How to Calculate Bra Size
Bra size has two parts: band size (around the ribcage) and cup size (difference between bust and band). Getting both right is essential for comfort and support.
Measurement Method
Step 1 — Band size:
Measure snugly under bust (cm or inches)
Round to nearest even number (or add 1 if odd)
US band sizes: 28, 30, 32, 34, 36, 38, 40...
Step 2 — Cup size:
Bust = measure at fullest point of chest
Difference = Bust - Band size (inches)
0" = AA | 1" = A | 2" = B | 3" = C
4" = D | 5" = DD/E | 6" = DDD/FInternational Size Conversion
US 34B = UK 34B = EU 75B = FR/ES 90B
US 34D = UK 34D = EU 75D
Cup letters vary between US and EU:
US DD = UK DD = EU E
US DDD = UK E = EU FSister Sizes
Bands +2, cups −1 letter = same volume:
34C = 36B = 32D (all same cup volume)
If band too tight: try same cup, larger band
If band too loose: try same cup, smaller bandCalculate bra size: Free Bra Size Calculator
Bra Size Conversion Quick-Reference Table
| US/CA | UK | EU | FR/ES/BE | IT | AU/NZ |
|---|---|---|---|---|---|
| 32A | 32A | 70A | 85A | 1A | 10A |
| 34B | 34B | 75B | 90B | 2B | 12B |
| 36C | 36C | 80C | 95C | 3C | 14C |
| 38D | 38D | 85D | 100D | 4D | 16D |
| 34DD (E) | 34E | 75E | 90E | 2E | 12E |
| 36DDD (F) | 36F | 80F | 95F | 3F | 14F |
How Bra Sizing Works
Bra size has two components: band size (the number) and cup size (the letter). Band size in inches (US/UK) = the measurement around the ribcage under the bust, rounded to the nearest even number, plus 4 inches (some brands use the measured size directly). Cup size = difference between the fullest bust measurement and the band size: a 1-inch difference = A cup; 2 inches = B; 3 inches = C; 4 inches = D; 5 inches = DD/E; and so on.
Sister sizing allows fitting across adjacent band/cup combinations while maintaining the same cup volume: 34C, 36B, and 32D have the same cup volume — a 36B is a "sister" of 34C but looser in the band. This is useful when a brand's 34 band is unavailable but the 36 fits through sister sizing. EU sizing converts US band size: EU = (US band − 4 + 16) × 5 ÷ 2, approximately.
Common Mistakes
- The +4 inch rule controversy: The traditional "add 4 to the underbust measurement" formula was designed for non-stretchy vintage bras. Modern bras with elastic bands fit correctly without adding 4 — measure your underbust and use that number as your band size directly. Many sizing experts now recommend the direct measurement method.
- Sizing variation between brands: Bra sizes are not standardised globally or even within countries. A 34C in one brand may fit like a 32D in another. Always try before buying, or consult the brand's specific fit guide when ordering online.
- Neglecting cup size when the band is changed: If you change band size, the cup size letter must change correspondingly to maintain fit. Going from 34C to 36B (both D-cup equivalent volume) will fit differently in the cup than 36C.
Frequently Asked Questions
Signs of correct fit: (1) Band is level all around and lies flat against the ribcage — not riding up. (2) Wires sit flat against the ribcage at the sides and front, not on breast tissue. (3) Centre gore (front middle panel) lies flat against the sternum. (4) Cups contain all breast tissue without bulging or gaping. (5) You can fit two fingers under the band with slight resistance. If the band rides up, go down a band size (and up a cup letter to maintain volume).
In the US system, after D comes DD (≈E), DDD (≈F), DDDD (≈G). In the UK system, the letters continue sequentially: D, DD, E, F, FF, G, GG. So UK E = US DD; UK F = US DDD. EU uses a combined letter+number system with no DD. When ordering internationally or reading sizing guides, confirm which convention is used — cups above D vary significantly across naming systems.
Body size changes with weight fluctuation, pregnancy, breastfeeding, hormonal changes, and age-related changes in breast tissue composition. Getting remeasured annually or after any significant body change (±5 kg weight change, pregnancy, post-nursing) is recommended. Studies suggest a majority of women wear an incorrect bra size — typically too large a band and too small a cup — often due to outdated measurements or attachment to a familiar size.