How to calculate percentages: discounts, increases and VAT
The four most common percentage operations, with formulas and worked examples.
Rule of three calculator
Direct and inverse rule of three, step by step.
Runs in your browser
X
If 3 corresponds to 4,50, what corresponds to 7?
X
10.5
X = (4,50 × 7) / 3 = 10.5
What it does: Rule of three calculator
The rule of three finds an unknown value from three known ones when two quantities are proportional. This calculator solves both direct and inverse proportions and shows the operation used.
Use it to scale recipes, work out unit prices, change map scales or estimate times.
How to use it
- Choose whether the proportion is direct or inverse.
- Enter A and B (the known relationship) and C (the new value).
- Read X and the formula behind it.
Examples
Direct: buying in bulk
3 kg cost 4.50; how much do 7 kg cost? X = 4.50 × 7 / 3 = 10.50.
Inverse: sharing work
4 people take 6 days; how long will 3 take? X = 4 × 6 / 3 = 8 days.
Limits, formulas and assumptions
- Direct: X = B × C / A.
- Inverse: X = A × B / C.
- It only works if the relationship is truly proportional; many real cases (bulk discounts, fixed fees) are not.
Frequently asked questions
How do I know whether it's direct or inverse?
Ask what happens if you double one quantity: if the other doubles too, it's direct; if it halves, it's inverse.
Can I use decimals?
Yes, with a decimal point or comma.
What is a compound rule of three?
One that relates more than two quantities. You can solve it by chaining several simple rules of three.
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