About this Bra Size Calculator
Studies show that up to 80% of women wear the wrong bra size. A properly fitting bra provides better support, comfort, and shape. This calculator uses standard measuring techniques to estimate your Band and Cup size.
Who this calculator is for
- People measuring at home before buying bras online or in-store.
- Shoppers who need a quick starting point for band, cup, and sister sizes.
- Anyone whose fit changes and wants to re-check their size.
When to use it
- Estimate a starting bra size from underbust and bust measurements.
- Compare sister sizes when the band or cup fit is slightly off.
- Shop across brands with a clearer idea of your likely size range.
How to use this calculator
- Wear a non-padded bra and use a soft measuring tape.
- Step 1 (Band): Measure tightly around your ribcage, just under your bust.
- Step 2 (Bust): Measure loosely around the fullest part of your bust.
- Enter both numbers to see your size.
Key assumptions
- Measurements are taken in inches using a soft tape measure.
- The calculator follows a modern +0 style sizing method.
- Brand, style, and shape differences can still change the final fit.
Accuracy and testing
Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.
How this calculator works
1. Band Size: Usually the Underbust measurement rounded to the nearest even number (sometimes +0, +2, or +4 inches depending on the sizing system - this calculator uses the modern +0/rounding method).
2. Cup Size: The difference between Bust and Band. (1 inch = A, 2 inches = B, 3 inches = C, etc.).
Inputs explained
Underbust (Band)
Measure snugly against the skin. Exhale slightly.
Overbust (Bust)
Measure straight across the back and over the nipples. Keep tape parallel to the floor.
Understanding your results
The result gives your US Size. It also lists Sister Sizes—sizes with the same cup volume but different band lengths. If your calculated size feels tight in the band, try the larger sister size.
Example scenarios
32C
Underbust 32, Bust 35. Difference = 3 inches (C cup).
34B
Underbust 34, Bust 36. Difference = 2 inches (B cup).
Key Features
- Standard US Sizing
- Sister Size Calculation
- Fitting Guide
Tips & Best Practices
- Measurements are just a starting point. Brands vary significantly.
- If the band rides up your back, it is too big. If the cups gape, they are too big.
- Check your size every 6 months, as weight changes affect fit.
Glossary of Terms
- Sister Size
- Alternative sizes that hold the same volume of breast tissue (e.g. 34C = 32D = 36B).
- Band Size
- The number part of the size (32, 34, 36). It provides 80% of the support.