Proportion Calculator

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

By Konstantin Iakovlev · Updated April 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 and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.

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. Consider electric vehicles: their share of new car sales is projected at roughly 25% globally in 2026, up from 15% in 2023, and reasoning proportionally from figures like these is how analysts forecast charging infrastructure needs.

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 carries out exactly that step for whatever value you leave blank.

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: Budgeting for a 2026 Marketing Campaign

  1. 1 A company plans to spend $75,000 on digital advertising in 2026 if their total marketing budget is $150,000. If their total marketing budget increases to $200,000, how much should they allocate to digital advertising to maintain the same proportion?
  2. 2 We set up the proportion: $75,000 / $150,000 = x / $200,000. Cross-multiplying gives us $75,000 * $200,000 = $150,000 * x. This simplifies to $15,000,000,000 = $150,000 * x.
  3. 3 Dividing both sides by $150,000, we find x = $100,000.
  4. 4 Therefore, to maintain the same proportion of digital advertising within their increased marketing budget, the company should allocate $100,000 to digital advertising in 2026.

Source: Khan Academy · Last updated: April 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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