Most people wear a band that is too loose and a cup that is too small, because the old "add four or five inches" method dates from stretch-free fabrics. This calculator uses the modern method that fitters and most brands now recommend: the band is your actual underbust measurement, and the cup comes from the difference between bust and band.
How to measure
- Underbust: wrap the tape directly under the breasts, level all the way round, snug but not digging in, after breathing out.
- Bust: around the fullest part, standing straight, tape level and just touching, not compressing. Wearing a non-padded bra gives the most consistent result.
How it is calculated
- Band = underbust in inches, rounded to the nearest even number (bands come in even sizes). 30.5 in rounds to 30; an odd whole number like 31 rounds down to 30, because a snug band gives most of the support.
- Cup = bust minus band, in inches. Each inch is one cup: 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD, 6 = DDD (US) or E (UK). Above D the letters diverge between countries, which is why the table shows several systems.
- EU band = underbust in centimetres rounded to the nearest 5, as European brands measure. French sizes are EU + 15; Australian band = US band − 22 with UK cup letters. Note that label conversion charts (US 34 = EU 75) still embed the old US "add four inches" rule, so a size measured here can differ from a chart conversion by one or two bands; the converter mode uses the charts, the measuring mode uses the tape.
Example
Underbust 76 cm (29.9 in), bust 91 cm (35.8 in). Band 30. Difference 5.8 in, about 6 cups: 30DDD in the US, 30E in the UK, 80F in the EU. If a 30 band feels too tight, the sister size 32DD has the same cup volume on a looser band.
Why the result may still need adjusting
Measurements give a starting point, not a verdict. Breast shape, how much tissue sits in the cup, and brand differences of half a size are normal. A good fit: the band is level and snug on the loosest hook, the centre piece lies flat against the sternum, and the underwire sits fully under the breast without gaps or spillage.
Frequently asked questions
How do I measure my bra size at home?
Two measurements with a soft tape: snug under the bust, and loosely around the fullest part. Enter both above. The band is the first number, the cup letter comes from the difference.
What is a sister size?
A size with the same cup volume on a different band: go up one band and down one cup letter, or the reverse. 32D, 34C and 36B all hold the same volume. Use it when the band is wrong but the cup is right.
Why does the calculator give a smaller band than I usually wear?
Because it uses the modern method without adding inches. Older charts added 4–5 inches to the underbust, which made sense for rigid fabrics decades ago and now produces loose bands that ride up. Try the calculated band; most people find it more supportive.
How do I convert a European bra size to US?
Switch to "Convert a size", pick EU and enter the band and cup: an EU 75C is a US 34C, an EU 80E is a US 36DD. The EU band is the underbust in centimetres rounded to 5, and cups run in a single letter sequence.
Are UK and US cup sizes the same?
Up to D, yes. Above that they differ: US goes DD, DDD, G, H; UK goes DD, E, F, FF, G. EU uses a single letter sequence with no doubles. The table shows all of them for your measurements.
How often should I re-measure?
Whenever weight changes by a few kilos, during and after pregnancy, and every year or so anyway. Bands also stretch with wear, so a bra that fit on the loosest hook should move to tighter hooks over time.
Sources
- ISO 3355: Sizing of bras (band and cup designations)
- Bra size conversion chart (Wikipedia, with references to national standards)
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked