Quick Bust Size Estimator
Enter your snug underbust measurement and your fullest bust measurement. This tool gives an estimated bra size in US/UK format plus sister sizes.
Tip: Keep the measuring tape level and snug, not tight. Measure while standing naturally.
How this bust size calculator works
A bra size estimate usually combines two numbers: band size (the number) and cup size (the letter). This calculator uses your underbust measurement to estimate the band and the difference between full bust and underbust to estimate cup volume.
The formula is a practical starting point, not a perfect final answer. Different brands, materials, and bra styles can fit differently. Even with correct measurements, two people with the same size may prefer different fits.
Step-by-step measuring guide
1) Measure underbust
Wrap a soft tape measure around your ribcage directly under the bust. Keep the tape parallel to the floor and snug. This measurement anchors your band size.
2) Measure full bust
Measure around the fullest part of your bust with the tape comfortably level. Do not pull too tight. If you are between numbers, record the decimal or fraction.
3) Use consistent units
Enter both values in the same unit (inches or centimeters). The calculator converts values internally for cup estimation and displays a clear result.
Understanding band size and cup size
Band size and cup letter work together. A larger cup letter does not always mean a larger bust unless the band stays the same. For example, 34D and 36C can have similar cup volume but different band tension.
- Band size: Based on ribcage measurement; should feel secure and supportive.
- Cup size: Based on the difference between full bust and underbust.
- Sister sizes: Alternative combinations with similar cup volume but different band fit.
Common fitting problems and quick fixes
Band rides up in the back
Usually means the band is too loose. Try a smaller band and compensate with one cup size up if needed.
Cups gape at the top
This may happen if cups are too large, the style shape does not match your body, or straps are too loose. Try a different cup size or a style with less upper-cup coverage.
Spillage or pressure at cup edge
This often means cup volume is too small. Consider moving up one cup size and reassess comfort.
Straps digging in
Straps should stabilize, not carry most support. If straps dig deeply, check whether your band is too loose or cups are off.
Important notes about bra sizing systems
US, UK, and EU sizing standards are not identical. Brand charts may also vary. This calculator provides a practical estimate:
- US/UK band estimate in even-number increments.
- US and UK cup letters (with UK double-letter progression in larger cups).
- EU band approximation in 5 cm increments.
Always check the brand size guide before purchasing, especially when buying online.
Best practices for better accuracy
- Measure twice and use the average if results differ.
- Measure when your body is relaxed and upright.
- Use a non-stretch tape measure.
- Re-check sizing every 6–12 months or after body changes.
- Try multiple bra styles: full coverage, balconette, plunge, and sports bras can fit differently.
Frequently asked questions
Is this calculator medically diagnostic?
No. It is a clothing fit estimator only and does not provide medical advice.
What if I get an odd or unexpected size?
Start with the suggested size, then test sister sizes and style variations. Fit comfort matters more than the label.
Can I use this for sports bras?
Yes, as a starting point. Sports bra sizing often uses S/M/L or compression levels, so brand charts are especially important.
Final takeaway
A good bust fit combines measurement, comfort, and style preference. Use this bust size calculator to find a strong starting point, then fine-tune with real-world try-ons. Small adjustments can make a big difference in support and comfort.