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The inverse rule of three is the counterpart to the direct one: here two quantities are linked antiproportionally, more of one means less of the other. The classic example is workers and time: if four workers need six days for a task, three workers complete the same task in more days, not fewer. The same holds for pumps and pumping time, speed and travel time, or the number of guests and supply per person.
The calculation runs entirely locally in your browser, in pure JavaScript - nothing is uploaded and nothing is stored. With the inverse rule of three the product of the two quantities stays constant: quantity A times its value gives a fixed total amount of work or capacity. The sought result is that product divided by the new quantity. Example: four workers times six days is 24 worker-days; divided by three workers that is eight days. Change an input and everything updates instantly.
An honest note: this calculator is meant for exactly the inverse (antiproportional) case. If more of A also means more of the result - for example amount and price - use the direct rule of three calculator. For plain percentage questions the percentage calculator fits better. The results are unitless numbers whose meaning you set yourself.
Specifications
Specifications
Input formats
Form inputs (no file)
Processing
Locally in your browser (JavaScript)
File upload
None
In 3 steps
Enter quantity A and its value (time, effort).
Enter the new quantity.
Read off the result and the constant product.
Limitations: The inverse (antiproportional) rule of three via the constant product: more A, less result. The direct case (more A, more result) is covered by the rule of three calculator. The results are unitless numbers whose meaning you set yourself.
FAQ
Are my inputs uploaded?
No. The calculation runs entirely locally in the browser (pure JavaScript); nothing is sent or stored.
When do I use the inverse rule of three?
Whenever more of one quantity means less of the other: more workers need less time, more pumps less pumping time, higher speed less travel time.
How is it calculated?
The product of quantity A and its value stays constant. The result is that product divided by the new quantity - in short: A times value, divided by the new quantity.
What is the difference to the direct rule of three?
With the direct rule of three more of A means more of the result (proportional). With the inverse one more of A means less of the result (antiproportional) - a different calculation.