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How Bra Size Calculator (US, UK, EU & AU) works
The Bra Size Calculator uses modern fitting standards to calculate your recommended band size and cup size across four international sizing systems: US, UK, EU (continental European), and AU/NZ (Australian/New Zealand). Unlike legacy methods that added 4–5 inches to the underbust measurement, modern fitting standards use the actual snug underbust measurement to determine band size, providing a starting point for trying garments.
The calculation requires three measurements taken at specific body positions: the snug underbust (firm but not tight tape directly below the bust), the loose underbust (relaxed tape at the same position), and the fullest standing bust measurement (tape parallel to the floor at the fullest point). The difference between the bust and the underbust determines the cup volume: each inch of difference corresponds to one cup size letter.
Beyond your primary size, the calculator also recommends "sister sizes" — adjacent combinations of a larger band/smaller cup and a smaller band/larger cup — that share the same cup volume and may offer better fit, comfort, or availability.
How to use Bra Size Calculator (US, UK, EU & AU)
1. Select your measurement unit
Toggle between inches and centimetres. All input fields and results update automatically when you switch units.
2. Take and enter your snug underbust
Wrap the tape measure firmly (but not painfully tight) directly under your bust, at the base of your ribcage. Enter this measurement.
3. Take and enter your loose underbust
Take the same measurement with the tape relaxed (2–3 cm of slack). This is used to cross-reference the best band size for your ribcage.
4. Take and enter your fullest bust
Measure around the fullest part of your bust, keeping the tape parallel to the floor. Stand upright and relaxed. Enter this measurement.
5. View your size and sister sizes
Review your recommended primary size across US, UK, EU, and AU systems, plus sister size recommendations for alternative fit options.
Key features and technical specifications
Modern Fitting Standards
Uses contemporary underbust-direct measurement method (not the outdated +4/+5 inch method) for significantly improved accuracy.
US, UK, EU & AU/NZ Sizing
Converts your measurements to all four major international sizing systems simultaneously so you can shop globally without confusion.
Sister Size Recommendations
Recommends adjacent sister sizes (larger band/smaller cup and smaller band/larger cup) that share the same cup volume for fit flexibility.
Units Toggle (in / cm)
All measurements accept both imperial inches and metric centimetres with seamless conversion.
100% Private Calculation
No body measurements or personal data leave your browser. Calculations run entirely in local JavaScript.
Use measurements as a fitting starting point
Measure with a level tape and compare the result with the specific brand’s size chart. Regional conversions are approximate because cup progressions and garment construction vary. Sister sizes change the band while approximating cup volume; they do not guarantee equal comfort or support. Check the band, cups and straps together when trying a garment.
Finding your starting bra size
Determine an accurate starting size for bra shopping, especially when switching brands or sizing systems.
Shopping internationally or online
Convert between US, UK, EU, and AU sizing systems when shopping from international retailers or using size conversion charts.
Exploring sister sizes for better fit
Find equivalent cup-volume alternatives if your primary size is unavailable, overly expensive, or uncomfortable in a particular brand or style.
Frequently asked questions
What is a sister size?
Sister sizes are bra sizes that have the same cup volume but different band and cup letter combinations. Going one band size up while going one cup letter down (e.g., 34C → 36B), or one band size down while going one cup letter up (e.g., 34C → 32D), gives you the same amount of cup space. Sister sizes are useful when your true size is unavailable or when you need more or less band support.
What is the difference between US and UK sizing?
US and UK labels often share the numerical band but differ in cup progression, especially above D. The interface uses one conventional conversion table: for example, a 34 band maps to approximately EU 75 and AU/NZ 12. Brand charts can differ, so compare the specific garment’s chart and fit. A regional label conversion is not an inch-to-centimeter conversion.
Why do my measurements give different results across brands?
Bra sizing is not standardised across manufacturers. The same nominal size (e.g., 34C) can vary substantially in actual cup volume, band stretch, and shape between brands. Use your calculated size as a starting point and expect to try 1–2 sister sizes when exploring a new brand. Online communities like r/ABraThatFits on Reddit have extensive brand-specific fitting guides.
Should I measure while wearing a bra?
No — always measure without a bra or while wearing a non-padded bralette to get accurate underbust and bust measurements. Padded cups inflate the bust measurement and produce an inaccurate cup size result.
Do I need an internet connection, and are my inputs uploaded?
An internet connection is required to open tools and refresh a temporary session. Processing stays on your device; the handshake sends a random challenge, not files or text inputs. Libraries, fonts or models may download. Local processing cannot remove risks from an untrusted device or extension.
Further reading
These references explain the topic and its limitations. They do not endorse or clinically validate this calculator.