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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.
Calculation details
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What does this calculator estimate?
Bra size calculator that converts your band and bust measurements into sister sizes across US, UK, and EU charts. Free, runs in your browser, no signup.
- A 34C, 32D, and 36B hold the same cup volume — cup letter alone means nothing without the band.
- 80% of support should come from the band; it should be level and snug on the loosest hook.
- UK sizing letters run differently above D (DD/E vs US D/DD) — UK labels use the double-letter ladder.
How this calculator works
Formula
Band = underbust rounded to even; cup = bust minus band, 1 inch per letter (1=A, 2=B, 3=C...). Sister sizes shift band and cup in opposite directions (34C = 32D = 36B) because cup size is relative to band.
Sources
Sources support the method, terminology, or planning context. Editable assumptions remain your responsibility.
Assumptions to verify
- Manufacturer charts vary; conversion outputs are the standard-mapping midpoint.
- Measurements are body measurements unless the tool labels them as garment measurements.
- Between-size results round per the rule stated in the FAQ.
- This tool provides general estimates for planning and education only; it is not medical advice and not a substitute for a qualified healthcare professional.
- Always consult your doctor or another healthcare provider about your specific situation.
Frequently asked questions
Why does the calculator give a different size than the store fitted me?
Store fittings often add +4 inches to the underbust, an obsolete convention from 1930s construction. Modern sizing uses the actual underbust rounded to even, which typically lands 1-2 band sizes smaller with a bigger cup letter.
What if I fall between sizes?
Round the direction that matches use: clothing and shoes generally round up.
Cite this tool
BoringToolsKit. “Bra Size Calculator.” boringtoolskit.com/bra-size-calculator/ (reviewed 2026-09-20). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Results are planning estimates based on your inputs; verify assumptions and current rules before making decisions.