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Proportion calculator

Find the missing number in a : b = c : x by cross-multiplying, or share a total out in a ratio, with the steps. Free, with no account or uploads.

Find the missing number in a proportion

Fill in three of the four boxes and leave the one you want empty. It is found by cross-multiplying.

What do you want?
Find xShare out a total

Leave the box you want worked out empty.

The missing number

x = 12

How it is worked out

  1. Write the ratios as fractions that are equal.\frac{3}{4} = \frac{9}{x}
  2. Cross-multiply: the top of each times the bottom of the other are equal.3 \times x = 4 \times 9
  3. Divide to get x on its own.x = \frac{4 \times 9}{3} = 12
How it works and help
Every fraction is kept exact, step by step, on your device. Nothing you type is sent anywhere.

A little help along the way

Follow these steps. Everything is worked out exactly in your browser; nothing is sent anywhere.

  1. Say what you want to doWork with fractions is the usual. The others simplify a ratio, find x or share out a total, turn decimals and fractions into each other, and work with picture sizes.
  2. Type the numbersFractions as 3/4, mixed numbers as 1 1/2, ratios with colons as 12 : 18, and repeating decimals with brackets as 0.1(6).
  3. Read the answer and every stepThe answer is drawn as a textbook would, with each step under it.

Questions people ask

How do I solve a proportion?
Write the two ratios as fractions that are equal and cross-multiply: in 3 : 4 = 9 : x, 3 × x = 4 × 9, so x = 36 ÷ 3 = 12. The page shows these steps with your numbers.
Does x have to be the last box?
No. Leave any one of the four boxes empty and that one is worked out.
How do I share an amount in a ratio?
Choose Share out a total. 1,200 in the ratio 2 : 3 is 480 and 720: the parts add to 5, one part is 240, and each share is its parts times 240.

This page is the Fraction and ratio calculator, opened for this one job; every other tool it has is one press away. Each of its settings is explained on the Fraction and ratio calculator's own page.

Read the step-by-step guide

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