BYTETOOLS

Proportion Calculator

Solve proportions online: enter three values of a/b = c/d and the calculator finds the missing one by cross-multiplication, or checks if two ratios are proportional.

a/b = c/d — leave exactly one box blank (or type x) to solve for it, or fill all four to check the proportion.

9
Missing value (c)
Cross-multiply
Method

Working

c = (a × d) ÷ b = (3 × 12) ÷ 4 = 9

What is the Proportion Calculator?

The ByteTools Proportion Calculator solves the classic equation a/b = c/d for whichever value is missing.

  • Solves for any of the four terms in a/b = c/d
  • Cross-multiplication working shown step by step
  • Proportionality check mode when all four values are given
  • Handles decimals and negative numbers
  • Clear divide-by-zero warnings
  • 100% private — runs entirely in your browser

How to use the Proportion Calculator

  1. 1

    Enter values into three of the four boxes of a/b = c/d.

  2. 2

    Leave the unknown box blank or type x in it.

  3. 3

    Read the solved value and the cross-multiplication working.

  4. 4

    Or fill all four boxes to check whether the two ratios are proportional.

  5. 5

    Copy the result with one click.

About the Proportion Calculator

The ByteTools Proportion Calculator solves the classic equation a/b = c/d for whichever value is missing. Leave any one of the four boxes blank (or type x) and it finds that value by cross-multiplication — the rule that in a true proportion, a × d always equals b × c.

Fill in all four values instead, and the tool switches to checking mode: it compares the two cross products and tells you whether the ratios are genuinely proportional. This covers both of the tasks students and everyday users need — solving for an unknown in recipes, map scales, unit prices and similar problems, and verifying a claimed ratio.

The calculation runs entirely in your browser, with the worked cross-multiplication shown so you can see exactly how the answer was derived. Nothing is uploaded, and it works offline.

Frequently asked questions

How do you solve a proportion?

Use cross-multiplication: if a/b = c/d, then a × d = b × c. To find a missing value, multiply the two numbers diagonal to it and divide by the remaining number. For example, in 3/4 = x/12, x = (3 × 12) ÷ 4 = 9.

What is cross-multiplication?

It is the rule that two equal fractions have equal cross products. Multiplying both sides of a/b = c/d by b × d gives a × d = b × c. It works because you are simply clearing the denominators from both sides of the equation.

How do I check if two ratios are proportional?

Compare their cross products. The ratios 6/8 and 9/12 are proportional because 6 × 12 = 72 and 8 × 9 = 72. If the two products differ, the ratios are not equivalent. Enter all four numbers and the calculator does this comparison for you.

What are proportions used for in real life?

Scaling recipes, converting map distances, calculating unit prices, resizing images, adjusting medicine doses by weight and converting currencies all use proportions. Any 'if 3 costs this, what do 5 cost?' problem is a proportion.

Why does the calculator say it cannot solve my proportion?

Solving for a term requires dividing by one of the known values, and division by zero is undefined. For example, finding c in a/b = c/d requires dividing by b — if b is zero, no unique solution exists and the tool explains that instead of showing a wrong answer.

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