Bra Size Calculator

Introduction

A woman walks into a department store and asks for a fitting. The fitter measures her over a padded bra, adds four inches to her underbust, and sends her home with a 36B. She wears it for years, assuming the discomfort is normal. It is not. The band rides up, the straps dig in, and the cups gap at the top. She is one of the estimated 80% of women wearing the wrong bra size.

A 2025 peer-reviewed study published in the Journal of Ergonomics found that every single participant failed an objective bra fit assessment, with 77 fit issues identified across just 24 women. A separate report from 10 months of professional fittings in East Sussex found that only 6% of women were wearing their correct size. Over 70% needed a larger cup, and 50% needed a smaller band. This calculator helps you find your actual size using the modern measurement method, without the outdated add-inches approach that many stores still use.

Inputs Required

• Bust Measurement: The circumference around the fullest part of your chest
• Underbust Measurement: The circumference snugly around your ribcage just below the bust
• Unit: Inches or centimeters

Outputs Provided

• US Bra Size: Band size (number) and cup letter (A, B, C, D, etc.)
• UK / Australian Size: Equivalent in the UK sizing system
• EU Size: European numeric size equivalent
• Sister Sizes: Alternative sizes with the same cup volume

How the Calculation Works

Bra sizing has two components: the band and the cup. The band is based on your underbust measurement rounded to the nearest even number. The cup is determined by the difference between your bust and underbust measurements.

For example, if your underbust is 33 inches and your bust is 37 inches, the difference is 4 inches, which corresponds to a D cup. The band rounds up to 34. Your size is 34D.

The older method of adding 4 or 5 inches to the underbust measurement is outdated. It was developed in the 1930s when bra fabrics had no stretch and bands needed to be larger to compensate. Modern bras use elastic materials that provide their own give. The 2025 Journal of Ergonomics study specifically noted that self-measurement and selection methods reduce the likelihood of achieving correct fit, which is why using accurate measurements with a modern formula matters.

How to Use the Calculator

1. Select your preferred unit (inches or centimeters)
2. Measure around the fullest part of your chest and enter the bust measurement
3. Measure snugly around your ribcage just below the bust and enter the underbust measurement
4. View your recommended size and international equivalents

For best results, measure without a bra or with a non-padded, well-fitting bra. Measure in front of a mirror to ensure the tape is level across your back. If you need help converting other measurements, try our Shoe Size Conversion tool or Height Calculator.

Example Calculations

Example 1: Common Size

• Bust: 38 inches
• Underbust: 33 inches
• Difference: 5 inches = DD cup
• Band: 34
• Result: 34DD (US), 34DD (UK), 75DD (EU)

The average US bra size is 34DD according to recent sales data. If this person had been fitted using the old add-4 method, her band would be 38 and her cup would be C, giving 38C. That is a completely different fit. The band would be too loose and the cup too small.

Example 2: Smaller Frame

• Bust: 34 inches
• Underbust: 28 inches
• Difference: 6 inches = DDD/F cup
• Band: 28
• Result: 28F (US), 28F (UK), 60F (EU)

This size is harder to find in mainstream stores, which often start at band size 32. The sister sizes (30E and 32DD) may be more readily available while providing a similar cup volume with a slightly different band fit.

Real-World Scenarios

Online Shopping from a European Brand

Rachel, a 29-year-old marketing manager in New York, wants to buy a bra from a French lingerie brand. She knows her US size is 34DD but the brand's site uses EU sizing. The calculator converts her to 75DD instantly. Without this conversion, she might have ordered a 90DD, which is the EU equivalent of a 40 band, two sizes too large. The EU band size is typically 15 to 16 cm more than the US band size in centimeters.

Post-Pregnancy Remeasurement

Aisha, a 34-year-old mother in London, had her second child four months ago. Her pre-pregnancy size was 36C. She has been wearing the same bras but they are uncomfortable and the cups overflow. She measures herself and finds she is now a 34E. Her ribcage narrowed slightly while her cup size increased significantly. The 2025 East Sussex fitting report found that women in 40+ band sizes experienced the largest fitting corrections, but post-pregnancy changes affect all sizes.

Sports Bra Selection for a Runner

Megan, a 26-year-old marathon runner in Portland, needs a high-impact sports bra. She has been wearing a 36C sports bra from a big-box store that uses the add-4 method. After measuring herself, she finds she is actually a 32DD. The smaller band provides much better support during running because the band carries most of the weight, not the straps. The sister size feature shows her that 30E and 34D are alternatives if 32DD is not available in the brand she wants.

Understanding Sister Sizes

Sister sizes are bra sizes that share the same cup volume but have different band sizes. For example, 34D, 36C, and 32DD are sister sizes. If a 34D feels tight in the band but fits in the cup, try a 36C. If the band fits but the cup is too big, try a 32DD. Understanding sister sizes helps when your exact size is out of stock or when a particular brand's sizing runs small or large.

The general rule for sister sizes: go up one band size and down one cup size, or down one band size and up one cup size. The cup volume stays approximately the same because the difference between bust and underbust changes proportionally.

Common Mistakes to Avoid

• Measuring over a padded bra: Padding adds volume and produces an inaccurate bust measurement. Measure without a bra or in an unpadded bra.
• Pulling the tape too tight or too loose: The underbust should be snug but not compressing. The bust should be measured with the tape relaxed and level. A 1 inch error in the underbust changes both the band size and the cup calculation.
• Using the outdated add-inches method: The older method of adding 4 or 5 inches to the underbust is no longer the standard. It produces a band that is too large and a cup that is too small. This is one of the main reasons so many women wear the wrong size.
• Ignoring brand variation: Sizing varies between manufacturers. A 34D in one brand may fit like a 34DD in another. Use this calculator as a starting point and try sizes on when possible.
• Not remeasuring after body changes: Weight gain or loss, pregnancy, breastfeeding, and hormonal changes all affect bra size. The 2025 study found that women's self-assessment of fit matched objective findings only 51% of the time, meaning you cannot rely on how a bra feels if your size has changed without you realizing it.

Limitations of This Calculator

This calculator uses the standard US measurement method and provides conversions to UK and EU systems. It does not account for breast shape (shallow versus projected), breast asymmetry (which affects approximately 50% of women), or the firmness of breast tissue. Two women with the same measurements can wear different sizes depending on these factors. Brand-specific sizing variations can also shift your size by a full cup. This tool gives you a starting point. Trying on bras in your calculated size and adjusting based on fit is still necessary. This calculator does not replace a professional bra fitting.

Authoritative Research and Resources

• Highlighting the Need for Change Through an Analysis of Bra Fit Quality (Journal of Ergonomics, 2025) - A peer-reviewed study of 24 women that objectively assessed bra fit and found all participants failed the assessment, with 77 fit issues identified. The study highlights the gap between subjective and objective bra fit and calls for educational interventions.
• The Great Bra Sizing Mismatch Report (The Braologist, 2025) - An independent report based on 10 months of professional bra fittings in East Sussex, finding only 6% of women wore the correct size. Over 70% needed a larger cup and 50% needed a smaller band.
• Analysis on the Difference Between Practical Brassiere Size and Real Breast Volume (Plastic and Reconstructive Surgery Global Open, 2023) - A clinical study of 163 women showing only 56.4% wore the correct cup size, with larger-breasted women more likely to wear incorrectly sized bras.

For other size conversion tools, try our Shoe Size Conversion calculator or Height Calculator.