Ratio Calculator

A ratio is a comparison of two quantities expressed as a : b or a/b. Ratios appear everywhere — in cooking (2 cups flour : 1 cup sugar), finance (debt-to-equity ratio), maps (1 : 50,000 scale), and image dimensions (16 : 9 aspect ratio). Our ratio calculator handles all three core ratio operations: simplifying to lowest terms, scaling by a factor, and solving for a missing value in a proportion — all instantly in your browser with no signup required.

1. 1. Choose a Mode: Select Simplify (reduce a ratio to lowest terms), Scale (multiply both parts by a factor), or Find Missing Value (solve a : b = c : ? for the unknown). Each mode shows only the inputs you need.
2. 2. Enter Your Values: Type your numbers into the input fields. The ratio calculator accepts integers and decimals. For the missing value mode, enter three known values and the tool solves for the fourth using cross-multiplication.
3. 3. Get Instant Results: Click Calculate and instantly see the simplified ratio, scaled values, or missing number — along with the decimal equivalent, percentage representation, and a visual proportion bar. All processing happens locally in your browser.

• What is a Ratio?: A ratio compares two quantities by division, written as a : b or a/b. For example, 3 : 4 means "for every 3 of A, there are 4 of B." Ratios can be simplified to their lowest terms by dividing both parts by their Greatest Common Divisor (GCD). Our ratio calculator uses the Euclidean algorithm to find the GCD and reduce any ratio to its simplest form.
• Simplifying a Ratio: To simplify a ratio a : b, find the GCD of a and b, then divide both parts by it. For example, 12 : 8 → GCD(12, 8) = 4 → 3 : 2. The simplified ratio is equivalent to the original — it represents the same relationship in the smallest whole numbers. Our ratio calculator handles decimal inputs by scaling to integers before applying the GCD.
• Scaling a Ratio: Scaling a ratio multiplies both parts by the same factor, producing an equivalent ratio. For example, 3 : 4 scaled by 5 gives 15 : 20. Scaling is used in recipe conversion, map reading, and any situation where you need to maintain a proportion while changing the actual quantities.
• Finding a Missing Value (Cross-Multiplication): In a proportion a : b = c : d, if one value is unknown, cross-multiply to solve: a × d = b × c. Rearranging: d = (b × c) / a. For example, 3 : 4 = 9 : ? → d = (4 × 9) / 3 = 12. This is the same as solving the equation a/b = c/d for the unknown variable.

Our ratio calculator accepts decimal inputs (e.g. 1.5 : 2.5) and simplifies them correctly by scaling to integers before applying the GCD algorithm. For example, 1.5 : 2.5 → 3 : 5. Very large or very small decimals may produce large integer ratios due to floating-point precision limits. For the missing value mode, the calculator uses exact floating-point arithmetic — results are accurate to approximately 15 significant digits.