Proportion Calculator
Use your own numbers after reviewing the formula and worked example.
Quick answer
A proportion states that two ratios are equal. Cross multiplication converts the proportion into a simple equation.
Formula or method
Keep units, percentages and time periods consistent. The formula is useful as a transparent check on the calculator result and helps explain why changing one input changes the answer.
Step-by-step
- Write the ratios in the same order.
- Cross multiply.
- Solve the resulting equation.
- Substitute the answer back to check.
Worked example
For 3/5 = x/20, 3×20 = 5x, so x = 12.
How to interpret the result
Proportions appear in scaling, rates, maps, recipes and percentage questions.
When the answer is used for a purchase, loan, payroll decision, construction order, school grade, health estimate or other real-world choice, verify the assumptions that matter in that context. Accurate arithmetic still depends on accurate inputs.
Common mistakes
- Reversing only one ratio.
- Cross multiplying incorrectly.
- Mixing units without converting.
Frequently asked questions
Can I solve by scaling instead?
Yes, when the scale factor is obvious.
What if a denominator is zero?
The ratio is undefined.
Are percentages proportions?
Yes; percentages compare to 100.