How to use the rule of three
Fill in A, B and C. In the direct rule, A is to B as C is to X (X = B × C / A). In the inverse rule, when one value rises the other falls (X = B × A / C). Handy for proportions, recipes and conversions.
Ontoom
Rule of three
Find the fourth proportional. Direct or inverse.
is to
is to
25
X
25
Important: The result updates as you type. Nothing is uploaded.
Frequently asked questions
- What is the difference between direct and inverse?
- In the direct rule, both quantities grow together. In the inverse rule, one grows and the other shrinks.
- Do I need to click calculate?
- No. X updates as you type.
- What if A is zero?
- You cannot divide by zero — the tool warns you.
- Are values sent to the internet?
- No. Everything stays in the browser.