Bra Size Calculator
Estimate a reference bra band and cup size (US and UK) from a snug underbust measurement and a loose bust measurement.
How It's Calculated
Formula
\text{Band} = \text{round}(\text{Underbust}) + \begin{cases} 4 & \text{even} \\ 5 & \text{odd} \end{cases} \qquad \text{Cup Diff} = \text{round}(\text{Bust}) - \text{Band}This calculator estimates a reference bra size using the traditional band-and-cup sizing method taught across brand-neutral lingerie-fitting guides — not any single brand's or retailer's proprietary chart. Band size comes from your snug underbust measurement, rounded to the nearest whole inch, with 4 inches added if that number is even or 5 inches added if it's odd. Cup size comes from the difference, in whole inches, between your loose bust measurement and the computed band size: each inch of difference steps up one cup letter (0" = AA, 1" = A, 2" = B, and so on). US and UK cup lettering are shown separately because they diverge starting at the "DD" step — UK continues DD, E, F, FF, G, GG while US continues DD, DDD, DDDD, G, H, I. Actual fit varies by brand, garment style, and body shape — treat this as a starting reference point, not your exact or guaranteed size.
Worked Examples
32in underbust, 38in bust
- Underbust rounds to 32 (even), so band size = 32 + 4 = 36
- Bust rounds to 38; cup difference = 38 − 36 = 2 inches
- 2 inches of difference = cup B in both US and UK tables
- Estimated reference size: 36B
31in underbust, 42in bust (US/UK cup letters diverge)
- Underbust rounds to 31 (odd), so band size = 31 + 5 = 36
- Bust rounds to 42; cup difference = 42 − 36 = 6 inches
- 6 inches of difference: US cup letter is DDD, UK cup letter is E
- Estimated reference size: 36DDD (US) / 36E (UK)
Frequently Asked Questions
Is this my exact bra size?
No — this is an estimated reference size from a generic, widely-taught sizing formula, not a professional fitting or a guarantee. Actual fit varies significantly between brands, garment styles, and fabric stretch, so treat this as a useful starting point for shopping, not a final answer.
Why are the US and UK cup letters different for the same measurements?
US and UK cup-letter progressions are identical through 'D' but diverge starting at the next step: UK continues DD, E, F, FF, G, GG, while US continues DD, DDD, DDDD, G, H, I. Both progressions represent the same underlying inch-by-inch cup difference — only the letter labels differ, and this calculator shows both.
How should I measure underbust and bust?
Underbust: wrap the tape snugly (not tight) around your ribcage directly under your bust, keeping it level. Bust: wrap the tape loosely around the fullest part of your bust, without compressing. Measuring while wearing a well-fitting, unpadded bra tends to give the most consistent results.
Why did my measurements return an error?
This calculator requires your bust measurement to be strictly greater than your underbust measurement (the formula needs a positive difference to compute a cup size), and both measurements must fall within a supported range of 20-70 inches. A cup difference beyond 12 inches (past US 'K' / UK 'HH') is also outside this generic formula's supported range — a professional fitting is the better next step at that point.