BraSizeChecker Methodology
We believe transparency builds trust. On this page, we outline the exact mathematical algorithms, measurement rounding rules, cup progressions, sister size logic, and international conversion standards used by BraSizeChecker.com to generate your sizing recommendations.
1. What the Calculator Uses
BraSizeChecker processes four primary inputs to determine your baseline starting size:
Underbust Measurement
Snug ribcage circumference directly beneath the bust (measured in inches or centimeters).
Bust Measurement
Loose circumference around the fullest point of the chest (measured in inches or centimeters).
Selected Unit System
Inches (in) or Centimeters (cm). Centimeter inputs are converted using 1 inch = 2.54 cm.
Sizing Convention
Primary baseline defaults to US/CA sizing, with secondary breakdowns for UK, EU, FR, AU, and JP.
2. Band Calculation Methodology
Bra bands are designed to provide 80% of a bra's structural support. BraSizeChecker uses the Modern Direct Underbust Sizing method for its primary US and UK band calculations:
US & UK Band Sizing (Modern Direct Method)
Your rounded underbust measurement in inches determines your band size. If your underbust is an odd integer, +1 inch is added to reach the nearest even band size (e.g., 31 inches → 32 band). Minimum supported band is 28.
US & UK Traditional (+4 Method) Reference
For secondary reference, the traditional +4 method is also displayed. If the rounded underbust is even, +4 is added; if odd, +5 is added (e.g., 30 underbust → 34 band).
European Band Sizing (EN 13402)
EU band sizes are calculated by rounding underbust centimeters to the nearest multiple of 5 (e.g., 73 cm → 75 EU band). Minimum EU band is 60.
French & Australian Band Sizing
French (FR/BE/ES) band sizes equal the EU band size +15 cm (e.g., 75 EU = 90 FR). Australian (AU/NZ) dress sizes equal 8 + (US Band - 30).
3. Cup Calculation Methodology
Cup size is calculated from the difference (Δ) between your full bust measurement and your calculated band size:
Difference (Δ) = Bust (inches) - Band (inches)
| Difference (Δ) | US Cup Progression | UK Cup Progression | EU Cup Progression |
|---|---|---|---|
| 0 inches (0 cm) | AA | AA | AA |
| 1 inch (2.5 cm) | A | A | A |
| 2 inches (5.0 cm) | B | B | B |
| 3 inches (7.5 cm) | C | C | C |
| 4 inches (10.0 cm) | D | D | D |
| 5 inches (12.5 cm) | DD / E | DD | E |
| 6 inches (15.0 cm) | DDD / F | E | F |
| 7 inches (17.5 cm) | G | F | G |
| 8 inches (20.0 cm) | H | FF | H |
4. International Size Conversions
Because manufacturers across North America, Europe, Australia, and Asia use different size conventions, BraSizeChecker maps your baseline measurements across 6 verified national standards.
Explore our full Bra Size Chart or convert known size labels using our Bra Size Converter Tool.
5. Sister Sizes Calculation
Sister sizes preserve identical cup volume capacity across band size steps. The algorithm follows an inverse step rule:
- Tighter Sister Size: Subtract 2 inches from band, add 1 cup letter index step (e.g. 34C → 32D).
- Looser Sister Size: Add 2 inches to band, subtract 1 cup letter index step (e.g. 34C → 36B).
Calculate your exact sister sizes using our interactive Sister Size Calculator.
6. Why Bra Size Results Can Vary
No automated formula can predict a 100% perfect fit across every bra brand because real-world lingerie manufacturing includes variable factors:
Fabric & Elasticity
Firm, unyielding lace bands feel tighter than soft, stretchy microfiber bands of the exact same labeled size.
Cup Styles & Cuts
Demis, balconettes, full-coverage, and plunge bras shape bust tissue differently even within the same cup size.
Wire Curvature
Underwire width and arch height vary between brands, altering side support and apex positioning.
Individual Tissue Density
Shallow vs projected breasts or firm vs soft breast tissue fill cup shapes differently.
7. Limitations & Fit Disclaimer
The calculated size generated by BraSizeChecker is an educational recommendation and starting baseline size rather than a guarantee of fit. We encourage using your calculated size as a starting point when trying on bras and adjusting based on our Fit Guide.
8. Reference Standards
Our sizing methodology references established international garment and sizing frameworks:
- European Standard EN 13402: Size designation of clothes — Part 3: Measurements and intervals.
- ASTM D7013 / D5586 Guidelines: Standard body measurement guides for apparel sizing.
- Lingerie Industry Sizing Conventions: Standard US, UK, and European bra sizing matrices.