Bra Size Calculator
Underbust and bust to US band and cup, metric supported.
What it does
Support starts with the band, not the cup. A band that rides up or slips means the cups can never sit right, whatever the letter says. Two tapes and a mirror beat years of guessing sizes.
The tool above rounds underbust to an even band and maps the bust gap to a cup. A 32 in underbust with a 36 in bust gives band 32 and a 4 in gap, which is D, so 32D. The metric match is 81 and 91 cm toward the same size.
Cups step one letter per inch on this US scale: A at one, B at two, C at three, D at four, DD at five, and DDD at six. Bands snap to even numbers because that is how most US bands are sold.
Brands still cut their own way. Sister sizes trade band for cup, fabrics stretch, and styles fit differently across the same label. Treat the output as the first size to try, then adjust in the fitting room.
How to use
- Pick imperial (in) or metric (cm).
- Measure snug under the bust.
- Measure at the fullest point.
- Enter both numbers: 32 and 36 in.
- Read band, cup, and full size.
- Try on and check sister sizes.
Formula
- under
- Underbust around the rib cage, in or cm.
- bust
- Fullest bust, in or cm.
- diff
- Bust minus band, rounded to whole inches.
The method follows university extension fitting guides: underbust around the rib cage sets the band, fullest bust sets the cup, and the rounded difference sets the letter. Tape reads to half inches, bands snap to even numbers, and metric tapes convert at 2.54 cm per inch before the same steps.
Worked example
Example: 32 under, 36 bust
Imperial defaults: 32 in underbust, 36 in bust. These are also the default values, so the page opens on this answer.
- Underbust: 32 in.
- Bust: 36 in.
- Band: round 32 to even, stays 32.
- Difference: 36 - 32 = 4 in.
- Cup: 4 maps to D.
- Size: 32D.
- Tool shows 32D with a 32 in band and a D cup (4 in difference).
- Metric match: 81 and 91 cm give the same 32D.
Reference table
| Under | Bust | Size |
|---|---|---|
| 30 | 32 | 30B |
| 32 | 35 | 32C |
| 34 | 38 | 34D |
| 36 | 41 | 36DD |
Limits
- US band and cup scale; EU and UK differ.
- Estimate only, not a professional fitting.
- Ignores shape, spacing, and style cuts.
- Pregnancy and nursing change fit fast.
Common mistakes
- Wearing a loose band with tight straps.
- Measuring over padded bras.
- Pulling the bust tape too tight.
- Skipping sister sizes.
- Keeping one size for years.
Try it yourself
Quick checks. Answers sit tucked under each question.
34 under, 37 bust?
Band 34 with a 3 in gap, so 34C.
86 and 96 cm in metric?
86 cm with 96 cm gives band 34 and a 4 in gap, so 34D.
Questions
How is 32D found?
Round underbust to the nearest even number for the band, then subtract from bust for the cup. Here 36 - 32 = 4, which is D.
What do the cup letters mean?
One inch is A, two B, three C, four D, five DD, and six DDD on this US scale.
Do sizes vary by brand?
Yes. Brands cut differently, so sister sizes like 34C can fit like 32D with a looser band.
How do I measure correctly?
Re-measure snug under the bust and at the fullest point, with the tape level and no padding.
Why does the tool reject my numbers?
Bust must exceed underbust; swap or re-check the two tapes when the tool flags it.
Band or cup first when trying on?
Band first for support, then cup without spill or gaps, then a final mirror check.