Enter your underbust and bust and get your size in Australian, UK, US, European and French sizing — plus your sister sizes.
Tip: Measure without padding, standing straight. Underbust = directly under the bust, pulled snug but not tight.
| System | Band | Cup | Full size |
|---|
Convert a size you already wear, without measuring.
Written and reviewed by Mohsin Iqbal · Last reviewed 4 August 2026 · Sizing conventions cross-checked against published international conversion charts
You need two numbers: your underbust and your bust. The band comes from the first, the cup from the difference between them.
| Measurement | Where | How tight |
|---|---|---|
| Underbust | Directly beneath your bust, around the ribcage | Snug and level. Exhale, then read |
| Bust | Across the fullest part | Loose — the tape should not compress |
Measure in a non-padded bra, or none at all. A padded bra adds two to four centimetres to the bust reading, which is enough to shift the cup by a letter or two. Keep the tape parallel to the floor all the way around, arms relaxed at your sides, and stand normally rather than holding a posture.
Band = underbust, rounded to the nearest even inch
Cup = bust − underbust, one letter per inch of difference
The underbust sits directly beneath the bust and should be snug. The bust is taken across the fullest part and should not compress. Both stay parallel to the floor.
| Step | Working | Result |
|---|---|---|
| Underbust | 76 cm ÷ 2.54 | 29.9 in |
| Band | rounded to the nearest even | 30 |
| Bust | 92 cm ÷ 2.54 | 36.2 in |
| Difference | 36.2 − 29.9 | 6.3 in |
| Cup | 6 inches of difference | E (UK) |
| Size | — | UK 30E · AU 8E · US 30DDD · EU 65F |
Band numbers and cup letters both change between countries, and neither changes in a way you can guess.
| System | Band is | Example band | Cup letters |
|---|---|---|---|
| Australia | A small even scale | 12 | Same as UK |
| United Kingdom | Underbust in inches | 34 | A B C D DD E F FF G GG |
| United States | Underbust in inches | 34 | A B C D DD DDD G H I |
| Europe / Japan | Centimetres, to the nearest 5 | 75 | A B C D E F G H (single letters) |
| France / Spain | The EU number plus 15 | 90 | Same as EU |
Reading across: AU 12 = UK 34 = US 34 = EU 75 = FR 90. All five describe the same band.
The cup letters are where it goes wrong. They match through D and then diverge. A UK DD is a US DD, but a UK E is a US DDD, and a UK F is a US G. Assuming the next letter across is the same size is the single most common conversion error, and it grows worse the larger the cup.
A sister size holds roughly the same cup volume on a different band. Go down a band and up a cup, or up a band and down a cup.
| If you wear | Sister size down | Sister size up |
|---|---|---|
| 32C | 30D | 34B |
| 34D | 32DD | 36C |
| 36DD | 34E (UK) | 38D |
| 30E (UK) | 28F | 32DD |
This is genuinely useful when a bra is almost right. If the cups fit but the band rides up, try the sister size down — a smaller band with a larger cup letter. If the band is comfortable but the cups gape or overflow, change only the cup.
| What you notice | Usually means |
|---|---|
| The band rides up at the back | Band too loose — go down a band, up a cup |
| Straps dig into your shoulders | The band is not carrying the weight; usually too loose |
| The underwire sits on breast tissue | Cup too small |
| The centre panel does not lie flat | Cup too small, or the wrong shape for you |
| Cups gape or wrinkle | Cup too large, or the wrong style |
| You are on the tightest hook already | Band too loose — a new bra should fit on the loosest |
Most of the bra’s support should come from the band, not the straps. If your straps are doing the work, the band is usually the thing to change.
A calculator turns two measurements into a starting size. It cannot account for breast shape, tissue firmness, how a particular style is cut, or the fact that brands differ — sometimes by a full size, in the same country.
Treat the result as the size to try first, not the answer. Sizes also change: weight change, pregnancy, breastfeeding, hormonal changes and simple ageing all shift measurements, which is why re-measuring every six to twelve months is worth doing rather than assuming a size from years ago still applies.
If you consistently cannot find a comfortable fit, a professional fitting is worth more than any online tool. Fitters see how a bra actually sits on you, which no set of numbers can.
The band and cup tables on this page were cross-checked on 4 August 2026 against published international conversion charts. The conventions they establish are:
There is no single global standard for bra sizing. The closest thing is the European standard EN 13402, which is voluntary rather than mandatory, and which defines bra sizes from underbust and bust girth in centimetres — the basis of the EU numbering used above. Its body measurement definitions were superseded by EN ISO 8559-1:2020. National systems in the UK, US and Australia developed separately and were never harmonised with it, which is the root of the differences on this page.
Because the standard is voluntary and national systems differ, brands vary — sometimes by a full size, sometimes between product lines of the same brand. Where a brand publishes its own size chart, use that rather than any general conversion, including this one.
How do I calculate my bra size?
Measure your underbust snugly and your bust loosely across the fullest part. The band is your underbust in inches rounded to the nearest even number. The cup is the difference between the two measurements, roughly one letter per inch. A 30 inch underbust with a 36 inch bust gives a 30 band and a six inch difference, which is a UK E cup.
How do I measure my bra size at home?
Use a soft tape measure, wear a non-padded bra or none, and stand naturally with arms relaxed. Measure the underbust snugly and level, exhaling first. Then measure the bust loosely across the fullest part without compressing. Keep the tape parallel to the floor for both.
What is the difference between band size and cup size?
The band is the number and comes from your underbust measurement — it carries most of the support. The cup is the letter and comes from the difference between your bust and underbust. They are independent, which is why 32C and 36C are quite different bras.
What does 34C mean?
A band that fits a 34 inch underbust, with a bust roughly three inches larger. The letter is not an absolute volume — a 34C and a 38C have the same three inch difference but quite different cup volumes, because the cup is proportional to the band.
What is a sister size?
A different band and cup combination holding roughly the same cup volume. Going down a band means going up a cup, so 34D and 32DD are sister sizes. It is useful when the cup fits but the band does not, or the reverse.
Is bra sizing different in Australia?
Australia uses a small even numbered band scale rather than inches — AU 12 is the same band as UK 34 and US 34. Cup letters follow the UK convention. So an Australian 12C is a UK 34C and a US 34C.
Is a UK bra size different from a US size?
The band numbers are the same but the cup letters diverge above D. A UK DD is a US DD, but a UK E is a US DDD and a UK F is a US G. Assuming the letters match is the most common conversion mistake, and the gap widens as cups get larger.
Why are European bra sizes such different numbers?
European and Japanese bands are in centimetres rounded to the nearest 5, so a UK 34 becomes EU 75. French, Belgian and Spanish brands then add 15 to the European number, making the same band an FR 90. The cup letters stay the same between EU and French sizing.
Should I measure with or without a bra?
Without one, or in a thin non-padded bra. Padding adds two to four centimetres to the bust reading, which can shift the cup result by one or two letters. A sports bra or minimiser will also distort the measurement.
How accurate are bra size calculators?
They give a reliable starting point from two measurements, but they cannot account for breast shape, tissue firmness or how a specific brand cuts its cups. Brands vary by up to a full size, so use the result as the size to try on first rather than a final answer.
What if I am between two sizes?
Try both, and try their sister sizes too. If your underbust falls between band numbers, the smaller band usually gives better support since bands loosen with wear. A new bra should fit comfortably on the loosest hook, leaving room to tighten later.
How often should I re-measure?
Every six to twelve months, and after any significant change — weight change, pregnancy, breastfeeding, or hormonal changes. Measurements shift more than most people expect, and a size from several years ago is often no longer right.