Solve for the missing value in any proportion using cross-multiplication. Handles fractions, decimals, and units. Also called a ratio calculator, proportions solver, or proportion solver.
Cross-multiplication (ad = bc) results for common proportion setups. Click “Use this” on any row to pre-fill the calculator below.
| a | b | c | d (solve) | Result | Apply |
|---|---|---|---|---|---|
| 2 | 3 | 8 | ? | 12 | Use this → |
| 1 | 4 | 5 | ? | 20 | Use this → |
| 3 | 5 | 9 | ? | 15 | Use this → |
| 4 | 7 | 12 | ? | 21 | Use this → |
| 5 | 8 | 25 | ? | 40 | Use this → |
A proportion calculator — also called a proportions calculator, ratio calculator, proportions solver, proportion solver, proportion converter, cross-multiplication calculator, or proportionate value calculator — solves for the missing value in any proportion of the form a/b = c/d. Enter three known values and the calculator returns the fourth using cross-multiplication (ad = bc), with optional step-by-step checks.
Common uses include scaling recipes (2 cups flour serves 4, how much for 9?), calculating percentages, finding equivalent ratios for map scales and unit conversion, solving similar-triangle problems in geometry, comparing rates (miles per hour, dollars per pound), and any proportion word problem from grade 6 through college algebra. This calculator handles decimals, fractions, mixed numbers, and units.
Cross-checked against authoritative formulas (IRS, NIST, ACSM) and updated for 2025/2026 tax years and rates.
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Each calculator includes the formula, plain-English explanation, and FAQs so you understand the result.
Estimates only. Calculator results are for educational and planning purposes — not professional advice. For binding decisions on taxes, investments, health, or legal matters, consult a qualified professional.
FAQ
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A proportion calculator is a free math tool that solves for the missing value in a proportion a/b = c/d, using cross-multiplication. Our proportion calculator lets you solve for any of the four variables and shows the calculation. The proportion calculator is used for recipe scaling, map scales, similar triangles, and unit conversion.
How it works
Our proportion calculator lets you solve for the missing value in a proportion with cross-multiplication, in three steps.
Click the A, B, C, or D button at the top to select which variable you want to find. The chosen variable is automatically cleared so you can leave it blank. The known three variables stay in their slots.
Type the three values you already know in their slots. Use any positive or negative numbers. The result can be a fraction, decimal, or large number - the calculator handles all of them with full precision.
Click Solve Proportion. The result shows the missing value, the verified proportion, and a cross-multiplication check (a x d should equal b x c within rounding). Use the share button to copy a link with your inputs pre-filled.
All of these queries are answered by the calculator above. Different people search for the same tool with different words.
Sponsored
We are solving a / b = c / d for D
Write a/b = c/d. Two pairs of values that scale together.
a \u00d7 d = b \u00d7 c. Three known values give you the missing one.
Plug back in. Both sides should give the same ratio.
Type the three known values from the proportion a/b = c/d. Leave the unknown (4th value) empty. The calculator accepts fractions, decimals, and integers.
a/b = c/d means ad = bc. We solve for the unknown using cross-multiplication, with steps shown.
See the missing value plus the verification: enter all four values back into a/b = c/d and check they are equal.
For related calculations, try our percentage calculator.