Enter your underbust and full-bust (overbust) measurements into the calculator to estimate a bra size (band + cup). You can also estimate measurements from a bra size or estimate a general clothing top size from a bust measurement.

Bust Size Calculator

Bra Size
From Bra Size
Clothing Bust

Enter underbust and full bust to get an estimated bra size, approximate international conversions, and sister sizes.

Recommended Bra Size:
International Conversions:
Sister Sizes:

Bra Size Formula (US/UK)

The calculator above estimates a US/UK bra size (band + cup) using underbust (UB) and full-bust/overbust (OB) measurements. A common modern approach is:

\begin{aligned}
\text{Band} &\approx \text{nearest even inch of } UB \\
\text{CupSteps} &\approx \text{round}(OB - \text{Band})
\end{aligned}

Variables:

  • UB is the underbust measurement (circumference) in inches (or convert cm to inches first)
  • OB is the overbust/full-bust measurement (circumference) in inches (or convert cm to inches first)
  • Band is the estimated US/UK band size (typically an even number in inches)
  • CupSteps is the estimated difference (in inches) used to select a cup (for example: 1=A, 2=B, 3=C, 4=D, 5=DD/E, 6=DDD/F, etc.)

Note: Your bust measurement itself is simply the full-bust/overbust circumference (OB). Underbust is used to estimate bra band and cup size, not to “average” into a bust measurement.

What is a Bust Size?

Bust size most commonly refers to the measurement around the fullest part of a person’s bust/chest and around the back (often called the full bust measurement). It is typically measured in inches or centimeters and is often used in determining clothing sizes. In bra sizing, the “bra size” is different: it includes both the band (related to the underbust) and the cup (related to the difference between full bust and band).

How to Calculate Bra Size (Band + Cup)?

The following steps outline how to estimate a US/UK bra size from underbust and full-bust measurements:

  1. Measure your underbust (UB) around the ribcage, under the bust.
  2. Measure your full bust (OB) around the fullest part of the bust/chest.
  3. If you measured in centimeters, convert to inches (1 in = 2.54 cm).
  4. Estimate the band size by rounding UB to the nearest even number of inches (or use the classic +4/+5 method if desired).
  5. Compute the difference in inches: OB − Band, then map that difference to a cup size (each ~1 inch is one cup step in US sizing).
  6. Combine band + cup to get the estimated bra size, then check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge:

Underbust measurement (UB) = 30 inches

Overbust measurement (OB) = 36 inches