Bra Size Calculator
Enter your measurements to estimate your bra size. This tool gives a strong starting point for shopping, then you can fine-tune by fit.
What does a bra size calculator actually do?
A bra size calculator uses two main measurements—your underbust and your full bust—to estimate two parts of a bra size: band size (the number) and cup size (the letter). In simple terms, the underbust helps determine band size, and the difference between bust and band helps determine cup size.
For example, when you see a size like 34C, the 34 is the band measurement class, and C reflects breast volume relative to that band.
How is bra size calculated? (Step by step)
1) Measure snug underbust
Wrap a soft tape around your ribcage directly under your bust. Keep it level and snug (not painfully tight). This measurement estimates the band foundation.
2) Measure full bust
Measure around the fullest point of your bust while standing straight, with the tape level all around. The difference between this and your band baseline drives cup estimation.
3) Estimate band size
Most modern calculators round underbust to the nearest even band size (for inch-based sizing systems), such as 28, 30, 32, 34, and so on.
4) Estimate cup size from difference
Cup letters are mapped from the difference between full bust and band. A bigger difference means a larger cup letter.
| Bust - Band Difference (inches) | Approximate Cup |
|---|---|
| 0 | AA |
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | DD / E |
| 6 | DDD / F |
Why cup letters can be confusing
Cup size is not a fixed volume by itself. A 34C and a 30C do not hold the same volume. Cup volume changes with the band, which is why sister sizes exist (for example, 34C, 32D, and 36B are close in cup volume but different in band fit).
Common measurement mistakes
- Holding the tape too loose at the underbust.
- Measuring over thick clothing.
- Letting the tape tilt up or down across your back.
- Using only one measurement and skipping bust difference.
- Assuming one brand's 34C fits exactly like every other brand's 34C.
How to check if the calculator result is correct on your body
Band fit
The band should feel firm and mostly horizontal around your torso. It should not ride up significantly in the back.
Cup fit
Breast tissue should sit inside the cups without major gapping or spilling. Minor variation can happen due to bra style.
Straps and comfort
Straps should support but not dig heavily. Most support should come from the band, not straps alone.
US/UK/EU differences
Size systems vary by region and by brand. A calculator can show equivalent estimates in multiple systems, but labels may still differ slightly. Always check each brand's own size chart when possible.
Bottom line
A bra size calculator is best used as a starting estimate. Accurate measurements + fit checking + trying sister sizes usually gives the best final result.