Proportion Calculator
Solve a / b = c / d for any unknown value with our free online proportion calculator. Enter three known values and leave the unknown blank.
How to Use This Calculator
- Select which variable (a, b, c, or d) you want to solve for using the buttons at the top.
- Enter the other three values (positive numbers) in the input fields.
- Click Solve Proportion to see the result and step-by-step solution.
- The cross-multiplication check verifies that the answer is correct.
Formula
Solve for any unknown:
- a = (b × c) / d
- b = (a × d) / c
- c = (a × d) / b
- d = (b × c) / a
Examples
Recipe Scaling
If 2 cups flour makes 24 cookies, how much for 36 cookies? 2/24 = x/36 → x = 3 cups.
Map Scale
1 inch = 10 miles. If a distance is 3.5 inches on the map: 1/10 = 3.5/x → x = 35 miles.
Unit Price
3 items cost $12. How much for 7 items? 3/12 = 7/x → x = $28.
Frequently Asked Questions
What is a proportion?
A proportion is an equation stating that two ratios are equal, expressed as a/b = c/d.
What is cross-multiplication?
Cross-multiplication is the method of multiplying the numerator of one ratio by the denominator of the other: a × d = b × c.
Can I use this for percentages?
Yes. Percentages are proportions where one ratio has a denominator of 100. For example, 25% = 25/100 = x/200.
What if the unknown is the same on both sides?
This calculator handles one unknown at a time. For equations with variables on both sides, use a different tool.