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Bra Size Calculator

Find your US bra size from your underbust and bust measurements, with the modern and traditional band methods and your sister sizes.

Results

ItemValue

Sister sizes

SizeFit
Bra sizes are not standard. Brands use different charts and cup letters, so treat the result as a starting point and try bras on. Many US brands go D, DD, DDD, then G, but some use E and F instead.

Quick Answer

Band size = underbust rounded to the nearest even inch, and cup size = bust − band, with each inch one cup letter (1 = A, 2 = B, 3 = C, 4 = D, 5 = DD). An underbust of 31.5 in and a bust of 36 in gives a 32D with the modern method, or a 36AA with the traditional add-4 method.

Last updated: 2 November 2026 ·How we calculate this: the US inch-based band and cup method

Table of Contents

How This Bra Size Calculator Works

Measure snugly around your ribcage just under the bust, then loosely around the fullest part of your bust. Enter both numbers in inches or centimeters and choose a band method. The calculator finds your band size, subtracts it from your bust, turns the difference into a US cup letter, and lists your sister sizes.

Quick explanation: each inch between bust and band is one cup size.

  • Band: the number, from your underbust.
  • Cup: the letter, from the bust and band difference.
  • Sister size: a size with the same cup volume on a different band.

How to Use the Bra Size Calculator

  1. Measure your underbust snugly, just under your bust.
  2. Measure your bust loosely at the fullest part.
  3. Choose inches or centimeters.
  4. Choose the modern or traditional band method.
  5. Press Calculate and read your size and sister sizes.

Core Formulas

  • Modern band: underbust rounded to the nearest even number
  • Traditional band: underbust + 4 if even, + 5 if odd
  • Cup difference: bust − band, rounded to a whole inch
  • US cup letters: 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD, 6 = DDD, 7 = G, 8 = H
  • Sister sizes: band + 2 and cup − 1, or band − 2 and cup + 1

Key Terms

  • Band size: the number in a bra size, based on the ribcage measurement.
  • Cup size: the letter in a bra size, based on bust minus band.
  • Underbust: the measurement around the ribcage just below the bust.
  • Sister size: a bra size with the same cup volume on a different band.
  • Gore: the center panel between the cups.

Worked Example

Your underbust is 31.5 in and your bust is 36 in.

  • Modern band: 31.5 rounds to the even number 32
  • Difference: 36 − 32 = 4 inches, which is cup D
  • Size: 32D, with sister sizes 30DD and 34C

Real-Life Examples

  • Shopping online: start with your calculated size, then check the brand's own chart.
  • A loose band: try the sister size with a smaller band, such as 32D instead of 34C.
  • Gaping cups: try one cup size down on the same band.
  • Metric sizes: enter centimeters. For example, 80 cm and 92 cm gives a 32D (US).

Modern vs Traditional Band Method

  • Modern: uses your real underbust size, rounded to an even number. Many fitters prefer it.
  • Traditional: adds 4 or 5 inches, which makes the band larger and the cup smaller.
  • Which to use: try the modern size first. If the band feels too tight, try the traditional result.

US Cup Sizes by Inch Difference

Many US brands use this order. Some use E and F after DD instead.

Bust − bandUS cupBust − bandUS cup
Less than 1 inAA5 inDD
1 inA6 inDDD
2 inB7 inG
3 inC8 inH
4 inD9 inI

Tips for Measuring

  • Wear a light, unpadded bra or none, so the tape sits close to your body.
  • Keep the tape level all the way around your back.
  • Underbust snug: firm but not so tight that it digs in.
  • Bust loose: the tape should rest on the fullest part without pressing.

Common Mistakes to Avoid

  • Measuring over a padded bra. It adds inches to the bust.
  • Letting the tape slip down your back. Keep it level.
  • Thinking D is always large. Cup size depends on the band. 30D is smaller than 38D.
  • Trusting one chart for every brand. Sizes vary, so check each brand.
  • Ignoring fit signs. A riding-up band is usually too loose.

Frequently Asked Questions

Measure snugly under your bust for the band, and loosely around the fullest part for the bust. Enter both here.

Subtract the band size from the bust measurement. Each inch is one cup: 1 = A, 2 = B, 3 = C, 4 = D.

That is the traditional method. Many fitters now use the underbust as it is, rounded to an even number. You can compare both here.

Many US brands use DDD and then G, but some use E and F. Check the brand's chart.

Sizes with the same cup volume on a different band, such as 30D, 32C, 34B and 36A.

They are sister sizes with about the same cup volume, but the 36B has a looser band.

The band should be snug and level, the cups should not gape or spill, and the center should sit flat.

Bra sizes are not standard, so brands use different charts.

Yes. Choose centimeters, and the result is shown as a US size.

Measure again after weight change, pregnancy or if your bras stop fitting well.

Your bust and band differ by less than an inch. Try AA or A cups, or a smaller band.

It gives US sizes. UK sizes use the same band numbers but name larger cups differently.

References & Sources

SourceUsed ForLink
Wikipedia, Bra size Band and cup methods, US cup letters and sister sizes. https://en.wikipedia.org/wiki/Bra_size