How to use
Fill in the three numbers you know: if A corresponds to B, what does C correspond to? The calculator finds the unknown X and shows the formula with your numbers plugged in. This is also known as the rule of three.
Direct and inverse proportions
Direct proportion — the more of one quantity, the more of the other: more items cost more, more servings need more flour.
X = B × C / A
Inverse proportion — the more of one quantity, the less of the other: more workers need fewer days, a higher speed means less travel time.
X = A × B / C
Examples
- 3 lb of apples cost $6. How much do 5 lb cost? Direct: 6 × 5 / 3 = $10.
- A recipe for 4 servings uses 300 g of flour. How much for 6 servings? 300 × 6 / 4 = 450 g.
- 4 workers finish a job in 12 days. How long will 6 workers take? Inverse: 4 × 12 / 6 = 8 days.
- At 60 mph a trip takes 3 hours. How long at 90 mph? Inverse: 60 × 3 / 90 = 2 hours.
FAQ
How do I know if it’s direct or inverse? Ask yourself: if the first quantity doubles, does the second one double too (direct) or halve (inverse)?
Can I use a proportion for percentages? Yes: 30 is to 150 as X is to 100, so X = 20%. The percent of total calculator does this directly.