What Is a Bra Size Calculator?
This calculator estimates your bra band size, cup size and full size from two simple measurements: underbust (directly under your bust) and bust (around the fullest point). It uses the traditional sizing formula found in most fit guides — band size from the underbust plus a standard allowance, and cup size from the difference between bust and band.
Because bra sizing conventions differ slightly between brands, countries and fit philosophies, treat the result as a well-informed starting point for trying on bras, not a guaranteed exact fit.
How to Read Your Results
Estimated Bra Size
Your full estimated size, combining the calculated band number and cup letter (for example, 34C).
Band Size
The number portion of your size, based on your underbust measurement plus a standard 4–5 inch allowance. This is meant to sit snugly around your ribcage for support.
Cup Size
The letter portion of your size, based on how much larger your bust measurement is than your band size — each full inch of difference moves up one cup letter.
Bust − Band Difference
The raw inch difference used to determine the cup letter, shown so you can see how the cup size was derived.
Real-World Example
Someone measures 30 inches underbust and 35.5 inches at the fullest point of the bust (rounded to 36 for the calculation):
| Measurement | Value |
|---|---|
| Underbust | 30 in |
| Band size (30 is even, +4) | 34 |
| Bust (rounded) | 36 in |
| Bust − band difference | 2 in → cup B |
| Estimated size | 34B |
A 2-inch difference between the rounded bust (36) and the band (34) lands on cup letter B in the standard one-inch-per-cup scale.
Tips for a Better Fit
- Measure without a bra or in a soft, unpadded one for the most accurate underbust reading.
- Try "sister sizes" if a calculated size doesn't fit well — for example, if 34C feels tight in the band, try 36B, which has a similar cup volume with a looser band.
- Bands stretch over time, so re-measure periodically, especially after weight changes.
- Cup letters aren't universal — sizing charts vary between US, UK and European brands, so always check a specific brand's size chart too.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team