Proportion Calculator

Two ratios are in proportion when their cross-products are equal. Because a × d = b × c, you can solve for any single missing value by dividing the product of the two known opposite terms by the remaining known term. This is the classic "rule of three".

A proportion a : b = c : d means a ÷ b = c ÷ d, so a × d = b × c. Rearranging this cross-product gives whichever value is left blank.

The tool solves a single linear proportion of the form a to b equals c to d by cross-multiplication. Leave exactly one box blank. It cannot solve if you divide by zero, and it does not handle percentages, compound ratios with more than two terms, or non-linear relationships. Results are decimal approximations, so recurring values are rounded for display.

For 2 : 3 = 4 : ?, cross-multiply to get d = 3 × 4 ÷ 2 = 6, so 2 : 3 = 4 : 6.

1. Enter the three known values of a : b = c : d.
2. Leave the box for the unknown value blank.
3. Press Solve to get the missing value and the steps.

• Leaving more than one box blank, so there is nothing to solve.
• Putting the values in the wrong order within each ratio.
• Dividing by a term that is zero, which has no solution.

The Proportion Calculator solves a proportion of the form a : b = c : d. Fill in any three values, leave the unknown box blank, and it cross-multiplies to find the missing number with a worked solution.

Students, teachers, cooks, makers and anyone scaling recipes, maps, models or measurements.

• Solves for any of the four values — just leave one blank.
• Step-by-step cross-multiplication you can follow.
• Cross-check confirms a × d equals b × c.
• Private — calculated entirely in your browser.

• Scaling a recipe up or down by ratio.
• Converting map or model scale distances.
• Solving rule-of-three word problems.