Rule of three calculator

Unknown value X -
Type -
Relationship -
Formula -
Substitution -
Step by step

    Use direct proportion when both values grow together. Use inverse proportion when one value grows while the other shrinks.

    See examples

    Direct proportion

    If 4 items cost 10, calculate how much 6 items cost using the same ratio.

    Inverse proportion

    If 4 people finish a job in 10 days, estimate how many days 8 people need when the work is shared evenly.

    Study and homework

    Check the formula, substitution, and result for proportional reasoning exercises.

    About this helper

    The rule of three solves a missing value when three related values are known. Direct proportion is used when both values increase or decrease together. Inverse proportion is used when one value increases while the other decreases, such as more workers needing fewer days for the same job.

    What is direct rule of three?

    Direct rule of three is used when the two quantities move in the same direction. If one doubles, the other doubles too.

    What is inverse rule of three?

    Inverse rule of three is used when the quantities move in opposite directions. For example, more workers may need fewer days for the same task.

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    No. The calculation runs in your browser and the site does not receive the numbers you type.