Math
Rule of Three Calculator
How to use this tool
- 1
Choose direct or inverse proportion.
- 2
Fill in A, B and C: A and B go together, C is the value you start from.
- 3
Read X and the sentence that restates it.
- 4
Follow the steps to check the reasoning, or copy the result.
Help
- Direct proportion: if A corresponds to B, then C corresponds to X, with X = B × C ÷ A. Use it when both quantities rise together, such as price and weight.
- Inverse proportion: if A corresponds to B, then X corresponds to C, with X = A × B ÷ C. Use it when one quantity rises as the other falls, such as workers and days.
- Decimals with a comma or a dot and Arabic digits are accepted. The calculation is exact, so 0.1 × 3 ÷ 0.3 gives exactly 1.
- The steps under the result show how to get there by hand: first bring it down to 1, then scale up to the new value.
Frequently asked questions
How do you do a rule of three?
Write the known pair, A and B, then the new value C. In direct proportion, divide B by A to get the value for 1, then multiply by C. For example, if 3 kg cost 12, then 1 kg costs 4 and 5 kg cost 20.
How do I know if it is direct or inverse?
Ask what happens when A doubles. If B doubles too, it is direct (price and quantity). If B is halved, it is inverse (workers and days, speed and travel time).
Can I use it for percentages?
Yes: treat 100 as A and the rate as B. To find 15% of 80, enter A = 100, B = 15 and C = 80. The result is 12.
Why does it say I cannot divide by zero?
In a direct rule A is the divisor, in an inverse rule C is. A value of zero there has no meaning, so enter a non-zero number.
Written and verified by the MaxyTools team · Updated on
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