Free Rule of Three Calculator
Solve any proportion instantly. If a relates to b in the same way that c relates to x, this calculator finds x. Also known as cross-multiplication or the rule of three, it is one of the most useful everyday maths tools.
How to Use
Enter the three known values (a, b and c) and the missing value x is solved automatically. The formula is x = b × c ÷ a.
- Solves a : b = c : x for x
- Shows the full working with the formula
- Detects direct vs inverse proportion
- Useful for scaling recipes, prices and dimensions
- Instant recalculation
When to use it
If 4 items cost $10, how much do 7 items cost? That is a rule-of-three problem. Use direct proportion when both values increase together (more items, more cost) and inverse proportion when one goes up while the other goes down (more workers, less time).
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