Bra Size CalculatorFree online other calculator — instant results, no sign-up
Estimate band and cup size (US).
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About the Bra Size Calculator
This calculator estimates US bra band and cup sizes based on snug underbust and bust circumference measurements in centimeters. It is designed for individuals looking for a starting point when selecting standard US bra sizing. Measurements are converted to inches and mapped to standardized cup letter increments.
- Underbust
- Snug ribcage circumference directly below the bust line, entered in centimeters (typically 60 to 120 cm).
- Bust
- Fullest circumference around the chest and breasts, entered in centimeters (typically 70 to 150 cm).
Formula and method
Band = round((u / 2.54) / 2) × 2 + (round(u / 2.54) mod 2) Diff = round((b / 2.54) − Band) Cup = CupList[clamp(Diff, 0, 8)]
- uUnderbust circumference in centimeters
- bBust circumference in centimeters
- BandCalculated band measurement in inches
- DiffRounded difference in inches between bust measurement and band size
- CupListIndexed cup array: [0: AA, 1: A, 2: B, 3: C, 4: D, 5: DD, 6: DDD, 7: G, 8: H]
The underbust is converted to inches (divided by 2.54) and rounded to establish the band size, with an adjustment added if the rounded underbust inch value is odd. The difference between the converted bust measurement and the band size is rounded to the nearest integer and clamped between 0 and 8. That index maps directly to a US cup letter ranging from AA to H.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Underbust76 cm
- Bust90 cm
Results
- Size (US)30DD
How to use the Bra Size Calculator
- 1Divide the underbust measurement in centimeters by 2.54 to convert it to inches.
- 2Calculate the band value using the rounding and parity formula to determine the base band size.
- 3Subtract the calculated band size from the total bust measurement in inches and round to the nearest whole integer.
- 4Map the resulting difference to the cup array (0 = AA, 1 = A, 2 = B, 3 = C, 4 = D, 5 = DD, 6 = DDD, 7 = G, 8 = H) and combine it with the band size.
Reading your result
The output displays an estimated US bra size combining the band number and cup letter (such as 30DD). A larger difference between bust and underbust corresponds to higher cup volume letters. This size represents a baseline recommendation for standard commercial US sizing.
Limitations
This calculator does not account for breast shape, density, ribcage flare, or asymmetry, which significantly alter fit. It also caps cup sizing at an 8-inch difference (H cup) and treats cup increments as linear whole-inch steps regardless of brand manufacturing variations.
Common questions
How should I measure my underbust and bust?+
Measure your underbust directly under your breasts around the ribcage, keeping the tape level and snug on an exhale. Measure bust circumference loosely over the fullest part of the chest, keeping the tape parallel to the floor.
Why does the cup size use letters like DD and DDD instead of E or F?+
This tool follows standard US sizing conventions, where a 5-inch difference is labeled DD and a 6-inch difference is labeled DDD (equivalent to UK E and F respectively).
What happens if my bust-to-band difference is zero or negative?+
The algorithm clamps the minimum difference to 0, which corresponds to an AA cup size.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.