Proportion Calculator
Enter three values and find the fourth, in direct or inverse proportion, with the math explained.
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Fill in A, B and C. E.g. if 3 lb cost $12, how much do 5 lb cost?
Direct: both quantities grow together (e.g. more pounds, higher price).
Quick answer: This proportion calculator uses the rule of three, also known as cross-multiplication. Fill in A, B and C ("A is to B as C is to X") and choose "Directly proportional" or "Inversely proportional". For direct proportions, X = B × C ÷ A; for inverse proportions, X = A × B ÷ C.
How to do it in 3 steps
- 1Choose Directly proportional or Inversely proportional.
- 2Fill in A and B (the known ratio) and C (the new value).
- 3Read the value of X and the math used.
Why use Pronto Arquivo
Direct and inverse
Switch with one tap and see how the result changes.
Math shown
The formula appears with your numbers, for studying or checking your homework.
Decimals supported
Use decimals as usual: 2.5 lb, $12.90.
No ads between the fields
The three fields and the result stay together on one screen.
Guides
Direct proportion example
If 3 lb of rice costs $18, how much do 5 lb cost? A = 3, B = 18, C = 5 → X = 18 × 5 ÷ 3 = $30.
Inverse proportion example
4 workers build a wall in 6 days. How many days for 8 workers? A = 4, B = 6, C = 8 → X = 4 × 6 ÷ 8 = 3 days.
How to tell direct from inverse
Ask yourself: if one quantity doubles, does the other double (direct) or halve (inverse)?
Frequently asked questions
What is the rule of three?
It's a way to solve a proportion when you know three values. Set up "A is to B as C is to X" and cross-multiply: X = B × C ÷ A.
What's the difference between direct and inverse proportion?
In a direct proportion, both quantities grow together (more items, higher price). In an inverse proportion, one grows as the other shrinks (more speed, less time).
Can I use it with percentages?
Yes: if 200 is 100%, what percent is 50? A = 200, B = 100, C = 50 → X = 25%. For percentage math, also try the percentage calculator.
What about the compound rule of three?
Not available yet. Solve it in steps: apply the simple rule for each quantity, one at a time.