Rule of three calculator
Solve direct and inverse proportions instantly. Enter three values and get the fourth.
What is the rule of three
The rule of three solves a proportion when you know three values and need the fourth. It is direct when both quantities grow together, and inverse when one rises while the other falls.
Direct rule of three
Two quantities are directly proportional when, as one rises, the other rises by the same factor. Example: if 3 kg of fruit cost £6, how much do 5 kg cost? More kilos, more cost → x = 5 × 6 / 3 = £10.
Inverse rule of three
They are inversely proportional when, as one rises, the other falls. Example: if 4 workers take 6 days for a job, how long do 8 workers take? More workers, fewer days → x = 4 × 6 / 8 = 3 days.
Frequently asked questions
How do I know if it is direct or inverse? If increasing one quantity increases the other, it is direct; if it decreases the other, it is inverse.
Does it work for percentages? Yes: a percentage is a direct rule of three out of 100.
Can I use decimals? Yes, enter decimal values and the result updates instantly.