How to Use the Ratio Calculator
Ratios are used everywhere from recipes to financial analysis. This calculator simplifies ratios and solves proportion problems in seconds.
Step-by-Step Instructions
- 1 Choose what you want to do: simplify a ratio or find a missing value.
- 2 Enter the known values.
- 3 View the simplified ratio or calculated missing value.
Understanding the Inputs
First Value (A)
The first term in your ratio, representing the initial value in the proportion. In the ratio 3:5, A is 3. For a recipe with a flour to sugar ratio of 2:1, A is 2 cups of flour.
How to find it: A is the leftmost or first number in your ratio or proportion. If you are comparing two ratios like "4:7 = 8:x," the first value of the first ratio (4) goes in A. Enter it exactly as written.
Why it matters: A anchors the first term of your ratio. Along with B, it defines the relationship that must be maintained in the proportion.
Type: number · Default: 0
Second Value (B)
The second term in your ratio, paired with A to define the ratio relationship. In the ratio 3:5, B is 5. If the ratio of boys to girls in a class is 3:5, B is 5 representing the number of girls.
How to find it: B is the second number written in your ratio, separated from A by a colon. In "16:9" (a common screen aspect ratio), B is 9. Enter the second value directly from your ratio.
Why it matters: B completes the known ratio that serves as the reference proportion. The calculator uses the relationship between A and B to find the missing value that preserves the same relationship.
Type: number · Default: 0
Third Value (C)
The third term in your proportion, usually the known value that corresponds to A in the second ratio. For the proportion 2:3 = 8:x, C is 8 (the value corresponding to A). For scale models, if 1 inch = 12 feet and your model is 6 inches, C is 6.
How to find it: C is the first number in the second ratio. When setting up A:B = C:D, C is the known value that pairs with A. In map scale problems, this is your measured distance on the map.
Why it matters: C is the known counterpart to A in the proportion. The calculator uses the rule that A divided by B equals C divided by D. An incorrect C value produces a wrong result for the missing term.
Type: number · Default: 0
Fourth Value (D)
The fourth term in your proportion, typically the value you are solving for. In "4:7 = 8:D," D is the unknown. If 3 eggs are needed per 2 cups of flour and you have 6 cups of flour, D would be 9 eggs.
How to find it: D is the last position in the proportion A:B = C:D. When you set Missing Value to D, leave this field blank or at zero and D is calculated.
Why it matters: D is most often the value you are solving for. The calculator uses cross multiplication (A times D equals B times C) to find its value.
Type: number · Default: 0
Missing Value
Selects which value in the proportion A:B = C:D is unknown and should be calculated. Choose C if you know A, B, and D but need C. Choose D if you know A, B, and C but need D.
How to find it: Select this option from the dropdown above the four values. Read your problem carefully: if the question asks "find x" in 3:5 = x:20, set missing to C. If it asks for the last value, set it to D.
Why it matters: Selecting the correct missing value tells the calculator which variable to solve for using cross multiplication. Choosing the wrong one returns an incorrect result because the formula changes which terms are known versus unknown.
Type: select · Default: c
Tips & Best Practices
- ✓ To simplify a ratio, divide both sides by their greatest common factor.
- ✓ Ratios are typically written in lowest terms (e.g., 4:6 simplifies to 2:3).
- ✓ In a proportion, the product of the extremes equals the product of the means.
by CalculatorPro Tools · Updated 2026-07-29