How to use this calculator
Choose simplification, scaling, or a missing-proportion calculation. Simplification divides by the greatest common divisor. Scaling multiplies both quantities by the same positive factor. In A/B = C/D, enter A, B and C to find D. Proportion mode can solve any one of A, B, C or D using the other three.
- Enter calculation, missing proportion quantity, first quantity (a) and review the remaining options.
- Select Calculate or change a value to update the result. Read the result label and units before copying.
- Use Reset to restore the worked starting values.
Formula and method
A and B are nonnegative integers and cannot both be zero. Proportion mode requires positive A, B and C so neither denominator is zero. Scaling accepts a positive decimal factor.
Worked example
12:18 simplifies to 2:3.
Questions about ratio calculator
Can one quantity in the ratio be zero?
Yes in simplify or scale mode: 0:5 simplifies to 0:1. Both quantities cannot be zero. Proportion mode solves A/B = C/D for the selected missing value; known values must satisfy the displayed positive-value restrictions.
What assumptions should I check?
A and B are nonnegative integers and cannot both be zero. Proportion mode requires positive A, B and C so neither denominator is zero. Scaling accepts a positive decimal factor.
Are my inputs saved?
Calculations run in your browser. We store only tool IDs for favorites and recent tools. A calculation link contains your inputs only when you explicitly choose to copy one.