Why +4 inches?
This is a traditional sizing method used by many US brands. Some modern fitting methods use the underbust measurement directly as the band size (e.g., 30" = 30 Band), which would result in a larger cup letter (e.g., 30DD).
Calculator
Enter your values
Analysis
Interpretation of the current calculator output
Enter values to see detailed analysis and insights.
How to Use
Step-by-step instructions
- 1Measure your underbust (snug, around ribcage).
- 2Measure your bust (fullest part).
- 3Enter the measurements in inches.
- 4The calculator will estimate your US size.
US Standard Sizing
Calculates band size by adding 4-5 inches to the underbust measurement (standard US method), then determines cup size by the difference between bust and band.
Cup = Bust - BandVariables:
BandUnderbust + 4/5 inchesCup1" diff = A, 2" = B, etc.Example
30" Underbust, 35" Bust
Inputs:
Underbust:30 inches
Bust:35 inches
Steps:
- 1.Band = 30 + 4 = 34
- 2.Diff = 35 - 34 = 1 inch
- 3.1 inch = A Cup
Result:
34A
Frequently Asked Questions
What are sister sizes?
Sister sizes are bra sizes that have the same cup volume but different band lengths. If a bra fits in the cup but is too loose/tight in the band, try a sister size.
Is this result the same for every brand?
No. Size charts, cup progression, materials, and bra styles vary by manufacturer. Treat the result as a starting point and compare it with the brand’s own chart.
How should a correctly fitted band feel?
A new band should sit level around the torso and feel snug on the loosest hook without restricting normal breathing. The center gore and cups should lie smoothly without painful pressure.