Bra Size Calculator
Find your bra size from underbust and bust measurements in cm — band, cup and full size, plus sister sizes that fit.
Measure snugly around the ribcage, just under the bust.
Measure around the fullest part of the bust.
Suggested size
30D
Band size
30 in
Cup size
D
Bust difference
3.94 in
Difference between bust and band
Sizing is a guide — every brand fits differently. The band size is the underbust rounded to the nearest even number of inches; the cup is based on the difference between bust and underbust.
Last updated: February 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Use this calculator when shopping for bras online or in Indian retail stores, where sizes follow the international band-and-cup system and trying on is often not possible before buying. A correct band measurement prevents the two most common fitting mistakes — bands that ride up and straps that dig in. The sister-size note helps when the exact size is out of stock, which is common in smaller towns, and when brands run slightly different fits. Parents helping teenagers with a first proper fitting, and women whose size changed after pregnancy or weight change, both benefit from the objective numbers instead of guessing. Measure in cm, enter the numbers, and walk into the store knowing two sizes to try.
How to Use This Calculator
- Step 1: Measure the underbust snugly around the ribcage, right under the bust, keeping the tape level.
- Step 2: Measure the bust around the fullest part of the chest, again with the tape level.
- Step 3: Enter both measurements in cm.
- Step 4: Read the band size, cup size and the combined size, then try it on with a sister size as backup.
Worked Example
With an 81 cm underbust and a 97 cm bust, the band size is 81 ÷ 2.54 = 31.9 inches, which rounds to 32. The difference between bust and underbust is 16 cm, or 6.3 inches, which rounds to a 6-inch cup difference — an F cup. The suggested size is therefore 32F. If it feels tight, the sister size 34E fits nearly the same volume with a looser band; if loose, 30G is the sister down.
Tips and Common Mistakes
- •Tip 1: Measure in a regular, unpadded bra or none at all — padding inflates the bust reading.
- •Tip 2: The band does the real support: it should sit level and snug, not ride up at the back.
- •Tip 3: Recheck every 6–12 months; weight and lifestyle changes shift sizes more often than expected.
- ✗Mistake 1: Do not add inches to the band — modern sizing uses the raw underbust measurement.
- ✗Mistake 2: Do not pick cup size from bust alone; the difference from the underbust is what decides the cup.
Frequently Asked Questions
How is bra size calculated?
Band size is the underbust in cm divided by 2.54, rounded to the nearest even inch. Cup is the difference between bust and underbust in inches: about 1 inch = A, 2 = B, 3 = C, 4 = D and so on.
Why do band sizes come in even numbers?
International sizing uses even band numbers (30, 32, 34, 36). An 81 cm underbust is 31.9 inches, which rounds to a 32 band. Indian brands generally follow this international scale.
My measurements give a size that doesn't fit — why?
Sizing varies between brands and styles, and sister sizes fit similarly (e.g. 32D ≈ 34C). Use the result as a starting point, then try both your calculated size and one band size smaller.
How do I measure correctly?
Measure the underbust snugly around the ribcage, and the bust around the fullest part, keeping the tape level. Measure in cm without a padded bra for the most consistent result.
How often should I remeasure?
Weight changes, pregnancy and age shift measurements. Remeasure every 6–12 months — most people wear a different size than they think, and an ill-fitting band does most of the support work.
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