Bra Sizes
Clothing Sizes · 7 specs
Direct Answer
Bra sizing standardizes dual-dimensional breast support combining underbust Band Size (30 to 44) with Cup Volume (AA to H). Cup size is calculated directly from the difference between overbust fullest circumference and snug underbust ribcage measurement, cross-mapped across US, UK, EU, French (FR), and Italian (IT) systems.
Source: ISO 3635 / BS 6183 / European Underwear Standards (EN 13402-4) · Last reviewed Aug 28, 2026
Specs
| Governing Standards | ISO 3635 / EN 13402-4 / BS 6183 |
| Band Size Calculation | Snug underbust ribcage circumference (inches) rounded to nearest even number |
| Cup Size Calculation | Fullest Overbust Measurement (in)–Band Size (in) = Cup Letter |
| Cup Step Increment | 1.0 inch (2.54 cm) difference per cup letter |
| US Cup Scale | AA (<1"), A (1"), B (2"), C (3"), D (4"), DD/E (5"), DDD/F (6"), G (7"), H (8") |
| Sister Sizing Rule | Cup volume stays equal if you go DOWN a band and UP a cup (e.g., 36B = 34C = 32D) |
| International Band Baseline | US/UK 32 = EU 70 = FR 85 = IT 2 | US/UK 34 = EU 75 = FR 90 = IT 3 |
Cup Sizing Calculation Guide
The cup letter represents the difference between your fullest overbust measurement and your band measurement:
| Bust–Band Difference | US Cup Size | UK Cup Size | EU Cup Size | Australia (AU) |
|---|---|---|---|---|
| < 1.0 inch (< 2.5 cm) | AA | AA | AA | AA |
| 1.0 inch (2.5 cm) | A | A | A | A |
| 2.0 inches (5.0 cm) | B | B | B | B |
| 3.0 inches (7.5 cm) | C | C | C | C |
| 4.0 inches (10.0 cm) | D | D | D | D |
| 5.0 inches (12.5 cm) | DD / E | DD | E | DD |
| 6.0 inches (15.0 cm) | DDD / F | E | F | E |
| 7.0 inches (17.5 cm) | G | F | G | F |
| 8.0 inches (20.0 cm) | H | FF | H | G |
International Band Size Conversion Chart
| Underbust Ribcage (in) | Underbust (cm) | US / UK Band | Europe (EU) | France / Spain (FR) | Italy (IT) |
|---|---|---|---|---|---|
| 26"–28" | 63–67 cm | 30 Band | 65 | 80 | 1 |
| 28"–30" | 68–72 cm | 32 Band | 70 | 85 | 2 |
| 30"–32" | 73–77 cm | 34 Band | 75 | 90 | 3 |
| 32"–34" | 78–82 cm | 36 Band | 80 | 95 | 4 |
| 34"–36" | 83–87 cm | 38 Band | 85 | 100 | 5 |
| 36"–38" | 88–92 cm | 40 Band | 90 | 105 | 6 |
| 38"–40" | 93–97 cm | 42 Band | 95 | 110 | 7 |
| 40"–42" | 98–102 cm | 44 Band | 100 | 115 | 8 |
The Sister Sizing Cross-Reference Principle
When a bra band feels too tight or loose but the cup volume fits, utilize Sister Sizing (equivalent cup volumes with different ribcage bands):
- Equivalent to 34C Cup Volume:
32D(tighter band) longleftrightarrow34C(standard) longleftrightarrow36B(looser band) longleftrightarrow38A. - Rule: If you go DOWN one band size, you must go UP one cup letter to maintain the identical cup volume.