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Bra Size Calculator & Sister Size Chart
Find your true bra size with modern direct measurements and sister sizes.
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About Bra Size Calculator & Sister Size Chart
Accurate bra size calculator based on modern snug underbust measurements (no +4 band sizing). Includes sister sizes (same cup volume), international conversions (US, UK, EU, AU, FR), and fit troubleshooting.
Why do over 80% of women wear the incorrect bra size?
Most commercial retail stores still use the obsolete "+4 sizing system" invented in the 1930s when bra fabrics lacked elastane/spandex. This outdated method adds 4 inches to your underbust, forcing women into bands that are too loose (e.g. 36B) and cups that are too small, causing back pain, slipping straps, and riding-up bands.
What are sister sizes and how do they work?
Sister sizes are bra sizes that share the exact same cup volume, but with different band circumferences. If a 34D bra fits in the cup but feels too loose in the ribcage, sister sizing down to a 32DD gives you the identical cup volume with a firmer, more supportive band.
How should a properly fitted bra feel?
80% to 90% of breast support must come from the snug underbust band sitting level across your ribcage, not the shoulder straps. The center underwire (gore) should lay flat against your sternum without resting on breast tissue.
📐 Methodology & Reference Accuracy:
Calculations, formulas, and conversions execute locally in your web browser. Output models are benchmarked against official reference standards and updated for 2026.
🔒 This tool runs entirely in your browser. Nothing is uploaded or stored.
Background reading: Bra Size Calculator on Wikipedia. More: all calculators.
Frequently Asked Questions about Bra Size Calculator & Sister Size Chart
What is the sister size for a 34C?
The tighter sister size is 32D (one band down, one cup up). The looser sister size is 36B (one band up, one cup down). All three hold the exact same volume of breast tissue.
Why does my bra band ride up in the back?
If the band arches up toward your shoulder blades, your band size is too large. You should try going down 1 or 2 band sizes and going up in cup size.
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Finding the right bra size is not simply a matter of choosing a number and letter from a chart. Your size depends on two main measurements: your underbust…
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