Proportion Calculator
Proportion Calculator
Solve for:
Cross multiply: A × D = B × C
Please enter values in all 3 fields.
Result
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What is a Proportion Calculator?
A proportion calculator is a tool that helps you solve for an unknown value when two ratios are equal. It is based on the mathematical concept that A/B = C/D, meaning the ratio of A to B equals the ratio of C to D.
For example, if you know that 3 out of every 5 students passed an exam, and there are 40 students total, a proportion calculator instantly tells you how many students passed.
How to Use This Calculator
- Select the unknown: Click A, B, C, or D to choose which value you want to find.
- Enter the 3 known values in the remaining fields.
- Click Calculate Proportion to get the answer instantly.
Formula Behind the Calculator
The proportion formula is based on cross-multiplication:
- Solve A: A = (B × C) / D
- Solve B: B = (A × D) / C
- Solve C: C = (A × D) / B
- Solve D: D = (B × C) / A
Since A × D = B × C, you can rearrange the equation to find any missing value as long as the other three are known.
Practical Examples
| Scenario | Known Values | Answer |
|---|---|---|
| Recipe scaling: 2 cups for 4 people, how many for 10? | A=2, B=4, D=10 | C = 5 cups |
| Map scale: 1 cm = 50 km, 3 cm = ? | A=1, B=50, C=3 | D = 150 km |
| Speed: 60 miles in 1 hour, 150 miles in ? hours | A=60, B=1, C=150 | D = 2.5 hours |
| Discount: $20 off $100, same rate on $350 = ? | A=20, B=100, D=350 | C = $70 off |
Where Are Proportions Used?
- Cooking & Recipes – Scaling ingredients up or down
- Maps & Architecture – Converting scale distances to real distances
- Finance – Calculating percentages, interest, and discounts
- Science & Medicine – Drug dosages, chemical mixtures
- Construction – Material ratios like concrete mix
- Photography – Resizing images while keeping aspect ratio
FAQs About Proportion Calculator
A proportion is an equation that states two ratios are equal. It is written as A/B = C/D, or equivalently A:B = C:D.
Cross multiplication means multiplying diagonally: A × D = B × C. This lets you solve for any unknown value in a proportion equation.
Yes. For example, "20 is what percent of 80?" becomes 20/80 = X/100, so X = 25%.
Division by zero is undefined in math. Make sure B, C, or A (whichever is in the denominator for the unknown you are solving) is not zero.
They are related but different. A ratio calculator simplifies ratios (e.g., 4:8 = 1:2), while a proportion calculator solves for an unknown value in two equal ratios.