Proportion Calculator: Solve a/b = c/d
This free proportion calculator finds the missing value in a proportion of the form a/b = c/d. Choose which of the four numbers you want to solve for, type the other three, and the answer appears with the cross-multiplication steps written out. It also shows the ratio in simplest terms, the decimal value of the ratio and a quick check that both sides match. Use it for recipe scaling, map and model scales, unit rates, percent problems and school homework. It works with decimals and negative numbers, and nothing leaves your browser.
Proportion Calculator
Find the missing number in a proportion, with cross-multiplication steps.
How to use the Proportion Calculator
- Choose which value to find: A, B, C or D.
- Type the three known numbers in the boxes.
- Read the answer and the step-by-step cross multiplication.
- Check the simplified ratio and decimal value.
- Press Copy results to save the answer.
The formula
For a/b = c/d the cross products are equal: a × d = b × c. To find D, divide B × C by A. The other three values are found the same way, by moving the unknown to one side.
Features
- Solves for any one of the four values
- Cross-multiplication steps shown
- Simplified ratio and decimal value
- Check that both sides are equal
- Works with decimals and negatives
- Copy and print results
Frequently asked questions
What is a proportion?
A proportion is an equation that says two ratios are equal, such as 2/3 = 4/6. If three of the numbers are known, the fourth can be found.
How do you solve a proportion?
Cross multiply: multiply the top of one fraction by the bottom of the other. For a/b = c/d, the cross products a × d and b × c are equal, so d = b × c ÷ a.
Can I use decimals or negative numbers?
Yes. The calculator accepts decimals and negative numbers. It shows an error only if the step needs a division by zero.
How can I use a proportion for recipes?
If 3 cups of flour serve 4 people, set 3/4 = x/10 and solve for x to scale the recipe to 10 people. The answer is 7.5 cups.
How do I check my answer?
Divide the top by the bottom on each side. If both decimals match, the proportion is correct. The calculator shows both values for you.