Ratio Calculator
Simplify any ratio to its lowest terms, or solve a proportion to find a missing value in A : B = C : D. Free, instant and completely private — every calculation runs in your browser.
Fill in three of the four boxes and leave the unknown one empty — the calculator finds it.
How to use the Ratio Calculator
- Choose a mode. Pick Simplify a ratio to reduce a ratio to lowest terms, or Solve a proportion to find a missing value.
- Enter your numbers. For simplifying, type two terms (a third is optional). For a proportion, fill in three of the four boxes in A : B = C : D and leave the unknown empty.
- Read the result. The simplified ratio or the solved value appears instantly, along with the decimal, fraction and percentage breakdown, ready to copy.
About this tool
This free ratio calculator does the two jobs people most often need. Simplifying reduces a ratio such as 8 : 12 to its lowest terms, 2 : 3, by dividing both sides by their greatest common divisor — and it handles decimals like 1.5 : 3 and three-part ratios like 4 : 6 : 8 as well. Solving a proportion uses cross multiplication: because A : B = C : D means A × D = B × C, the calculator rearranges that equation to find whichever value you leave blank, showing the exact working so you can follow the maths.
Ratios are everywhere — mixing paint, scaling a recipe, sharing money, adjusting screen aspect ratios, converting map scales and reading financial statements. Alongside the simplified ratio you get the equivalent fraction, the decimal value and the percentage split, so 3 : 2 also reads as 3/2, 1.5 and a 60% / 40% share. Everything runs locally in your browser and nothing you type is ever uploaded. For related conversions see the Fraction Simplifier, the Percentage Calculator and the GCD Calculator.
Frequently asked questions
How do you simplify a ratio?
Divide every term by the greatest common divisor of all the terms. For 8 : 12 the GCD is 4, so both terms divide down to give 2 : 3. When the terms contain decimals, the calculator first scales them up to whole numbers — for example 1.5 : 2 becomes 15 : 20, which simplifies to 3 : 4.
How does solving a proportion work?
A proportion states that two ratios are equal, A : B = C : D. Cross multiplying gives A × D = B × C, and solving that for the empty box gives the answer. For instance in 3 : 4 = 9 : x, x = (4 × 9) ÷ 3 = 12. Leave exactly one of the four boxes blank so the tool knows what to find.
Can I use decimals or three-part ratios?
Yes. In simplify mode you can enter decimals such as 2.5 : 5 and an optional third term to reduce ratios like 4 : 6 : 8 down to 2 : 3 : 4. Every term must be greater than zero, since a ratio compares real quantities.
What is the difference between a ratio and a fraction?
They are closely related. The ratio A : B compares two quantities, and the same relationship can be written as the fraction A/B or as the decimal A ÷ B. A ratio of 3 : 2 is the fraction 3/2, the decimal 1.5, and a 60% to 40% split of the whole — all of which this tool displays for you.