defined by two independent measurements: the underbust (which determines the band number) and the bust-to-underbust difference (which determines the cup letter). These two dimensions are converted separately and then combined.
The band number represents the underbust circumference. EU sizing simply rounds the underbust to the nearest 5 cm (e.g., 75 cm → EU 75). US and UK use the same number (both 34 for a 75 cm underbust), derived by the formula (EU − 10) ÷ 2.54 rounded to the nearest even number. French/Belgian/Spanish sizing adds 15 to the EU number (EU 75 → FR 90). Italian sizing uses its own scale: IT = (EU − 50) ÷ 5. Australian sizing follows (EU − 58) ÷ 2.54 rounded to the nearest even number. Japanese and Korean bands equal the EU number directly.
The cup letter represents the bust-to-underbust difference. This is where US, UK, and AU diverge significantly. All three use the same 1-inch (2.54 cm) step size, but their letter progressions differ: after D (4 inches difference), US uses DD/DDD/DDDD/H/I/J/K; UK uses DD/E/F/FF/G/GG/H/HH/J; and AU uses DD/E/F/G/H/I/J. This means US 34H = UK 34FF = AU 34G — completely different letters for the same physical measurement. The common mistake of treating US and UK cups as equivalent (US DD = UK DD) is only correct up to the 4-inch mark; beyond that, the letters diverge immediately.
EU/FR cup letters use a 2 cm step size instead of 2.54 cm, so they do not align perfectly with English-system cups. Japanese cups use a 2.5 cm step, starting from AA at 7.5 cm difference. Korean cups also use 2.5 cm steps, starting from AAA at 5 cm. The Japanese standard (JIS L4006) writes the cup letter before the band number — B75, not 75B — which is the opposite of all other major systems.
Data sources used in this converter: Wikipedia 'Bra size' article (the only source that mathematically defines all 8 systems with citations), cross-validated against calcipedia.org (Playwright-verified), unittables.com, and bra-calculator.com. IT cup letters are not fully defined in Wikipedia and are marked as pending a second authoritative source. All conversions are approximations — brand-level variation is significant, and the only reliable sizing method is trying on or consulting the brand's own chart.
Data Sources & Standards
All conversion data sourced from Wikipedia 'Bra size' (the only publicly available source mathematically aligning all systems) and cross-validated against 4 competitor sites.
All calculations run locally in your browser. No measurement data is sent to any server.
Frequently Asked Questions
- What is a sister size in bra sizing?
- A sister size is a bra size that shares approximately the same cup volume as your own size but with a different band size. For example, 34B, 32C, and 36A are sister sizes — they all have roughly the same cup volume. Moving to a smaller band (e.g., 34B → 32C) keeps the cup volume similar but gives a tighter band fit. Moving to a larger band (34B → 36A) keeps cup volume similar but gives a looser fit. Sister sizes are useful when your exact size is out of stock, but they are an approximation: cup volume is conserved in principle, but the overall fit experience changes because the band tightness affects uplift and support.
- How do I convert US bra size to UK?
- US and UK bra sizes share the same band number (US 34 = UK 34), but the cup letters differ beyond DD. For cup sizes up to D, US and UK are identical (A=A, B=B, C=C, D=D). From DD onwards, they diverge: US DD = UK DD, but US DDD = UK E, US DDDD = UK F, US H = UK FF, and so on. To convert, enter your US size in the converter above — it shows the correct UK cup alongside all other systems. The most common mistake is assuming US DDD = UK DDD, which is incorrect: UK does not use triple-D letters beyond DD.
- Is EU bra size the same as US?
- EU and US bra sizes are different in both band and cup. For bands: EU 75 = US 34 (formula: US = (EU − 10) ÷ 2.54, rounded to nearest even). For cups: EU uses a 2 cm step size while US uses a 2.54 cm (1 inch) step, so they do not align perfectly. Roughly: EU A ≈ US A, EU B ≈ US B, EU C ≈ US C, but the correspondence becomes approximate at larger cup sizes. Enter your EU size in the converter for exact results.
- What does 75B mean in Japanese bra sizing?
- In Japanese bra sizing (JIS L4006 standard), the cup letter comes before the band number — so B75 means cup B, band 75. The 75 refers to the underbust in centimeters (same as EU 75). The cups follow a 2.5 cm step starting from AA at 7.5 cm bust difference, so: AA, A, B, C, D, E, F, G. A Japanese B75 corresponds approximately to US 34B or UK 34B, though the cup measurement differs slightly due to the different step size (2.5 cm JP vs 2.54 cm US/UK).
- How do I measure my bra size at home?
- You need a soft measuring tape. First, measure your underbust: wrap the tape snugly around your ribcage directly below your bust and note the measurement in cm. Second, measure your bust: wrap the tape loosely around the fullest part of your chest (nipple line), keeping the tape level. The difference (bust − underbust in cm) determines your cup size. Enter both measurements in the 'Measure → Size' tab above to see your size in all 8 international standards instantly. For the most accurate result, measure without a bra or while wearing a non-padded bralette.
- Why do US and UK bra cup sizes differ?
- US and UK bra cup systems use the same 1-inch step size but different letter progressions beyond D. After D (4 inches difference), US uses the sequence DD, DDD, DDDD, H, I, J, K — adding letters before jumping to single letters. UK uses DD, E, F, FF, G, GG, H, HH, J — inserting double-letters for each step. This means US 34H = UK 34FF in terms of actual bust difference (8 inches / ~20 cm). The divergence starts at the 5-inch mark (DD), where both US and UK share the label DD. From 6 inches onwards, the letters are completely different. Australian (AU/NZ) cups form a third independent system: DD, E, F, G, H — no double letters except DD.
- What is the difference between calculated size and converted size?
- A calculated size comes from taking your actual body measurements (underbust and bust in cm) and computing which standard size they correspond to. A converted size starts with a known size label (e.g., US 34B) and converts it to another standard (e.g., EU 75B). Both methods in this tool ultimately go through the same conversion math — the difference is just the starting point. Calculated sizes may land between standard sizes, in which case the converter snaps to the nearest standard size and indicates this with a note. Converted sizes are exact matches within the data table.
- Do bra sizes vary by brand?
- Yes, significantly. Bra sizing varies more between brands than most other garment categories. Victoria's Secret, Aerie, Skims, and Uniqlo all use slightly different internal measurements for the same labeled size — sometimes by a full cup size. The formulas in this converter represent international standard sizing, not any single brand's interpretation. For a specific purchase, always verify against the brand's own size chart. The two most common brand-level differences are: (1) some US brands use DD→E instead of DD→DDD for cup progression; (2) some brands use the old '+4 inch' method for band sizing rather than the modern direct method.
- What is the old +4 bra measurement method?
- The '+4 method' was historically used in US bra fitting: take your underbust measurement in inches, add 4 (or 5 if the result is odd), and that gives your band size. For example, underbust 29 inches → 29 + 4 = 33 → round up to 34. This method was developed when bras used less elastic fabric and needed extra ease. Modern bras are made from stretch fabrics and fit closer to the body, making the +4 addition unnecessary. Most fitting experts today recommend the direct method: measure your underbust in cm and round to the nearest EU band (which is the same as the JP/KR band), or use the direct formula for US/UK bands. This calculator uses the modern direct method.
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