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.
A bra size calculator uses two measurements, underbust (band) and bust circumference, to determine your bra size across multiple sizing systems. In the US system, bra sizes combine a band number (28-48) with a cup letter (AA through N), where each inch of difference between bust and band equals one cup size. According to research published in the Ergonomics Journal, a significant percentage of women wear incorrectly fitted bras. Our free calculator covers 6 international sizing systems (US, UK, EU, FR, AU, and JP) and includes sister size alternatives for a better fit.
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 |
What Is a Bra Size Calculator?
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
How Our Calculator Works
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)
About Accuracy
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.
Important Notes About Results
What Can Affect Your Results
- !Inaccurate measurements lead to incorrect size recommendations
- !Measuring over thick clothing distorts results
- !Using a stretched or worn measuring tape affects accuracy
When to Try a Different Size
- ⚠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.
Research & Sources
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.
Common Bra Fitting Problems and Solutions
Band Rides Up in the Back
Cause: Band is too loose. Fix: Go down one band size and up one cup size (e.g., 36C → 34D). The band should sit level all the way around.
Cups Wrinkle or Gap
Cause: Cup is too large. Fix: Try a smaller cup size. If the cup gaps at the top, consider a demi or balconette style that provides a lower cut.
Spillage Over the Cups
Cause: Cup is too small. Fix: Go up one cup size. If you experience side spillage, look for bras with wider side panels or full-coverage cups.
Straps Dig In or Slip Off
Cause: Straps carry too much weight (band too loose) or are poorly positioned. Fix: Tighten the band first — straps should provide only 10-20% of support. For slipping straps, try a racerback or converter clip.
Underwire Pokes or Sits on Breast Tissue
Cause: Cup is too small or the wire shape does not match your breast shape. Fix: Go up one cup size. The underwire should sit flat against your ribcage and fully enclose the breast tissue.
Life Events That Change Your Bra Size
Your bra size is not fixed for life. These common life events can shift your measurements — re-measure after each one.
Weight Changes
Gaining or losing 5 kg (10+ lbs) typically affects both band and cup size. Breasts contain fatty tissue, so they respond to weight fluctuations.
Pregnancy and Breastfeeding
Breast size can increase 1-3 cup sizes during pregnancy and fluctuate further during breastfeeding. Re-measure at each trimester and after weaning.
Hormonal Changes
Starting or stopping hormonal contraception, perimenopause, and menopause all affect breast tissue density and size. Monthly cycle changes are normal too.
Fitness and Muscle Changes
Building chest and back muscles can change your band size. High-impact exercise may also reduce breast volume over time as fat decreases.
Aging
Ligament elasticity decreases with age, changing breast shape and position. Many women find they need a different cup shape (e.g., full-coverage instead of demi) as they age.
Surgery
Any breast or chest surgery (augmentation, reduction, mastectomy, reconstruction) requires a complete re-measurement. Consult your surgeon for fitting timing.
Beyond the Numbers: Shape and Brand Differences
Breast Shape Matters Too
Two people with identical measurements can need different bra styles. Breast shape, whether projected, shallow, wide-set, or close-set, affects how a bra fits even at the same calculated size. If your calculated size feels off in certain styles, try a different cup shape (full coverage, demi, or plunge) rather than changing the size itself.
Brand Sizing Varies
Bra sizing is not fully standardized across brands. Your calculated size is a reliable starting point, but expect some variation when shopping. Some brands run larger in the cup, others tighter in the band. When trying a new brand for the first time, consider ordering your calculated size along with one sister size to compare the fit.
Frequently Asked Questions
How do I calculate my bra size at home?
A bra size calculator is a free tool that determines your bra size from two measurements: underbust (band) and bust circumference. Measure your underbust snugly around your ribcage, then measure at the fullest point of your bust. Enter both numbers into the calculator above to get your size in US, UK, EU, and 3 additional international systems, plus sister size alternatives for a better fit.
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. For example, a US 34D equals UK 34D, EU 75D, FR 90D, and AU 12D. Use the conversion table provided with your results for your exact size.
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.
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