For informational purposes only. This calculator provides a starting size estimate based on standard measurement formulas used by major lingerie brands worldwide. Results are a starting point — actual fit varies by brand, style, and body shape. We recommend trying on bras or getting a professional fitting at a lingerie store. Formulas are based on research from OmniCalculator, Calculator.net, and guidelines from Victoria's Secret and Glamorise. Your measurements are never stored or shared.
Enter Your Measurements
Under-Bust
in
Snugly around your ribcage, directly under your bust
Full Bust
in
Loosely around the fullest part of your bust
Please enter both measurements.
Your Results
International Sizes
Sister Sizes
— same cup volume, different band
Remember: This is your starting size. Bra fit varies by brand, style, and body shape. Always try on bras and use the 7-point fit checklist on the right. Sources: OmniCalculator · Calculator.net
How to Measure
1
Get a soft measuring tape
Use a flexible fabric tape measure. Measure braless or wearing a non-padded bra for accuracy. Stand straight with arms at your sides.
2
Measure your under-bust (band size)
Wrap the tape snugly — but not tightly — directly under your bust, where a bra band sits. Keep it level and parallel to the floor. Exhale and note the number. Round to the nearest whole number.
3
Measure your full bust (cup size)
Wrap the tape loosely around the fullest part of your bust (across the nipples), keeping it parallel to the floor. Don't compress your breasts. Note the number.
4
Enter measurements above
Choose inches or centimeters, enter both numbers, and click Calculate. Your size in US, UK, EU, FR, and AU sizing appears instantly.
Put on your bra and check each point. A bra that passes all 7 is the right fit.
✓
Center gore lies flat
The panel between the cups should press flat against your sternum. If it floats away, your cups are too small — try a larger cup.
✓
Band is level all around
The back band should be parallel to the floor. If it rides up at the back, the band is too large — go down a band size.
✓
Two-finger band rule
You should be able to slip two fingers under the back band — not three or more (too loose) and not struggle to fit one (too tight).
✓
Cups fully contain breast tissue
No spillage over the top or sides (cup too small) and no gaps or wrinkling (cup too large). Cups should be smooth against your body.
✓
Straps don't dig or slip
Straps should be snug but comfortable — not digging into shoulders (too tight) or sliding off (too loose). The band does 80% of the support work.
✓
No red marks after wearing
Red marks from underwire or band indicate the cup is too small or the band too tight. Persistent marks after 30 minutes mean a poor fit.
✓
New bra fastens on loosest hook
A new bra should fasten comfortably on the loosest hook. As fabric stretches with wear, you'll tighten to the next hook. If you need the tightest hook immediately, the bra band is already too large.
How to Find Your Bra Size — The Complete Research-Backed Guide
Studies and surveys consistently show that 70–85% of women wear the wrong bra size — usually a band that is too large and cups that are too small. This is partly due to outdated sizing methods still taught in many stores, and partly because bra sizing varies significantly between countries and brands. Our calculator uses the modern measurement method recommended by leading lingerie brands and validated by independent size experts.
Why This Calculator Is Ethical & Trustworthy
Bra size is a health and wellness matter, not just a fashion concern. A poorly fitting bra can cause back pain, neck pain, shoulder grooving, and poor posture. Our tool is built on publicly verified formulas from credible sources:
1OmniCalculator Bra Size Calculator — peer-reviewed formula source citing Owen C., Pike N., Chokrane B., How To Actually Measure Your Bra Size For a Perfect Fit, Vogue, 2024. omnicalculator.com ↗
2Calculator.net Bra Size Calculator — US, UK, EU, FR, AU international sizing tables. Formula: Cup size = bust − band difference. calculator.net ↗
3Glamorise Fit Resources — 100-year lingerie brand measurement and fitting guide. glamorise.com ↗
4Lane Bryant / Cacique Bra Fit Guide — Band and cup measurement instructions with size chart. lanebryant.com ↗
5Victoria's Secret Bra Size Guide — Measurement method and fit video guide. victoriassecret.com ↗
6University of Portsmouth Research Group in Breast Health — Cited by MagicBra International: inconsistent bra fit is a leading cause of breast discomfort globally, traced to measurement errors. Reference ↗
7BraCalculator.net — Modern band-size formula (no +4 add method) validated against current elastic-band bra construction. brasizecalculator.com ↗
International Bra Size Conversion Chart
Bra sizes are not standardised globally. The same body can have 5 different size labels depending on the country. Use this chart when shopping internationally:
Sister sizes are bra sizes that share the same cup volume but use a different band length. This is because cup volume changes with band size — a larger band holds more volume per cup letter.
Your size: 34C — Sister sizes are 32D (tighter band, same cup volume) and 36B (looser band, same cup volume)
Use when: your exact size is sold out, or the band fits but not the cup, or vice versa
Rule: Go one band size up → go one cup letter down. Go one band size down → go one cup letter up.
Many older fitting guides (and some department stores) still teach the "+4 rule": add 4 inches to your underbust to get your band size. This rule was created in the 1940s when bras were made from rigid, non-elastic fabric that needed extra room to close.
Modern bras use stretch fabric and elastic. They are designed to fit snugly at your actual underbust measurement. Using the +4 rule puts most women in a band 2–3 sizes too large, causing:
The band to ride up at the back (providing no support)
Straps to carry all the weight, digging into shoulders
Back pain, neck pain, and discomfort
Cups that appear to fit but are actually the wrong size
Our calculator uses the modern direct measurement method: your actual underbust measurement rounded to the nearest even number = your band size. Source: The Saraya Store Guide · UnitTables Bra Size Converter
Frequently Asked Questions
How accurate is this bra size calculator?
This calculator provides a research-backed starting size based on the modern measurement method used by major brands. It is as accurate as any online calculator can be — but bra fit ultimately depends on brand, style, fabric, and individual body shape. Always try on bras using your calculated size as a starting point, then adjust using the fit checklist and sister sizes.
Are my measurements stored or shared?
Never. All calculations run locally in your browser using JavaScript. Your measurements are never sent to any server, stored, or shared with any third party. See our Zero Data Storage Policy.
Why does my bra size differ between brands?
There is no global standard for bra sizing. Each brand grades its patterns independently, so a 34C from one brand may fit differently than a 34C from another. This is why the calculator gives you a starting size — you may need to try a sister size or adjust by one cup letter depending on the brand.
Can my bra size change over time?
Yes. Bra size can change due to weight fluctuations, hormonal changes, pregnancy, breastfeeding, menopause, and natural aging. Experts recommend re-measuring every 6–12 months, or whenever your current bras start feeling uncomfortable. Source: BraCalculator.net
What is the difference between US and UK cup sizes?
US and UK bra sizes use the same band numbers but their cup letters diverge above D. US goes D → DD → DDD → DDDD, while UK goes D → DD → E → F → FF → G. So US DD = UK DD, but US DDD = UK E, and US G = UK F. When shopping UK brands for larger cup sizes, always check the cup letter column. Source: HauteFlair International Conversion
Should I get a professional bra fitting?
Yes, whenever possible. A professional fitting at a lingerie store accounts for breast shape, projection, tissue placement, and preference in a way no calculator can. Many stores including Victoria's Secret, Lane Bryant, Nordstrom, and independent boutiques offer free fittings. Our calculator is a convenient starting point — a professional fitting will always be more precise.