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Inverse rule of three

Solve an inverse proportionality problem.

Calculator

Known value (pair 1)
Value corresponding to a
New value (c ≠ 0)
Rounding

Results

x40.00
product (a · b)200.00
Formula
—

Examples

Click to prefill

Definition & method

In inverse proportionality, the product remains constant.

Formula
—
  1. Calculate the product: a · b.
  2. Set the equality: a · b = c · x.
  3. Isolate x: x = (a · b) / c.
If c increases, x decreases (and vice versa): c × x remains constant.
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About: Inverse rule of three

This tool allows you to easily solve an inverse proportionality calculation, when the product of two quantities remains constant.

Relevant situations

  • Time and workforceThe more people there are, the less time it takes.
  • SpeedsSame distance traveled at different speeds.
  • ProductionDistribution of a fixed load.

Result

The unknown value calculated automatically.

A calculation respecting the principle of constant product.

Common errors

  • Do not use this tool for direct proportionality.
  • Check that the product remains constant.

Tip

If one quantity doubles and the other is halved, you are indeed in an inverse case.