Bra Size Calculator
Band and cup from your bust and underbust measurements, with US, UK and EU equivalents, sister sizes, and what the fitting research says about the old +4 method.
Bra Size Calculator: with the default inputs, your starting size (us) is 32D.
Snug, directly under the bust, tape level all the way round, at the end of a relaxed breath out.
Around the fullest part, tape level and not pulled tight, standing upright in an unpadded bra.
The traditional method dates from stretch-free fabrics. Research finds it overestimates band and underestimates cup.
- Band size
- 32US and UK bands use the same number.
- Cup (US)
- D
- UK equivalent
- 32D
- EU equivalent
- 70D
- Bust minus band
- 4 inEach inch of difference is one cup.
- Sister size, one band smaller
- 30DDSame cup volume, tighter band.
- Sister size, one band larger
- 34CSame cup volume, looser band.
Assumptions
- The default band method is the raw underbust measurement rounded to the nearest even inch, which is what McGhee and Steele (2006) recommended over the traditional +4 rule.
- Cup size is one letter per inch of bust-minus-band difference — the standard convention, not a standard.
- Cup letter sequences above D are retail conventions in the US, UK and Europe. No standards body defines them, individual brands deviate from all three, and the US ladder shown here follows Wacoal's published sequence, which skips E and F.
- Band numbers are not measurements of the garment. Bras labelled band 36 from three manufacturers averaged 27 inches of actual circumference (Pechter 1998, reported in Ng et al. 2019).
- EU band numbers are converted as 2.5 × the US band minus 10 (US 34 ↔ EU 75); UK and US bands use the same number.
- Measurements should be taken at the end of a relaxed breath out — respiratory state alone changes the calculated size.
- This is a fitting aid, not a medical assessment. It cannot see breast shape or root width, which often decide fit more than the numbers do.
| Difference (inches) | US | UK | EU | |
|---|---|---|---|---|
| 0 or less | AA | AA | AA | |
| 0 or less | AA | AA | AA | |
| 1 | A | A | A | |
| 2 | B | B | B | |
| 3 | C | C | C | |
| 4 | D | D | D | ← you |
| 5 | DD | DD | E | |
| 6 | DDD | E | F | |
| 7 | G | F | G | |
| 8 | H | FF | H | |
| 9 | I | G | I | |
| 10 | J | GG | J |
One inch of difference is one cup. These letter sequences are the common retail conventions in each market — no standards body defines them, and brands deviate from all three by up to a full cup. Two live manufacturer charts disagree about whether a US DDD is a UK E or a UK F.
| Size | |
|---|---|
| 28DDD | Two bands down — try if the band rides up at the back |
| 30DD | One band down |
| 32D | Your calculated size |
| 34C | One band up |
| 36B | Two bands up — try if the band digs in |
Cup volume depends on both letters and band. Going down a band and up a cup keeps roughly the same cup volume, which is why these are worth trying when the cup fits but the band does not.
How this is worked out
The formula
Band (modern) = underbust measurement in inches, rounded to the nearest even inch Band (traditional) = underbust rounded to a whole inch, + 4 if even, + 5 if odd Cup = bust − band, one cup per inch of difference 1 in = A · 2 = B · 3 = C · 4 = D · 5 = DD · 6 = DDD (US) / E (UK) / F (EU) EU band = 2.5 × US band − 10 (US 34 ↔ EU 75) UK band = US band
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Underbust measurement
- Snug, directly under the bust, tape level all the way round, at the end of a relaxed breath out.in in, cm · from 20 to 70 · defaults to 32
- Bust measurement
- Around the fullest part, tape level and not pulled tight, standing upright in an unpadded bra.in in, cm · from 20 to 80 · defaults to 36
- Band method
- The traditional method dates from stretch-free fabrics. Research finds it overestimates band and underestimates cup.Modern — the underbust measurement itself (recommended) · Traditional — add 4 inches (5 if odd)
What you get back
- Your starting size (US)main answer
- Band size
- US and UK bands use the same number.
- Cup (US)
- UK equivalent
- EU equivalent
- Bust minus band
- Each inch of difference is one cup.
- Sister size, one band smaller
- Same cup volume, tighter band.
- Sister size, one band larger
- Same cup volume, looser band.
What this assumes
- The default band method is the raw underbust measurement rounded to the nearest even inch, which is what McGhee and Steele (2006) recommended over the traditional +4 rule.
- Cup size is one letter per inch of bust-minus-band difference — the standard convention, not a standard.
- Cup letter sequences above D are retail conventions in the US, UK and Europe. No standards body defines them, individual brands deviate from all three, and the US ladder shown here follows Wacoal's published sequence, which skips E and F.
- Band numbers are not measurements of the garment. Bras labelled band 36 from three manufacturers averaged 27 inches of actual circumference (Pechter 1998, reported in Ng et al. 2019).
- EU band numbers are converted as 2.5 × the US band minus 10 (US 34 ↔ EU 75); UK and US bands use the same number.
- Measurements should be taken at the end of a relaxed breath out — respiratory state alone changes the calculated size.
- This is a fitting aid, not a medical assessment. It cannot see breast shape or root width, which often decide fit more than the numbers do.
About this calculator
Two measurements, two subtractions, and a starting point — not an answer. Bra sizing is one of the few areas of clothing with no standards body at all, and this calculator is honest about that from the top.
How to measure
Underbust: snug, directly under the breast tissue, tape level and parallel to the floor. Measure at the end of a relaxed breath out — McGhee and Steele showed in 2006 that respiratory state alone shifts the calculated size.
Bust: around the fullest part, standing upright in a non-padded bra, tape level and not pulled tight.
Two methods, and why the default is the modern one
The traditional method adds four inches to the underbust (five if odd). It comes from an era of bras made without elastic. Fabric changed; the rule stayed.
The research is unkind to it. White and Scurr (2012) found it overestimated band size and underestimated cup size, worsening as band size increased. McGhee and Steele (2006) recommended the raw underbust measurement instead. Greenbaum's 2003 study of 102 women referred for breast reduction found every one was wearing the wrong size.
So the default here is the modern one: your underbust measurement, rounded to the nearest even inch. The +4 method is offered because the international cross-reference tables were built on it and older charts still circulate — but mainstream retailer instructions have quietly dropped it, and you should treat any chart that still teaches it as out of date.
Reading the results
The band does most of the work — most of the support comes from it, not the straps. It should stay level across your back on the loosest hook, leaving room to tighten as it stretches.
Cup letters are the messy part. Up to DD the US and UK agree. Above that they diverge: a 6-inch difference is DDD in the US, E in the UK and F in Europe. Trust the chart of whichever country the retailer is in.
Sister sizes are the most useful thing here. Cup volume depends on band as well as letter, so a 34D, a 32DD and a 36C hold roughly the same volume. If the cup is right but the band rides up, go down a band and up a cup. If the band digs in, go the other way.
How bad is the variation?
Worse than almost anywhere else in clothing. Pechter measured bras labelled band 36 from three manufacturers and found they averaged 27 inches of actual circumference — nine inches under the label. His 1998 conclusion has not aged: "The current popular system of determining bra size is inaccurate so often as to be useless."
Two problems hide behind that. Dimensional variation is the same printed number arriving as a different physical garment. Nomenclature variation is the same physical cup carrying a different letter between brands. The table here shows the three national ladders; brands deviate from all of them by up to a full cup.
What the number cannot do
It cannot see breast shape, root width, or how full you are at the top versus the bottom — all of which decide whether a style works. That is why fit checks beat arithmetic.
White and Scurr concluded that women should be taught to assess fit rather than chase an absolute size. Use this to pick the first three bras to try on, then let the mirror decide.
If a bra causes pain, leaves marks that do not fade, or if one side has changed noticeably, that belongs with a clinician rather than a size chart.
Frequently asked questions
▸How do I measure my bra size?
Measure snugly under the bust for the band and around the fullest part for the cup, both with the tape level. Round the underbust to the nearest even inch for the band, then subtract the band from the bust — each inch of difference is one cup.
▸Should I add 4 inches to my underbust?
Not any more. That rule comes from bras made without elastic. White and Scurr's 2012 study found it overestimates band size and underestimates cup, and McGhee and Steele recommended using the raw underbust measurement instead.
▸What are sister sizes?
Sizes with roughly the same cup volume on a different band: 34D, 32DD and 36C all hold about the same. Go down a band and up a cup if the band rides up; go up a band and down a cup if it digs in.
▸Why is a UK DD different from a US DD?
They are not — both are a 5-inch difference. It is the next sizes that split: a 6-inch difference is DDD in the US, E in the UK and F in Europe. Always check which country's chart the retailer uses.
▸Why does the same size fit differently between brands?
Because no standard defines bra sizing. Each brand cuts its own pattern, and shape, wire width and cup depth vary even when the label matches. That is why fit checks matter more than the letters.
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