MathsLive

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

C (optional — to solve A:B = C:?)

Fill in the fields above to see your result instantly.

Calculation method, coverage and version

Inputs used: A, B, C (optional — to solve A:B = C:?).
Calculation approach: The calculator applies the rules and assumptions described in “How it works” to the values you enter. Rounding can cause small differences from an official or provider calculation.
Coverage: Maths. UK nations, tax treatment, local rules and provider criteria can differ; check the official sources shown below before acting.
Content version: No page-specific review date is currently published. Treat the result as indicative and verify current rules independently.

Not included unless explicitly requested: personal circumstances not entered, provider discretion, future rule changes, professional fees and case-specific exceptions. Read the full calculator methodology.

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 →