Bra Size Calculator
The bra size calculator converts your underbust and bust measurements into a band size and cup letter, using either the modern method (round band to nearest even) or the traditional +4 method.
verified_userReviewed by the Calculopedia editorial teamLast updated 2026-08-15
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quizExample
How this calculator works, with real numbers (no JavaScript needed):
Inputs
- Measurement unit
- in
- Underbust (band)
- 31
- Bust
- 36
- Sizing method
- modern
Results
- Your bra size
- 32D
- Band size
- 32
- Cup letter
- D
- Cup difference
- 4
functionsThe formula
Finding a bra size only needs two measurements and a little arithmetic. Getting the numbers right is worth it: most of the discomfort people blame on bras actually comes from a band that is too loose and a cup that is too small, both of which are easy to fix once you know your true size.
Where the sizing system came from
Bra sizing as we know it was formalised in the 1920s and 1930s, when manufacturers and department stores standardised band-and-cup measurements to make off-the-rack garments fit a range of bodies. The classic cup letters — A, B, C, D and up — map a one-inch difference in bust circumference onto each step. The system has been tweaked countless times since, and different countries (US, UK, EU) use different scales, which is a large part of why the "same" size can feel so different between brands.
The two measurements
- Underbust — snug around the ribcage, directly under the bust. This gives the band number.
- Bust — around the fullest part of the bust. This sets the cup.
Step 1 — the band
Modern method: round your underbust to the nearest even number. Underbust 31" → band 32.
Traditional method (+4): add 4 (or 5 if the rounded number is odd) to the underbust. Underbust 31" → 31 + 5 = 36. Older sizing guides still use this; modern bras fit better with the rounded-even method.
The +4 method is a relic of an earlier era when undergarments were cut looser and bands contained more stretchy material. Modern bras use firmer, more elastic bands that are meant to sit snugly on the ribcage, so adding four inches generally produces a band that rides up and fails to support — which is why most modern fitters now recommend the rounded-even approach.
Step 2 — the cup
The cup is the difference between bust and band:
| Difference | Cup |
|---|---|
| < 1" | AA |
| 1" | A |
| 2" | B |
| 3" | C |
| 4" | D |
| 5" | DD |
| 6" | DDD / F |
| 7"+ | G and beyond |
Worked example
Underbust 31", bust 36", modern method:
Band = 32
Difference = 36 − 32 = 4" → D cup
Bra size = 32D
Sister sizes
Sizes shift together: 32D ≈ 34C ≈ 30DD. If a 32D is snug in the band but loose in the cup, try 34C. Fit is king — treat any calculated size as a starting point, and get fitted in person when you can. Because so much depends on the cut and fabric of a specific brand, the on-paper size is a launch pad, not a guarantee — the ultimate test is how the bra feels when worn.
helpFrequently asked questions
question_markHow do I calculate my bra size?
Round your underbust to the nearest even number for the band, subtract it from your bust measurement, and map the difference to a cup letter (1" = A, 2" = B, 3" = C, 4" = D).
question_markWhat is the difference between the modern and +4 methods?
The modern method rounds the underbust to the nearest even number. The traditional +4 method adds 4–5 inches to the underbust. Modern bras generally fit better with the modern method.
question_markWhat are sister sizes?
Sister sizes are adjacent sizes that fit the same cup volume — 32D, 34C and 30DD all hold the same cup. If one size is snug in the band, try a sister size.
question_markWhy does my size differ between brands?
Sizing varies by brand and country (US vs UK vs EU scales differ). Use this calculator as a starting point and always check each brand’s fit guide.