Free Bra Size Calculator
Calculate your perfect bra size in seconds. Get your size in US, UK, EU, and international sizing systems, plus discover your sister sizes for the best fit. No signup required.
International Bra Size Systems
| Region | Band Range | Cup Range | Notes |
|---|---|---|---|
| US | 28-48 | AA-N | Most common in North America |
| UK | 28-48 | AA-K | Double letters after D (DD, E, FF, etc.) |
| EU | 60-125 | AA-N | Band sizes in centimeters |
| FR | 75-140 | AA-N | Add 15 to EU for French sizing |
| AU | 6-26 | AA-K | Uses dress sizing for bands |
| JP | 65-100 | AAA-M | Cup letter comes before band number |
Definition and Scope
A bra size calculator is a measurement-based tool that determines proper bra sizing by analyzing the relationship between underbust (band) and bust circumference measurements, converting these to standardized sizing systems used across different regions.
Applies To
- ✓Adult women seeking accurate bra sizing
- ✓Finding correct bra sizes when shopping internationally
- ✓Understanding sister sizes for better fit options
- ✓Converting between US, UK, EU, FR, AU, and Japanese sizing systems
Does NOT Apply To
- ✗Maternity or nursing bra sizing (temporary size changes)
- ✗Post-surgical compression garments
- ✗Medical breast health assessments
- ✗Children or adolescents (developing bodies)
Edge Cases
- ⚠Asymmetric breasts may require different approaches (fit to larger side)
- ⚠Athletic compression needs may differ from everyday sizing
- ⚠Weight fluctuations, hormonal changes, and aging affect sizing over time
Methodology
How Results Are Derived
- Collect two measurements: underbust (band) and bust circumference
- Round underbust measurement to nearest even number for US/UK band size
- Calculate the difference between bust and band measurements
- Convert inch difference to cup letter (each inch = one cup size)
- Apply regional conversion formulas for international sizes
Assumptions Made
- •Measurements are taken at correct anatomical positions
- •The measuring tape is level and parallel to the floor
- •The person is standing in a relaxed, upright posture
- •Measurements represent typical body state (not immediately post-exercise, etc.)
Simplifications Applied
- •Uses standardized conversion formulas that may vary slightly between brands
- •Does not account for breast shape, projection, or root width
- •Assumes symmetric breast sizing (fit to larger side recommended for asymmetry)
Uncertainty Declaration
Results are starting points for finding your ideal fit. Bra sizing varies between brands and styles. Sister sizes offer alternative fits with the same cup volume. Always try before purchasing when possible.
Limitations and Known Issues
Failure Scenarios
- !Inaccurate measurements lead to incorrect size recommendations
- !Measuring over thick clothing distorts results
- !Using a stretched or worn measuring tape affects accuracy
When Results May Mislead
- ⚠When comparing results with vintage or non-standard sizing
- ⚠For specialty bras (sports, strapless, nursing) that may size differently
- ⚠When brands use proprietary sizing that deviates from standards
Version Notes
Conversion formulas based on current industry standards. Regional sizing systems may occasionally be updated by manufacturers.
External References
Brassiere Measurement
Wikipedia
Our sizing methodology aligns with internationally recognized measurement standards
Evaluation of Professional Bra Fitting Criteria
Ergonomics Journal (via PubMed)
Academic research on bra fitting informs our measurement guidelines
How to Measure Your Bra Size
Get a soft measuring tape
Use a flexible fabric measuring tape. If you do not have one, use a piece of string and measure it against a ruler afterwards.
Wear an unpadded bra or no bra
For the most accurate measurements, wear a non-padded, non-molded bra, or measure without a bra. Avoid wearing thick clothing.
Measure your underbust
Wrap the tape snugly around your ribcage, directly under your bust. Keep it level and parallel to the floor. This is your band measurement.
Measure your bust
Wrap the tape around the fullest part of your bust, usually across the nipples. Keep the tape level but do not pull it tight—it should just touch your skin.
Note both measurements
Write down both numbers in inches or centimeters. Make sure to note which unit you used.
Enter into the calculator
Input your underbust and bust measurements, select your unit preference, and click Calculate to see your bra sizes in multiple regions plus sister sizes.
Frequently Asked Questions
How do I calculate my bra size at home?
To calculate your bra size, measure your underbust (directly under your bust, snugly around your ribcage) and your bust (at the fullest point). Enter these measurements into our calculator to get your bra size in US, UK, EU, and other international sizes, plus sister size alternatives.
What are sister sizes in bras?
Sister sizes are bra sizes with the same cup volume but different band sizes. For example, if you wear 34C, your sister sizes are 32D (smaller band, larger cup) and 36B (larger band, smaller cup). Sister sizes are helpful when your usual band feels too tight or loose but the cup fits well.
Why does my bra size differ between brands?
Bra sizes vary between brands due to different sizing standards, manufacturing tolerances, and style designs. A 34C in one brand may fit like a 32D or 36B in another. Use calculated sizes as starting points and try on bras when possible, or order multiple sizes when shopping online.
How do I convert my US bra size to UK or EU?
US and UK band sizes are the same (32, 34, 36, etc.), but cup letters differ after D. Our calculator automatically converts to UK, EU, FR, AU, and Japanese sizes. For EU/FR, band sizes are in centimeters (US 34 = EU 75). Use the conversion table provided with your results.
What is the most common bra size?
The most commonly purchased bra size in the US is 34DD, though many women wear incorrect sizes. Studies suggest the average true bra size is closer to 34DDD or 36D. Using a calculator and proper measurements often reveals a different size than what many women currently wear.
How tight should a bra band be?
A properly fitting bra band should feel snug when fastened on the loosest hook. You should be able to fit two fingers under the band, but it should not ride up your back. The band provides 80% of support, so it should be firm but comfortable. If it rides up, the band is too loose.
How often should I measure my bra size?
Measure your bra size every 6-12 months, or whenever you notice fit changes. Factors like weight fluctuations, hormonal changes, aging, pregnancy, and fitness routines can all affect your size. Many women find their bra size changes throughout their adult life.
Is this bra size calculator free to use?
Yes, our bra size calculator is completely free with no signup required. Your measurements are processed locally in your browser and are never stored on our servers, ensuring complete privacy. Calculate as many times as you need.
Explore More Fashion Tools
Discover your perfect style with our free fashion calculators and quizzes.