Bra Size Calculator
Two tape measurements reveal your band, cup and sister sizes — plus US, UK and EU conversions. The fit fix most wardrobes need.
How bra sizing works
Your bra size has two parts. The band is your snug underbust measurement rounded to the nearest even number (for example, 31.5" becomes a 32 band). The cup is the difference between your bust and your band — each inch of difference is one cup letter.
| Band | Snug underbust (inches) | EU size |
|---|---|---|
| 30 | 27–29 | 65 |
| 32 | 29–31 | 70 |
| 34 | 31–33 | 75 |
| 36 | 33–35 | 80 |
| 38 | 35–37 | 85 |
| 40 | 37–39 | 90 |
| Bust − band | Cup |
|---|---|
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5 inches | DD / E |
| 6 inches | DDD / F |
Sister sizes
Sister sizes carry the same cup volume on a different band: 34C = 32D = 36B. They're the secret to fine-tuning fit — if the cups are right but the band rides up, drop a band size and go up a cup; if the band digs in, go up a band and down a cup.
US, UK & EU conversion
Band numbers are identical in the US and UK — a US 34 is a UK 34. The EU system measures the underbust in centimetres instead, so US 34 = UK 34 = EU 75. Cup letters can diverge above DD between systems, so always check the brand's own conversion chart for larger cups.
Measuring & fit tips
- The band does the heavy lifting. Most support comes from the band, not the straps — it should sit level and snug around your ribcage.
- Two-finger test. You should be able to slide two fingers under the band. More room means it's too loose; less means it's too tight.
- Check the centre gore. On an underwire bra it should sit flat against your sternum. If it floats, try a larger cup.
- Straps shouldn't dig or slip. They're for shaping, not support — if you're cranking them tight, the band is probably too big.
- Re-measure regularly. Bodies change — check your size yearly or after significant weight change, and replace bras once the elastic gives out.
Frequently asked questions
Why does my bra size differ between brands?
There's no enforced sizing standard, so one brand's 34C can be another's 34D. Your measurements stay constant — compare them against each brand's own size chart instead of trusting the label alone.
What are sister sizes and when should I use them?
Sister sizes hold the same cup volume on a different band: 34C = 32D = 36B. If the cups fit but the band rides up, try a smaller band and larger cup (32D); if the band digs in, go up a band and down a cup (36B).
How do US, UK and EU bra sizes convert?
Band numbers are the same in the US and UK, so a US 34 is a UK 34. The EU measures the underbust in centimetres, so a US/UK 34 converts to an EU 75. Cup letters can differ above DD, so check the brand's conversion chart for larger cups.
How tight should a bra band be?
The band provides most of the support, so it should feel snug and sit level around your ribcage. You should be able to slide two fingers underneath it, but it shouldn't ride up your back — if it does, try a smaller band (a sister size).