Free Breast Calculator (Bra Size Estimator)
Enter your measurements to estimate your starting bra size. This tool gives a practical baseline you can refine with fit checks.
Tip: Measure without a padded bra for best accuracy. This is an estimate, not a medical diagnosis.
How this breast calculator works
This calculator estimates bra size using two numbers: your underbust and your full bust. It first estimates your band size from the ribcage measurement, then estimates your cup size from the difference between bust and underbust.
Most sizing systems are not perfectly standardized between brands, so think of the result as a smart starting point. From there, comfort and fit signs help you fine-tune.
How to measure correctly
1) Underbust measurement
- Wrap the tape directly under your breasts, level all the way around.
- Keep it snug but not painfully tight.
- Exhale naturally before reading the number.
2) Full bust measurement
- Measure around the fullest part of your bust.
- Keep the tape parallel to the floor.
- Do not compress breast tissue with the tape.
Understanding band and cup sizes
A bra size combines band and cup (for example, 34C). The band represents ribcage support, while the cup reflects volume relative to band. Because of this, cup letters are not absolute across all bands.
For example, 32D and 34C can have similar cup volume. These are called sister sizes. The calculator includes sister-size suggestions so you can adjust for comfort.
Common fit checks after calculating
- Band: Should feel firm and level, not riding up your back.
- Cups: No major gaping or spillover at top/sides.
- Center gore: Should sit close to the sternum in most underwire styles.
- Straps: Should not dig deeply or slide off constantly.
Why your calculated size might vary by brand
Different brands use different patterns, wire widths, fabric stretch, and cup shape assumptions. That means your best fit may vary by one band size or one cup step depending on style.
Sports bras, plunge bras, balconette bras, and full-coverage bras can each fit differently even in the same labeled size.
Frequently asked questions
Is this calculator medically diagnostic?
No. This tool is for clothing fit and comfort only.
Can body changes affect size?
Yes. Weight changes, hormonal shifts, pregnancy, postpartum changes, and aging can all alter fit.
How often should I re-measure?
Every 6–12 months, or sooner if your current bras become uncomfortable.
Final note
Use this breast calculator as a practical first step. The best bra size is the one that gives comfortable support, natural shape, and confidence throughout your day.