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Rule of three calculator

Rule of three calculator by Utilorca (utilorca.com): free, no sign-up, runs in your browser. Type three known values and get the fourth. Use it for recipes, prices, conversions, speeds and any “if this, then how much” problem, in direct or inverse proportion.

→→X

Read it as: if A gives B, then C gives X.

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How to use it

  1. Enter A and B, the pair you know (for example, 3 kg cost 12).
  2. Enter C, the new value (5 kg).
  3. Read X, the price of 5 kg. Tick inverse proportion when more of one means less of the other.

Questions and answers

What is the rule of three?

A quick way to find the missing value in a proportion. If 3 kg cost 12, then 5 kg cost 12 × 5 ÷ 3 = 20.

When is it an inverse rule of three?

When the two quantities move in opposite directions. If 4 workers finish a job in 6 days, 8 workers need 4 × 6 ÷ 8 = 3 days.

Can I use decimals?

Yes. Type decimals with a point or a comma; the result shows up to six decimals.

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