Bra Size Calculator
Advanced bra size calculator using multiple bust and underbust measurements to estimate your US, UK, EU, FR and AU bra size, plus sister sizes.
Underbust measurements
Bust measurements
What the bra size calculator does
The bra size calculator estimates your band and cup size from several underbust and bust measurements rather than a single pair. Enter your units, choose the sizing system you want first (US, UK or EU), then type three underbust measurements taken snug, tight and relaxed, and three bust measurements taken standing, leaning forward and lying down. Press Calculate to get a recommended size, the equivalent in other systems, and the two sister sizes that fit the same cup volume on a different band.
Why six measurements instead of two
A tape measure gives a different reading depending on how hard you pull it and how you stand. Ribcages compress by an inch or more between a relaxed and a tight measurement, and breast tissue shifts by a similar amount between standing and leaning. Averaging several readings smooths that out and gets closer to the size that will actually fit, which is why bra-fitting communities recommend the multi-measurement method over the old “add four inches” rule.
How the size is worked out
- Band. The underbust readings are averaged and rounded to the nearest even number in inches, because bands come in even sizes (28, 30, 32 …). Centimetre inputs are converted first (1 in = 2.54 cm).
- Cup. The bust readings are averaged and the band measurement is subtracted. Each inch of difference is roughly one cup letter.
- Conversion. The US band and cup are mapped to UK and EU equivalents. UK cups run AA, A, B, C, D, DD, E, F, FF, G; US cups run AA, A, B, C, D, DD, DDD, G, H; EU bands are in centimetres, 60, 65, 70, 75, 80, 85 and so on.
| Bust minus band | US cup | UK cup | EU cup |
|---|---|---|---|
| Under 1 in | AA | AA | AA |
| 1 in | A | A | A |
| 2 in | B | B | B |
| 3 in | C | C | C |
| 4 in | D | D | D |
| 5 in | DD / E | DD | E |
| 6 in | DDD / F | E | F |
| 7 in | G | F | G |
| 8 in | H | FF | H |
Worked example with the default values
Underbust 32, 31 and 33 in average 32; bust 38, 39 and 37.5 in average 38.2. The difference is 6.2 in, so the calculator suggests a 32 band with a DDD (US) / E (UK) / F (EU) cup, written 32DDD, 32E or 70F. Its sister sizes are 30G (smaller band, larger cup) and 34DD (larger band, smaller cup), both of which hold the same cup volume.
Sister sizes
Cup letters are relative to the band. A D cup on a 36 band holds more than a D cup on a 32 band. Moving one band size down and one cup letter up keeps the volume the same: 34C, 32D and 30DD are sister sizes. This is useful when a style runs tight or loose in the band, or when a shop does not stock your exact size.
Checking the fit
- Band. Should be snug on the loosest hook when new, level all the way round, and stay put when you raise your arms. It provides about 80% of the support.
- Cups. No gaping at the top and no spillage at the sides or centre. The underwire, if any, should sit flat against the ribcage between the breasts.
- Straps. Adjusted so they hold without digging in; if they carry the weight, the band is too loose.
- Centre gore. Should lie flat against the sternum; if it lifts away, the cups are too small.
Common mistakes
- Mixing centimetres and inches. Set the unit selector before entering any numbers.
- Measuring over a padded bra. Take bust measurements in an unpadded bra or none at all.
- Adding inches to the band. Old charts added four or five inches; modern bands are meant to be worn snug at the actual underbust measurement.
- Assuming letters mean the same across brands. UK and US systems diverge above D; use the conversion rather than the letter alone.
Related tools
The body shape type calculator and waist to hip ratio calculator use the same tape measurements for other purposes. The shoe size conversion calculator handles the other common international sizing headache, and the healthy weight calculator and body fat calculator track changes that will eventually change your bra size too.
External references
Frequently asked questions
Why is my calculated size different from what I wear?
Most people wear a band that is too large and a cup that is too small, because of the old add-four-inches method. A snug band with a larger cup often fits better.
Is the result exact?
It is a strong starting point. Brands cut differently, and cup volume varies by style. Try the suggested size and its two sisters.
What if my measurements fall between sizes?
Try both. If the band feels too tight on the smaller size, go up a band and down a cup.
Do the letters mean the same in every country?
No. UK and US cups match up to D, then diverge; EU uses a different band scale entirely. The calculator shows all three.
How often should I re-measure?
Every six to twelve months, and after weight change, pregnancy or a change in exercise habits.