What Is a Bra Size Calculator?
A bra size calculator finds your correct band and cup size from two simple measurements: your underbust (band) and your bust. Many people wear the wrong size because they guess rather than measure, leading to discomfort and poor support. This calculator uses your measurements to suggest a better-fitting size and explains sister sizes so you can fine-tune the fit. Enter your measurements and the calculator returns a recommended size.
How to Use the Calculator
- Measure your underbust — snugly around your ribcage, just under the bust.
- Measure your bust — around the fullest part.
- Calculate — see your recommended band and cup size.
How Bra Size Is Determined
The band size comes from your underbust measurement, and the cup size comes from the difference between your bust and underbust. A larger difference means a larger cup. Because both measurements matter, two people with the same bust measurement can need very different sizes depending on their band.
Cup Size by Difference
| Bust − Underbust | Approx Cup (UK/US) |
|---|---|
| ~1 inch | A |
| ~2 inches | B |
| ~3 inches | C |
| ~4 inches | D |
| ~5 inches | DD/E |
Understanding Sister Sizes
Sister sizes have the same cup volume but a different band. If a band feels too tight or loose, you can go down or up a band size and adjust the cup letter in the opposite direction to keep a similar cup volume. This helps you find the most comfortable combination.
Signs of a Bad Fit
- Band riding up at the back — try a smaller band.
- Cups gaping or overflowing — adjust the cup size.
- Straps digging in — the band may be doing too little work.
- Underwire poking — likely the wrong cup or band.
Note: Sizing varies between brands and styles, so use this as a starting point and try sizes on where possible.
Frequently Asked Questions
How do I measure my bra size?
Measure your underbust snugly for the band, and your bust at the fullest point for the cup. The difference between the two determines your cup size.
How is cup size calculated?
Cup size comes from the difference between your bust and underbust measurements — roughly one cup letter per inch of difference.
What are sister sizes?
Sister sizes have the same cup volume with a different band size. Going down a band and up a cup (or vice versa) keeps a similar fit when the band feels off.
Why does my bra size differ between brands?
Sizing and cut vary between manufacturers and styles, so the same labeled size can fit differently. Always treat a calculated size as a starting point.
Is this bra size calculator free?
Yes — it is completely free, requires no signup, and suggests your band and cup size.