Proportion Calculator

Solve proportions and find the missing value in a ratio equation.

By Konstantin Iakovlev · Updated September 2026 · Source: Khan Academy

x (solved)

20.0000

a/b = c/x

Proportion3/5 = 12/x
Cross Multiply3 * x = 5 * 12
x20.0000

Use the Proportion Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only. Results are estimates based on the information you provide and the assumptions described on this page.

How It Works

Solving for a missing value in a proportion is a core math skill with a surprising reach, and this calculator returns the answer in one step. Proportions let you scale recipes, convert between units, and read economic trends.

At its core, a proportion says that two ratios are equal, written a/b = c/d. Suppose you need to find x in 3/5 = x/10. Cross-multiplication does the work: multiply the numerator of one ratio by the denominator of the other, set the two products equal, and solve for the unknown. The calculator solves a/b = c/x for the fourth term, x = (b × c) / a, from the a, b and c you enter. If your unknown sits somewhere else, rearrange first: 3/5 = x/10 is the same proportion as 5/3 = 10/x, so enter a = 5, b = 3, c = 10 and get x = 6.

Consistent units are essential, so keep the same measure on both sides of each ratio rather than comparing inches to centimeters without converting first. Watch the denominators as well, because division by zero is undefined and a zero on the bottom of either ratio has no solution.

Example: Keeping a Marketing Budget Split

  1. 1 A company spends $75,000 of a $150,000 marketing budget on digital advertising. If the budget grows to $200,000, how much should go to digital advertising to keep the same share?
  2. 2 Set the proportion up so the unknown comes last: 150,000 / 75,000 = 200,000 / x. Enter a = 150000, b = 75000 and c = 200000 (type the numbers without commas).
  3. 3 Cross-multiplying gives 150,000 × x = 75,000 × 200,000 = 15,000,000,000, so x = 15,000,000,000 / 150,000 = 100,000. The calculator shows x = 100,000.0000.
  4. 4 The company should put $100,000 into digital advertising. Entering the numbers the other way round, 75,000 / 150,000 = 200,000 / x, gives x = 400,000, which answers a different question: the total budget at which $200,000 would be the digital share.

Source: Khan Academy · Last updated: September 2026

Frequently Asked Questions

How do I solve a proportion for a missing value?
In the proportion a/b = c/d, cross-multiply to get a*d = b*c, then solve for the unknown. For example, if 3/4 = x/20, then 3*20 = 4*x, so 60 = 4x, and x = 15.
What is the difference between a ratio and a proportion?
A ratio compares two quantities (3:4). A proportion is an equation stating that two ratios are equal (3:4 = 6:8). Proportions are used to solve for unknown values when two quantities maintain a constant ratio.
How are proportions used in real life?
Proportions are used in recipe scaling, map reading, converting units, calculating dosages, resizing images, mixing solutions, and any situation where you need to maintain a constant relationship between two quantities at different scales.

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