Proportion Calculator: Cross Multiply Method + Examples
Proportions answer everyday questions: “If 3 items cost $12, what do 7 cost?” or “How much flour for 30 cookies instead of 12?” A free proportion calculator finds the missing number in a/b = c/d so you skip the algebra when you are in a hurry.
This guide explains cross-multiplication with worked examples, then shows how to verify homework or kitchen math with the ToolMars solver. For related fraction work, see our mixed number guide.
Solve any missing value in a/b = c/d
Free cross-multiply proportion calculator — instant, private, no signup.
What Is a Proportion?
A proportion says two ratios are equivalent — a is to b as c is to d. With three values known, you can always find the fourth.
How to Cross-Multiply
Example 1: Solve x / 4 = 6 / 8
- Cross-multiply: x × 8 = 4 × 6 → 8x = 24
- Divide: x = 3
Example 2: Recipe scaling — 2 cups flour makes 12 cookies. How much for 30?
- 2 / 12 = x / 30 → 12x = 60 → x = 5 cups
The same steps work no matter which letter is missing — a, b, c, or d.
Everyday Examples
Where proportions show up
- Recipes: Scale ingredients up or down by serving count.
- Maps: 1 inch = 5 miles — a 3.2-inch route = 16 miles.
- Shopping: 3 items for $15 → 7 items at the same rate = $35.
- Check any scenario in the ToolMars proportion calculator in seconds.
Ratio vs Proportion
A ratio compares two amounts (3:5). A proportion states two ratios are equal (3/5 = 6/10). One comparison is a ratio; setting two comparisons equal creates a proportion you can solve.
Solve any missing value in a/b = c/d
Free cross-multiply proportion calculator — instant, private, no signup.
Conclusion
Cross-multiplication turns proportion problems into one-step algebra. Enter any three values in the ToolMars Proportion Calculator and get the missing fourth — whether you are scaling a recipe, reading a map, or checking homework.
Frequently Asked Questions
How do you solve a proportion?
Cross-multiply (a × d = b × c), then isolate the unknown.
Ratio vs proportion?
Ratio = one comparison. Proportion = two equal ratios.
Can the calculator find any missing value?
Yes — enter three values, leave one unknown.
Real-life uses?
Cooking, maps, unit rates, shopping, and geometry.
What if a number is zero?
Division by zero is undefined — recheck inputs.
Proportions with fractions?
Cross-multiply the same way, or convert to decimals first.
Unit rate vs cross-multiply?
Both work — unit rate scales from one unit; cross-multiply solves directly.