Proportion calculator
Enter three values and find the fourth, in direct or inverse proportion.
X
If 3 corresponds to 12, then 5 corresponds to 20.
How this is calculated →- Working12 × 5 ÷ 3
- Constant4
How to solve a proportion
Direct (both quantities grow together): A is to B as C is to X, so X = B × C ÷ A. If 3 lb cost $12, 5 lb cost 12 × 5 ÷ 3 = $20.
Inverse (one grows as the other shrinks): X = A × B ÷ C. If 4 painters take 6 days, 8 painters take 4 × 6 ÷ 8 = 3 days.
Frequently asked questions
How do I know if it is direct or inverse?
Ask: if one quantity doubles, does the other double (direct) or halve (inverse)? More items, more cost: direct. More workers, less time: inverse.
Can I use it for percentages?
Yes. If 200 is 100%, what is 30? X = 30 × 100 ÷ 200 = 15%. The percentage calculator does this directly.
What is cross multiplication?
It is the same idea: in A/B = C/X, multiply across (A × X = B × C) and divide to isolate X.