Calculators
Proportion Calculator
Solve any missing term in a proportion a/b = c/d using three positive values, locally in your browser.
Solve a / b = c / d. Leave exactly one term blank. Enter positive known values from 1e-12 to 1e12. Calculations stay in your browser.
Find a missing value with cross multiplication.
A proportion states that two ratios are equal: a/b = c/d. Cross multiplication gives ad = bc. Solve a = bc/d, b = ad/c, c = ad/b, or d = bc/a. For example, 2/3 = 4/d gives d = 6. Leave exactly one term blank and enter the other three as positive numbers between 1e-12 and 1e12 inclusive. Decimals and scientific notation are accepted. Zero and negative values are excluded to avoid undefined ratios and ambiguous inverse solutions. Results are rounded to 12 significant digits using floating-point arithmetic. Use consistent units for corresponding terms; this calculator does not convert units. All calculations run locally without uploading or storing inputs.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
How do I solve a proportion?
For a/b = c/d, cross multiplication gives ad = bc. Solve a = bc/d, b = ad/c, c = ad/b, or d = bc/a. For example, 2/3 = 4/d gives d = 6.
Which values can I enter?
Leave exactly one term blank. Each known value must be positive and between 1e-12 and 1e12 inclusive. Decimals and scientific notation are accepted; zero and negative terms are excluded.
Are the results exact?
Results use floating-point arithmetic and are displayed to 12 significant digits. Corresponding quantities must use consistent units.
Are my inputs uploaded?
No. Calculation and copying happen locally in your browser without uploading or storing inputs.