# Bra Size Calculator

> Bra size calculator that converts your band and bust measurements into sister sizes across US, UK, and EU charts. Free, runs in your browser, no signup.

- **Canonical URL:** https://boringtoolskit.com/bra-size-calculator/
- **Category:** Health
- **Evidence level:** authored-methodology
- **Reviewed:** 2026-09-20
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Band = underbust rounded to even; cup = bust minus band, 1 inch per letter (1=A, 2=B, 3=C...). Sister sizes shift band and cup in opposite directions (34C = 32D = 36B) because cup size is relative to band.

## Assumptions

- Manufacturer charts vary; conversion outputs are the standard-mapping midpoint.
- Measurements are body measurements unless the tool labels them as garment measurements.
- Between-size results round per the rule stated in the FAQ.
- This tool provides general estimates for planning and education only; it is not medical advice and not a substitute for a qualified healthcare professional.
- Always consult your doctor or another healthcare provider about your specific situation.

## Common questions

### Why does the calculator give a different size than the store fitted me?

Store fittings often add +4 inches to the underbust, an obsolete convention from 1930s construction. Modern sizing uses the actual underbust rounded to even, which typically lands 1-2 band sizes smaller with a bigger cup letter.

### What if I fall between sizes?

Round the direction that matches use: clothing and shoes generally round up.

## Sources

- [Aggregated manufacturer sizing conventions](https://boringtoolskit.com/methodology/)

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