BH Size Calculator
Use your underbust and full bust measurements to estimate your starting bra size. This gives a practical baseline you can refine by trying different brands and styles.
Finding the right bra or BH size can make an immediate difference in comfort, posture, and confidence. Yet many people are still wearing bands that ride up or cups that don’t fully support them. A simple calculator can save time by giving you a reliable starting point before you shop.
What this BH size calculator does
This tool estimates a starting bra size using two inputs:
- Underbust measurement to estimate the band size
- Full bust measurement to estimate cup volume
The result is not a final diagnosis of fit. Different brands shape their cups differently, and materials stretch at different rates. Think of the output as your “first size to try,” then adjust based on real fit feedback.
How to measure correctly
1) Underbust
Wrap a soft measuring tape around your ribcage directly under your bust. Stand normally, exhale gently, and pull the tape snug but comfortable. This measurement drives band fit, which does most of the support work.
2) Full bust
Measure around the fullest part of your bust while the tape remains level front to back. Keep it close to the body without flattening tissue. This value is used with band size to estimate cup letter.
3) Check your tape position
Most sizing errors happen when the tape drifts upward at the back or is pulled unevenly. A mirror helps. If possible, take two readings and use the average.
How the calculator estimates size
The sizing logic is straightforward:
- Convert your measurements to inches (if you entered centimeters)
- Round underbust to the nearest even band size
- Calculate difference: full bust - band size
- Map that difference to cup letters (A, B, C, D, etc.)
For example, if your underbust rounds to 34 and your full bust is about 37, the difference is 3, which often maps to a C cup, giving an estimated size of 34C.
Common fit signs and what to change
Band rides up in back
Usually means the band is too loose. Try one band size down (and adjust cup via sister sizing).
Cups wrinkle or gap
Can indicate cups are too large, or cup shape mismatch. Try one cup size down, or a different cup shape (balconette, plunge, full-cup, etc.).
Spillage at top or sides
Usually a cup that is too small. Go up one cup size first, then reassess band tension.
Straps digging in
Often a support problem from the band, not the straps. If straps carry all the load, your band may be too loose.
Why sister sizes matter
Sister sizes keep similar cup volume while changing band tightness. Examples:
- 34C has similar cup volume to 32D and 36B
- 36DD has similar cup volume to 34E (or 34DD depending on brand labels)
If your cup feels right but the band is too tight, try one band up and one cup down. If band is too loose, do the opposite.
Important notes before buying
- Brand sizing can vary significantly
- Sports bras and everyday bras may fit differently
- Body changes over time; re-measure periodically
- Comfort and support are more important than the label
This calculator is educational and practical for shopping decisions, but it does not replace a professional fitting if you have persistent discomfort or specialized support needs.
Quick FAQ
Is this calculator for US sizes?
Yes, the primary output is a US-style estimate. It also shows an approximate EU band number to help with international labels.
Why does my measured size feel different between brands?
Brands use different patterns, materials, and cup shapes. Use your estimated size as a baseline, then refine by fit.
Can I use centimeters?
Absolutely. Select centimeters in the calculator and it will convert automatically.