Bra Size Calculator

Estimate band and cup size from underbust and bust measurements.

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Unit

Underbust

Bust

Result

Enter values to calculate.

Summary

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Detail

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Estimate your band and cup size from underbust and bust measurements, in inches or centimeters.

How It Works

How Bra Size Calculator Works

The band size is based on your underbust measurement, rounded to the nearest whole number and adjusted up to the nearest even number, since band sizes are conventionally sold in even sizes.

The cup size comes from the difference between your bust and underbust measurements: each full inch of difference corresponds to one cup size up the scale (AA, A, B, C, D, DD, DDD, and beyond), rounded to the nearest inch.

Worked Example

See It In Action

An underbust measurement of 32 inches and a bust measurement of 36 inches gives a 4-inch difference, resulting in an estimated size of 32D.

Real-World Use Cases

Who Uses Bra Size Calculator and Why

Getting a starting size estimate before shopping online, when you can't try garments on in person first.
Re-measuring after a body change (weight change, pregnancy, postpartum) to check whether your usual size still matches the calculator's estimate.
Understanding why two bras labeled with different numbers (like 32D and 34C) might fit similarly, via the sister-sizing explanation in the FAQ.
Getting a rough estimate before a professional fitting, to have a starting point to compare against.

Common Mistakes

Mistakes to Avoid

Measuring underbust over a padded or ill-fitting bra rather than directly against skin or over a well-fitting non-padded bra, which throws off both the band and cup calculation.
Letting the tape measure sag or angle instead of keeping it level and parallel to the floor for both measurements — this is called out directly in the calculator's own guidance since it commonly skews results.
Assuming the calculated size will match exactly across every brand — the calculator itself notes that sizing varies noticeably between manufacturers and countries.

Pro Tips

Tips for Best Results

Take both measurements in one sitting, back-to-back, so posture and breathing don't shift between the underbust and bust readings.
If your result feels off when you try it on, try a sister size (one band size up or down with the opposite cup adjustment) before assuming the calculator was wrong — it's a very common real-world fit issue.

Troubleshooting

Fixing Common Problems

The size I got doesn't match what fits me in a store. — Try the sister-size adjustment first (band size ±1 with cup adjusted in the opposite direction), and remember sizing charts vary by brand and country — this estimate is a starting point, not a guarantee for every manufacturer.

Glossary

Terms Explained

Sister size: An alternate size with the same cup volume as your calculated size, found by moving the band size up or down one step while adjusting the cup letter in the opposite direction.

Band size: The number in a bra size, based on underbust circumference, rounded to the nearest even number to match standard manufacturing sizes.

FAQ

Frequently Asked Questions

Why does the band size round to an even number?

Bra bands are conventionally manufactured and sold in even sizes (32, 34, 36, and so on), so the calculator rounds your measurement to the nearest even number to match what's actually available in stores.

How should I take these measurements accurately?

Measure underbust snugly around your ribcage directly under the bust, and measure bust around the fullest part while wearing a well-fitting, non-padded bra, keeping the tape level and parallel to the floor for both.

Why might my actual size differ from this estimate?

Sizing varies noticeably between brands and countries, and this formula is a general approximation — always try a size on or check a specific brand's size chart before buying, especially online.

What is "sister sizing"?

If a size doesn't fit quite right, a sister size (moving one band size up or down while adjusting the cup size in the opposite direction) often gives the same cup volume with a different band fit — worth trying if your first estimate feels off.