Converters
Bra Size Calculator
Enter your underbust and bust measurements to estimate your band and cup size in five sizing systems, with sister sizes to try.
How to measure
- Use a soft tape measure and stand straight, arms relaxed. A light, unpadded bra or no bra both work.
- Underbust: wrap the tape round the ribcage just below the bust, level all the way round, and pull it firm but comfortable. Breathe out normally before reading it.
- Bust: measure round the fullest part, keeping the tape level across the back and resting lightly rather than pulled tight.
- Measure twice. If the two readings differ, use the average.
Why methods and brands disagree
There is no single standard. UK and US sizes grow one cup per inch of difference between bust and band, while European, French and Australian cups are set in 2 cm steps and bands are labelled differently (UK 34 = EU 75 = FR 90 = AU 12). The +4 method was long the norm on US store charts; from around 2011 a campaign nicknamed the “war on plus four” argued that it fits most people poorly, and many fitters now start from the underbust itself. Letters beyond D also vary: US brands may use DD, DDD and G where UK brands use DD, E and F.
Treat the result as a starting point. A good fit usually means a band that sits level and does most of the supporting, cups without gaps or spilling, and a centre panel that rests flat. If something doesn’t feel right, try the sister sizes or the next band on the same hook.
Your measurements are calculated in your browser and are never sent or stored.
Works in your browser. No account needed.
Frequently asked questions
How is bra size calculated?
The band comes from the underbust measurement and the cup from the difference between the bust and the band. With the modern method, a snug underbust of 30 inches gives a 30 band; a 35 inch bust is 5 inches more, and in UK sizing each inch is one cup (A, B, C, D, DD), so the estimate is 30DD. The same size is 30DD/E in the US, 65E in EU sizing, 80E in French sizing and 8DD in Australia.
What is the difference between the modern method and the +4 method?
The traditional method adds 4 inches to an even underbust (5 to an odd one) to get the band, so the same 30 inch underbust becomes a 34 band and the cup comes out smaller — 34A instead of 30DD. It was long standard on US store charts, but a campaign from around 2011 argued that it fits most people poorly, and many fitters now start from the underbust itself. Because some brands still size with +4, the calculator offers both.
What are sister sizes?
Sister sizes hold roughly the same cup volume on a different band. Going one band size down means going one cup up, and one band size up means one cup down: 32D, 34C and 36B are sisters. They help when a bra’s cups fit but the band is too loose or too tight, or when your usual size is out of stock.
How do UK, US, EU, French and Australian bra sizes compare?
Bands are labelled differently: a UK or US 34 is an EU 75, a French 90 and an Australian 12. UK and US cups match up to D, then the UK uses DD, E, F, FF and the US often DD, DDD and G. European and French cups have no double letters, so UK DD is EU E. Retailers’ conversion charts follow these pairings, but brands vary, especially at larger cup sizes.
Why does the European EN 13402 measurement give a different size?
EN 13402 labels the band as the underbust rounded to the nearest 5 cm and sets cups by the bust minus underbust difference in 2 cm steps, starting with A at 12–14 cm. That scale lines up broadly with the traditional +4 method, so a tape-measure EU size often shows a larger band and smaller cup than the modern UK method. The calculator shows it for comparison.
Are my measurements stored or sent anywhere?
No. The calculation runs entirely in your browser; nothing you enter is uploaded, logged or saved, and closing the page clears it.
About this tool
This bra size calculator turns two measurements — a snug underbust and the fullest part of the bust — into an estimated band and cup size, in inches or centimetres. The result appears in five systems at once: UK, US, EU (international), French, Belgian and Spanish, and Australian and New Zealand sizing, together with a table of sister sizes. It is a starting point for trying bras on or ordering online, not a substitute for how a particular bra fits.
By default it uses the modern underbust method: the band is the snug underbust rounded to the nearest even inch, and each inch of difference between the bust and the band adds one cup letter (AA, A, B, C, D, DD, E, F, FF, G in UK sizing). The traditional +4 method, which adds 4 or 5 inches to the underbust, is available as an option because some brands and store charts still use it; it produces a larger band and a smaller cup for the same body. When your underbust falls between two bands, the calculator suggests the neighbouring band with its matching cup as well.
Other systems are converted from the UK size the way retailers’ conversion charts do it: UK 30 is EU 65 and French 80, UK 34 is EU 75, French 90 and Australian 12, and European cups follow the same order without double letters, so UK DD is EU E. For comparison the calculator also shows the size you would get by measuring directly to the European standard EN 13402, which rounds the underbust to the nearest 5 cm and steps cups every 2 cm. The sources are the published size tables summarised in Wikipedia’s bra size article, including the EN 13402 cup table and the band equivalence table.
Sizing is not standardised across brands, styles or even colours of the same bra, and bodies change with the menstrual cycle, weight changes, pregnancy and age. A well-fitting bra usually has a band that sits level and does most of the supporting, cups without gaps or overflow, and a centre panel that rests flat. If the estimate doesn’t feel right, try a sister size, or the other band when you are between two. Your measurements never leave your device.