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♥ Free UK sizing tools

My Bra Size
My Bra Size

Our method

How bra sizes work, and how our calculator works

Every rule we use, where it comes from, and what our tools cannot tell you.

Sources checked

A tape measure extended across a wooden table
Photo: Mark Owen Wilkinson Hughes (opens in a new tab) on Unsplash

How UK bra sizes work

A UK bra size has two parts. The number is the band, based on your underbust in inches. The letters are the cup, based on how much bigger your bust is than the band.

Cup letters are relative to the band: a 34D cup holds more than a 32D cup. That is why sister sizes exist.

34: the band
Your underbust in inches, rounded up to an even number. When the bra is on, the band should sit firm and level around your ribcage.
DD: the cup
How much larger your bust is than the band, at roughly one cup step per inch. UK cups use double letters: D, DD, E, F, FF, G, GG, H and so on.
How to read UK size 34DD: 34 is the band, DD is the cup.

The method we use

We use the tape method published by two UK retailers, Brastop and Boux Avenue. Their guides agree on every rule they both state, and Boux Avenue’s worked example (39 − 34 = 5 inches, a DD cup) settles the one point Brastop leaves open: you subtract the rounded band, not the raw underbust.

  1. Units. Inches are used as entered. Centimetres are divided by 2.54 (the definition of an inch). No UK brand we checked publishes a centimetre band table, so this conversion is our choice, not a brand rule.
  2. Band. Round the underbust up to a whole inch. If that number is odd, round up again to the next even number. Nothing is added: Brastop says “do not add anything extra on top”.
  3. Bust. Round to the nearest whole inch. Exactly half rounds up.
  4. Difference. Rounded bust minus band.
  5. Cup. One cup step per inch, using Brastop’s published table: 1 A, 2 B, 3 C, 4 D, 5 DD, 6 E, 7 F, 8 FF, 9 G, 10 GG, 11 H, 12 HH, 13 J, 14 JJ, 15 K.

Some US charts add 4 or 5 inches to the underbust to get a band. We do not, because the UK retailers that publish a method say not to, and a UK study found the traditional tape method tended to give too large a band and too small a cup.

Rounding at the boundaries

You enterWhat happensResult
Underbust 30 Already a whole, even number Band 30
Underbust 30.1 Rounds up to 31, which is odd, so up to 32 Band 32
Underbust 31 Odd, so up to 32 Band 32
Bust 36.49 Rounds down to 36 36
Bust 36.5 A half rounds up to 37 37
76.2 cm Exactly 30 inches Band 30
76.3 cm Just over 30 inches, so 31, then 32 Band 32

Supported sizes

Bands: 28 to 48, even numbers only. This matches the UK brand conversion tables we checked (28 to 46 at Curvy Kate and Brastop, 48 at Elomi).

Cups: the UK order, with double letters and no I:

  1. AA
  2. A
  3. B
  4. C
  5. D
  6. DD
  7. E
  8. F
  9. FF
  10. G
  11. GG
  12. H
  13. HH
  14. J
  15. JJ
  16. K
  17. KK
  18. L

The inch table stops at K (15 inches). KK and L follow the UK order used by Elomi and Bravissimo, but no source maps inches to them, so we flag those results.

Edge cases

Bust not larger than underbust
We ask you to check both measurements; no size is calculated.
Rounded bust equals the band (0 inches)
No published rule covers this. We say so and suggest trying AA or A and checking the fit.
Band below 28 or above 48
Outside the UK brand tables we checked. We point you to brand guides and a fitting instead of guessing.
Difference of 16 or 17 inches
We continue the UK cup order to KK and L and clearly flag that this goes beyond the published table.
Difference over 17 inches
Beyond the cups we can support (L). We recommend a specialist fitting.
Measurements under 20 or over 70 inches
Treated as a likely typing or unit mistake, and we ask you to check.

Sister sizes and label conversion

Sister sizes move the band by one size (2 inches) and the cup one step the other way through the UK order, double letters included. So 32DD has sister sizes 30E and 34D. Brastop, Boux Avenue and Pour Moi all describe the same rule.

Label conversion changes the label on an existing size; it never converts your measurements. Bands: US is the same as UK, EU is 60 for UK 28 and rises by 5 for each UK band, French is EU plus 15, and Australian is UK minus 22. Cups come from published brand charts. Where brands disagree (EU labels above UK FF, and US aliases such as DDD or F), our bra size converter shows both.

What our tools cannot do

  • A tape cannot see breast shape. Two people with the same measurements can need different sizes.
  • Brands and styles fit differently. A size is a starting point to try, not a guaranteed fit.
  • No professional bra fitter has reviewed our tools. We will say so here if that changes.
  • We give no medical advice. If a bra causes pain that does not ease, or you notice breast changes, speak to your GP.
  • We do not publish accuracy percentages, because we have no data that would support one.

How we research and test our guides

Our buying guides are written by Jenisha Jonchhe, a fashion writer and designer. Each guide is researched from brand and retailer size guides, published fit advice and how each style is constructed, and brand sizing and product details are checked against current manufacturer or retailer information before publication.

  • Tested where possible. Jenisha examines or tries bras herself where she can, checking the band, cups, wires, straps and fabric against the checks in our bra fitting guide.
  • Research and testing are kept separate. A guide calls a product tested only if it has been. Everything else is labelled as research.
  • Search links are not endorsements. The Amazon links open search results, which change all the time, so a product appearing there has not necessarily been tested.
  • Commission never changes advice. See our affiliate disclosure.

Your measurements stay with you

All the sizing runs in your browser. We do not store your measurements or fit-checker answers, send them to a server, or put them in web addresses. When one tool hands your result to another, only the size label (for example 32DD) travels, in the part of the address after “#”, which browsers do not send to websites. See our privacy notice.

Sources

Checked . Brands change their charts, so check the retailer’s own size guide before you buy.

  1. S1 Brastop: How to measure your bra size (opens in a new tab) Tape method; inches-to-cup table (A to K); rounding rules
  2. S4 Boux Avenue: How to measure bra size (opens in a new tab) Tape method; worked example 39 − 34 = 5 (DD); sister sizes; limits of measuring
  3. S3 Brastop: Sister sizes explained (opens in a new tab) Sister sizes and their limits
  4. S22 Pour Moi: Sister sizes (opens in a new tab) Sister sizes worked example (32F and 34E)
  5. S12 Elomi: Elomi size guide (UK) (opens in a new tab) Conversion up to KK; size range
  6. S18 Bravissimo: Bra sizing explained (US site, conversion charts) (opens in a new tab) UK to US, EU and French conversion up to L
  7. S5 Curvy Kate: Bra fitting size guide (opens in a new tab) Conversion table; EU labels that skip the letter I
  8. S9 Freya: Size guide (UK) (opens in a new tab) International conversion; UK sizing statement; size range
  9. S26 White & Scurr, Ergonomics 55(6), 2012: Evaluation of professional bra fitting criteria for bra selection and fitting in the UK (opens in a new tab) Tape methods compared with professional fitting
  10. S27 McGhee & Steele, Journal of Science and Medicine in Sport 13(6), 2010: Optimising breast support in female patients through correct bra fit (opens in a new tab) How often measured sizes differ from a fitted size

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