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Ratio Calculator

Simplify A:B or scale a ratio up while keeping it equivalent.

Short answer

Enter A and B to see A:B simplified. Add a C value (in the third box) and we'll solve for D so A:B = C:D.

Inputs

Fill in the fields above to see your result instantly.

How it works

Divide both terms by their greatest common divisor (GCD) to simplify. To solve A:B = C:D for D, cross-multiply: D = (C × B) ÷ A.

Worked example

12:18 → GCD is 6 → simplified to 2:3. If we want 2:3 = 10:?, then ? = (10 × 3) ÷ 2 = 15.

Who should use this

  • •Recipe scaling
  • •Map and model builders
  • •Mixing paint, fuel or chemicals

Common mistakes

  • ×Mixing up which term is A vs B when scaling
  • ×Rounding too early — keep full precision until the end

Frequently asked questions

Is a ratio the same as a fraction?▾

They're related — a ratio A:B is equivalent to the fraction A/B but is usually used to compare two quantities of the same kind.

Related calculators

Fraction Calculator →Percentage Calculator →