Bust Size Calculator – Find Your Bra Size from Your Bust
Enter your underbust and bust measurements below, and we’ll give you your estimated US and UK bra size. Your bust measurement is the key to finding your cup letter – it's fast, private, and works for everyone.
Bust Size Calculator
This is your estimated US band and cup size.
Need just your cup letter? Try the Cup Size Calculator or convert to international sizes with the Conversion Tool.
What Is a Bust Measurement?
Your bust measurement is the circumference around the fullest part of your chest – usually across the nipples. It’s one of the two key numbers you need to find your bra size. The other is your underbust (the band). The difference between your bust and your band gives you your cup letter.
For example, if your bust is 38 inches and your band is 34 inches, the difference is 4 inches – which is a D cup. So your bust measurement directly decides your cup size.
How to Measure Your Bust Correctly
Step 1: Stand Up Straight
Keep your feet flat, shoulders relaxed, and arms at your sides. This gives you the most accurate reading.
Step 2: Use a Soft Tape Measure
Wrap it around the fullest part of your bust – the tape should be level and not too tight. It should rest gently against your skin without compressing.
Step 3: Take the Measurement
Look straight ahead in a mirror to make sure the tape is level. Take two or three measurements and use the average for best accuracy.
Step 4: Enter It in the Calculator
Now enter your bust measurement (and your underbust) into the tool above. We'll do the rest – your band and cup size will appear in seconds.
Why Your Bust Measurement Matters
Without your bust measurement, you can’t know your cup size. The band gives you the number, but the bust gives you the letter. Together, they tell you exactly which bra size to try on.
Remember: your bust measurement alone doesn’t give you a bra size – you need the underbust too. That’s why our calculator asks for both. It’s the most reliable way to get your perfect fit.
What Affects Your Bust Measurement?
- Weight changes – even a few pounds can change your bust size.
- Hormonal changes – pregnancy, nursing, or your menstrual cycle can temporarily change your bust size.
- Brands and styles – different bras fit differently, so your bust measurement stays the same, but the bra size you need might vary.
That’s why we recommend measuring yourself every few months – especially before buying new bras.
Sister Sizing – The Same Bust, Different Letter
If your usual size feels off, you can try a sister size – a different band and cup combination that holds the same cup volume. For example, if your bust measurement gives you a 34C, you could also try a 32D (smaller band, larger cup) or a 36B (larger band, smaller cup). These all have the same cup volume but may feel more comfortable depending on the brand.
Use our Sister Size Calculator to explore your options.
Want to double-check your band size?
Try the Band Size Calculator →Why Trust This Calculator?
- It’s free – no sign‑up, no charges.
- It’s private – all calculations happen in your browser.
- It’s accurate – we use the same formula professional fitters use.
- We keep it updated – our charts are checked regularly to match current brands.
Your result is a great starting point. Every bra fits a little differently, so always try on a few styles to find the one that feels best.
Ready to find your size?
Enter your bust and underbust measurements in the calculator above right now.
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