Bra Size Calculator
Two measurements — underbust and bust — give your size in US, UK, EU and French sizing, plus your sister sizes. Uses the measurement method, not the old “add 4 inches” rule that gets the band too big and the cup too small.
Enter your measurements
How to measure
Underbust (band)
Wrap the tape snugly around your ribcage directly under the bust, level all the way round. Breathe out. Or use our printable tape.
Bust (cup)
Measure around the fullest part of the bust, standing relaxed, tape level and not compressing.
Read your size
The calculator does band and cup for you and shows US, UK, EU and French sizes at once.
Use sister sizes to fine-tune
If the band feels off but the cup is right, move to a sister size — same cup volume, different band.
Flexible body tape for an accurate underbust and bust measurement.
View on Amazon →Add a little room to the band without changing the cup.
View on Amazon →A smooth, supportive everyday bra once you know your size.
View on Amazon →How this is calculated
We use the measurement method, not the old +4/+5 rule. The band is your snug underbust measurement in inches (to the nearest even number); the cup is the difference between your standing bust measurement and the band, one cup per inch. US cup lettering above D varies by brand (DD vs E, DDD vs F), so treat letters as a guide and use sister sizes to fine-tune the band while keeping the same cup volume.
Sources: Measurement method per r/ABraThatFits calculator methodology (band = underbust, cup = bust − band); Standard international band scale (US/UK inches → EU/FR), cross-checked retailer charts 2026; Cup-letter divergence US vs UK vs EU documented on major lingerie size charts (2026).
Frequently asked questions
How do I measure my bra size at home?
Measure your underbust (ribcage, snug) and your bust (fullest part, relaxed). Your band is the underbust in inches to the nearest even number; your cup is the difference between bust and band, one cup per inch.
Why shouldn't I add 4 inches to my band size?
The old +4 method came from a time of stiff, non-stretch fabrics. With modern bras it gives a band that's too loose and a cup that's too small. Use your actual snug underbust measurement as the band.
What is a sister size?
A sister size has the same cup volume in a different band — go down a band and up a cup, or up a band and down a cup. For example 34D, 32DD and 36C all share the same cups.
Why do US cup letters differ from UK?
Above a D cup the lettering diverges: US doubles letters (DD, DDD) while the UK inserts (DD, E, F, FF) and the EU runs straight through (D, E, F, G). The calculator shows all systems so you can match correctly.