Proportion Calculator

1. Choose Direct proportion (a/b = c/d) or Inverse proportion (a × b = c × d).
2. Choose Solve for a, Solve for b, Solve for c, or Solve for d.
3. Enter the three known values in the fields labelled a, b, c, and d. Leave the selected unknown blank.
4. Optionally enter labels in Unit for a and b and Unit for c and d.
5. Click Calculate and read the formula, substitutions, and assumptions. Use Copy result or Download result (.txt) to keep the text.

Direct proportion example

Direct proportion keeps the two ratios equal. For a/b = c/d, the calculator cross-multiplies and rearranges for the selected unknown. Select Solve for d and enter:

a = 2
b = 3
c = 8
d = (unknown)

d = 12

For this input, the solved value is d = 12. The Steps list shows the governing formula, the rearrangement, the substitution, and the solved value.

Inverse proportion uses a × b = c × d. Select Inverse proportion (a × b = c × d) when paired quantities move in opposite directions while their product stays constant. The same Steps list records the formula and substitution for the selected unknown.

Numbers and units the Proportion Calculator accepts

Enter a whole number such as 7, a decimal such as 2.5, or a fraction such as 3/4. Terms must be zero or positive. Leave the selected unknown blank. When you change the selected Solve for option, its field is cleared and disabled.

Unit for a and b and Unit for c and d add labels to the solved value and equation output. They do not convert units, so values in each pair must already use compatible units. A zero can cause an error when the selected rearrangement divides by it, and a fraction with a zero denominator is rejected.

What the result shows

The result includes the exact fraction, a mixed-number form, and a decimal form. Repeating decimals can use notation such as 0.(3); a longer repeating cycle can be shortened with an ellipsis. Entered values are treated as exact, so the calculator does not add measurement error or rounding assumptions.

Changing the proportion type or selected unknown hides the previous result until you click Calculate again. The calculation and result text are produced locally in the browser, and the calculator workflow does not submit the values you enter.

When to use another calculator

Use this page when one of four proportion terms is missing. The Ratio Calculator simplifies a ratio, solves an equivalent ratio, or divides a total by a ratio. The Fraction and Ratio Calculator handles fraction operations alongside ratio work. For a general one-variable equation, use the Algebra Calculator. The Math Calculators page lists the other math tools.

What to check when Calculate shows an error

Check that each known field contains a whole number, decimal, or fraction. The parser rejects unsupported symbols, negative terms, and fractions with a zero denominator.

Check the selected Solve for option as well. A zero is allowed as an entered term, yet the calculation can still fail when the formula would divide by that zero. The error message identifies the field or condition to correct.

Frequently asked questions