Bra Size Calculator
Calculate Bra Size
How to Use This Bra Size Calculator
Bra sizing conventions vary wildly between manufacturers and regions—guessing your size often leads to painful underwires or poor support. I keep a soft tape measure handy and run both numbers through this tool before ordering online from US, UK, or EU retailers.
- Select Unit. Choose inches or centimeters to match your tape measure.
- Enter Underbust Measurement. Measure snugly just under the bust line, level around the ribcage.
- Enter Bust Measurement. Measure loosely around the fullest part of the bust—typically while wearing an unpadded everyday bra.
- Press Calculate. Results show US/Canada, UK, Europe, and Australia/New Zealand sizes.
For foot sizing across regions, see Shoe Size Conversion. For general body measurements, try the Height Calculator.
Bra Size Formulas and Practical Applications
Bra size is a band number plus a cup letter—the cup is not a fixed volume, it is the difference between bust and underbust relative to the band. Think of the band as the anchor and the cup as how much room the fabric needs above it.
US/UK Band = Round Underbust (in) to Nearest Even Inch
Cup Index = Round(Bust − Underbust in inches)
EU Band ≈ Round Underbust (cm) to Nearest 5 cm
Example: underbust 31" rounds to band 32; bust 35" gives a 3" difference → US cup C → size 32C. AU band uses US band minus 22.
Standard Units and Conversion Tables
Inch difference between bust and underbust maps to cup letters. US and UK diverge after D—this calculator applies each region's cup chart.
| Difference (inches) | US Cup | AU Cup | UK Cup |
|---|---|---|---|
| <1 | AA | AA | AA |
| 1 | A | A | A |
| 2 | B | B | B |
| 3 | C | C | C |
| 4 | D | D | D |
| 5 | DD/E | DD | DD |
| 6 | DDD/F | E | E |
| 7 | DDDD/G | F | F |
Frequently Asked Questions
Does a C cup mean the same thing on every band size?
No. The cup letter represents volume relative to band, not a fixed volume. A 32C holds less than a 38C. Sister sizing—going down a band and up a cup—keeps similar cup volume.
Should I measure while wearing a bra?
For bust measurement, an unpadded everyday bra gives the most consistent results. Underbust should be measured snugly, directly under the bust line.
Why do US and UK cup letters differ at larger sizes?
US and UK systems diverge after D—US may use DD/E, DDD/F while UK continues DD, E, F, FF. This calculator maps inch difference to each regional cup chart.
How is the band size calculated?
US and UK band sizes round underbust inches to the nearest even number. EU band uses underbust centimeters rounded to the nearest 5. Australia/New Zealand band is US band minus 22.
Are calculator results the same as trying bras on?
No. Brands use different fits and wire shapes. Use these results as a starting size, then adjust based on how the bra sits on your body.