What Is the Bra Size Calculator?
The bra size calculator finds your band and cup size from your underbust and bust measurements, then converts it to UK, US, EU, French and Australian sizes. It uses the modern measuring method recommended by professional fitters, rather than the outdated “add 4 inches” rule, and it lists sister sizes so you can adjust between brands.
Surveys by fitters regularly suggest that a large share of people wear the wrong bra size, most often a band that is too big and a cup that is too small. Getting the band right is the single biggest improvement to comfort and support.
How Does the Bra Size Calculator Work?
The band comes from your underbust measurement in inches, rounded to the nearest even number. The cup comes from the difference between your bust and that band: every inch is one cup step (1 in = A, 2 in = B, 3 in = C and so on). If you add a leaning bust measurement, the calculator averages it with your standing bust, which gives a better result for fuller or softer breast tissue.
For EU sizes, the band is your underbust in centimetres rounded to the nearest 5, and the cup is based on the bust-to-underbust difference in 2 cm steps. French bands are EU + 15 and Australian bands are UK − 22.
How to Use the Bra Size Calculator?
- Choose inches or centimetres.
- Measure your underbust snugly around the ribcage directly under the bust, after breathing out.
- Measure your standing bust loosely around the fullest part.
- Optionally bend forward 90° and measure your leaning bust.
- Press Find my bra size to see your size, conversions, charts and sister sizes.
Bra Size Calculator Inputs
| Input | Required | How to measure |
|---|---|---|
| Underbust | Yes | Snug, level, directly under the bust |
| Standing bust | Yes | Loose, fullest part, standing upright |
| Leaning bust | Optional | Bent forward 90°, fullest part |
How to Measure Underbust and Bust Correctly
Wear a non-padded, unlined bra that fits reasonably well, or measure without a bra.
- Underbust: wrap the tape around your ribcage directly under the bust. Pull it snug, breathe out, keep it level at the back.
- Standing bust: stand straight and measure around the fullest part loosely, without compressing.
- Leaning bust: bend forward at the waist so your back is parallel to the floor and measure around the fullest part again.
Bra Size Formula
Band = underbust (in) rounded to the nearest even number
Cup difference = bust (in) − band
EU band = underbust (cm) rounded to the nearest 5; FR = EU + 15; AU = UK band − 22
| Difference | UK cup | US cup |
|---|---|---|
| 1 in | A | A |
| 2 in | B | B |
| 3 in | C | C |
| 4 in | D | D |
| 5 in | DD | DD / E |
| 6 in | E | DDD / F |
| 7 in | F | G |
| 8 in | FF | H |
| 9 in | G | I |
| 10 in | GG | J |
Bra Size Calculation Example
Someone measures a 30 in underbust and a 36 in standing bust.
- Band = 30 (already even).
- Difference = 36 − 30 = 6 in.
- Six steps gives a UK E cup, which is a US DDD/F.
Her size is UK 30E / US 30DDD. The old +4 method would have given a 34B band, which is far too loose and too small in the cup.
Understanding Your Bra Size Result
Band Size
Your band number provides about 80% of a bra’s support. It should feel snug on the loosest hook and sit level around your body.
Cup Size
Cup letters are relative to the band: a 32D and a 38D hold very different volumes. Your result shows UK and US cup letters, which differ from DD upward.
International Sizes
EU, French and Australian equivalents for brands from those regions.
Sister Sizes
Going one band up and one cup down (or the reverse) keeps a similar cup volume. Use sister sizes when a band feels too tight or too loose.
Factors That Affect Your Result
- Breast tissue softness and shape, which the leaning measurement helps capture.
- Your menstrual cycle, pregnancy and breastfeeding.
- Weight changes.
- Brand sizing and bra style (plunge, balconette, full cup).
Benefits of Using the Bra Size Calculator
- Uses the modern method fitters recommend.
- Includes a leaning measurement for fuller busts.
- Five international size systems.
- Sister sizes and a comparison with the outdated +4 method.
Limitations of the Bra Size Calculator
Measurements give a starting size, not a guaranteed fit. Breast shape, root width and projection affect fit and cannot be captured by a tape. Sizing varies by brand and style, so try your calculated size and its sister sizes.
Tips for Getting Accurate Results
- Measure in front of a mirror to keep the tape level.
- Measure at a consistent point in your cycle.
- Use the leaning bust if your breasts are full or soft.
- Re-measure after any significant weight change.
Bra Size Calculator vs. Other Methods
| Method | Band rule | Accuracy |
|---|---|---|
| Modern method (this tool) | Underbust rounded to even | High starting point |
| Traditional +4 method | Underbust + 4–5 in | Often too loose |
| Professional fitting | Fitter assessment | High, depends on fitter |
| Trial and error | None | Slow and costly |
Who Should Use the Bra Size Calculator?
Anyone shopping for bras online, people whose bands ride up or straps dig in, anyone whose size has changed after weight change or pregnancy, and people buying from international brands.
Frequently Asked Questions
What is a bra size calculator?
It is a tool that works out your band and cup size from your underbust and bust measurements and converts it to international sizes.
How does the bra size calculator work?
It rounds your underbust to the nearest even band and uses the difference between bust and band to find the cup letter.
How accurate is the bra size calculator?
It gives a reliable starting size. Because brands and breast shapes vary, try sister sizes if needed.
What measurements do I need?
Underbust and standing bust. A leaning bust measurement is optional.
Can my result change over time?
Yes. Weight change, hormones, pregnancy and age can change your bra size.
Why can different calculators give different results?
Some still use the +4 method, and cup letters above DD differ between UK and US systems.
Is the bra size calculator free?
Yes, it is free and private.
Can I use the bra size calculator on mobile?
Yes, it works on any phone or tablet.
Bra Size Calculator Conclusion
The right bra starts with a snug band and the correct cup. Measure your underbust and bust carefully, use the modern method result as your starting point, and keep sister sizes in mind when switching brands or styles.