Ratio Calculator
Solve ratio problems step-by-step with our free calculator. See formulas, worked examples, and clear explanations. Get results you can export or share.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
A/B = C/D → D = B × C / A
Enter three of the four terms in a ratio comparison A:B = C:D. Cross-multiplying (A × D = B × C) and solving for the unknown term gives you a ratio equivalent to the first, scaled to a new size.
Worked Examples
Example 1: Scaling a recipe
Problem:A recipe uses a flour-to-water ratio of 5:3. You have 12 cups of flour — how much water do you need?
Solution:Set up 5:3 = 12:D. Cross-multiply: 5 × D = 3 × 12 = 36, so D = 36/5 = 7.2.
Result:You need 7.2 cups of water
Example 2: Reading a map scale
Problem:A map has a scale of 1:25,000. A measured distance on the map is 4.4 cm — what is the real-world distance?
Solution:Set up 1:25,000 = 4.4:D. D = (25,000 × 4.4)/1 = 110,000 cm = 1.1 km.
Result:Real-world distance = 1.1 km
Frequently Asked Questions
What does it mean to find an equivalent ratio?
An equivalent ratio scales every term of a ratio by the same factor, so the underlying proportional relationship stays identical — 2:3, 4:6, and 20:30 are all equivalent ratios because each is 2:3 multiplied by 1, 2, and 10 respectively. Ratio Calculator finds the missing fourth term (D) that keeps A:B equivalent to C:D, which is exactly the technique used to scale a ratio up or down to a new size.
What is the difference between a ratio and a fraction?
A ratio compares two (or more) quantities directly, like 3:4, while a fraction represents a part relative to a whole, like 3/4. Every ratio A:B can be rewritten as the fraction A/(A+B) if you want the share of the total, or simply as A/B if you want the relative comparison — Ratio Calculator uses the A/B = C/D cross-multiplication form common to both interpretations.
How do I simplify a ratio to its lowest terms?
Divide both terms of the ratio by their greatest common factor (GCF). For example, 24:36 has GCF 12, so dividing both terms by 12 gives the simplified ratio 2:3. A ratio is in lowest terms once its two terms share no common factor other than 1 (i.e., they are coprime).
Why does the calculator need exactly three values to find a ratio?
A ratio comparison (A:B = C:D) is a single equation with one degree of freedom once three of the four terms are known — cross-multiplying (A×D = B×C) lets you solve directly for the missing term. With only two values you can express a ratio but can't yet find an equivalent one; you need a third reference value to anchor the scale.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy