Proportion Calculator

A proportion is a statement that two ratios are equal, written a/b = c/d. For example, 3/4 = 6/8 because both ratios equal 0.75. When you know three of the four terms, you can solve for the fourth.

It uses cross multiplication. From a/b = c/d the cross product a times d equals b times c. To isolate the unknown, it rearranges that identity: for the missing d it computes (b times c) divided by a, and the same pattern applies to a, b, or c.

Enter any three of the four terms (a, b, c, d) and leave the one you want to find blank. The calculator fills in the missing value, shows the completed proportion, and reports both ratios as decimals so you can check they match.

Solving for a term requires dividing by another term, so that divisor cannot be zero. For example, solving for d in 0/4 = 6/d has no finite answer because it would require dividing by a, which is 0. You also need exactly one blank field, not two.

Yes. The calculator accepts any finite real numbers, including negatives and decimals, so 2/5 = c/12 returns c = 4.8. Results are rounded to six decimal places to keep long fractions readable.

Yes. Cross multiplication is the formal version of the rule of three you learn in school. Both find a fourth proportional value from three known terms, which is handy for scaling recipes, converting map distances, or adjusting mixture ratios.