US Bra Size Calculator
Enter your underbust and full bust measurements to estimate your US bra size.
Note: This is an estimate. Brand, style, breast shape, and personal comfort all affect final fit.
How the US bra size calculator works
A US bra size usually combines a band size (number) and a cup size (letter), such as 34C or 36DD. The band size is based on your ribcage measurement, while cup size is based on the difference between your full bust and your band size.
This calculator follows a practical, modern estimate:
- Round your underbust to the nearest even band size.
- Compute the bust-to-band difference in inches.
- Map that difference to a US cup letter.
The goal is to give you a reliable starting point for shopping, then refine through fit testing.
How to measure correctly (step-by-step)
1) Underbust
Wrap a soft tape measure around your ribcage directly under your bust. Keep the tape level and snug, but not painfully tight. Exhale naturally before reading the number.
2) Full bust
Measure around the fullest part of your bust, typically at nipple level. Keep the tape straight across your back and avoid pulling it too tight.
3) Enter measurements into the calculator
Choose inches or centimeters, enter both numbers, and calculate. If you use centimeters, the calculator converts to inches automatically.
US cup difference guide (quick reference)
- 0"–1" difference: AA / A range
- 2" difference: B
- 3" difference: C
- 4" difference: D
- 5" difference: DD (E)
- 6" difference: DDD (F)
- 7"+ difference: G and above
Different brands may label larger cups differently, especially beyond DDD/F. Always compare size charts when shopping.
Common bra fitting mistakes
- Band too loose: Most support comes from the band, not the straps.
- Straps over-tightened: Tight straps can cause shoulder discomfort without improving support.
- Wrong cup volume: Gaping or spilling usually means cup size mismatch (or shape mismatch).
- Ignoring sister sizes: If band feels off, sister sizes can preserve similar cup volume.
- No scoop-and-swoop: Positioning breast tissue into the cups can change fit significantly.
Understanding sister sizes
Sister sizes have similar cup volume but different band sizes. If your calculated size is close but not perfect:
- Go down one band and up one cup (example: 34C → 32D).
- Go up one band and down one cup (example: 34C → 36B).
The calculator provides sister-size suggestions to make this easier.
Fit checklist after you calculate
- The band should feel firm on the loosest hook when new.
- The center gore should sit close to your chest (for most wired bras).
- Cups should contain tissue without gaping or overflow.
- Straps should stay put without digging in.
- You should be able to move and breathe comfortably.
FAQ
Is this calculator accurate?
It is accurate as a starting estimate for US sizing. Final fit depends on brand, bra style, breast shape, and comfort preference.
Can I use this if my measurements are in cm?
Yes. Select centimeters, and the calculator handles conversion automatically.
Why does one brand's 34D fit differently than another's?
Sizing standards vary by manufacturer. Cup shape, wire width, and fabric stretch all affect feel and fit.
How often should I re-measure?
Re-measure every 6–12 months, or sooner after major weight changes, pregnancy, hormonal shifts, or changes in activity level.
Final thoughts
A good bra size calculation should save time, reduce returns, and make shopping less frustrating. Use the number you get here as your baseline, then test nearby sister sizes for best comfort and support. The best-fitting bra is the one that feels stable, comfortable, and nearly invisible in daily wear.