Proportion Calculator
A proportion says two ratios are equal: A/B = C/D. Choose which of the four terms is missing, enter the other three, and the calculator solves for it and writes out the completed proportion.
- Proportion
- 3/4 = 9/12
- Cross product (A × D = B × C)
- 36
Saved setups
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Your recent calculations
Results you calculate here are kept on this device so you can come back to them.
Formula
How to use it
- Pick the term you want to solve for.
- Enter the three values you know in their positions.
- Read the missing value and check the completed proportion.
Worked examples
3/4 = 9/D
- Missing value
- 12
- Proportion
- 3/4 = 9/12
- Cross product (A × D = B × C)
- 36
2 cups of rice serve 5 people; how many cups for 12?
- Missing value
- 4.8
- Proportion
- 2/5 = 4.8/12
Setting it up correctly
Keep the same kind of quantity in the same position on both sides. If 2 cups of rice serve 5 people, then for 12 people write cups over people on both sides: 2/5 = C/12, so C = 2 × 12 ÷ 5 = 4.8 cups.
Most wrong answers come from flipping one side — writing 2/5 = 12/C instead.
When a proportion is the wrong tool
Proportions only work when the two quantities rise and fall together. If more of one means less of the other, the product stays constant instead: 4 workers taking 6 days is 24 worker-days, so 8 workers take 3 days, not 12.
Questions people ask
How do I solve 3/4 = 9/x?
Cross-multiply: 3 × x = 4 × 9 = 36, so x = 12.
If 2 cups serve 5 people, how many cups serve 12?
4.8 cups. Set up 2/5 = x/12 and solve: x = 2 × 12 ÷ 5.
What is the rule of three?
It is another name for solving a proportion: given three numbers where a is to b as c is to x, the fourth is x = b × c ÷ a.