What makes a proportion 'inverse'?

Two quantities are inversely proportional when their product stays constant. Double one and the other halves.

How is this different from the direct rule of three?

In the direct version both numbers rise or fall together. Here they move in opposite directions.

A car at 60 km/h takes 2 hours. At 120 km/h it takes 1 hour. Speed doubled, time halved — classic inverse proportion.

Will it handle fractional results?

Yes. Use the precision slider to show as many decimal places as you need, up to 10.

Rule of Three (Inverse)

Solve inverse-proportion problems instantly. Enter three values and find the missing fourth.

First Relationship

Value A

Unit (optional)

Value B

Unit (optional)

Second Relationship

Value C

Unit (optional)

Value X (result)

Unit (optional)

Decimal Places

Features

Inverse-proportion solver

Handles cases where quantities move in opposite directions

Transparent working

Step-by-step display of the formula and arithmetic

Precision slider

Choose 0 – 10 decimal places for the result

Unit labels

Optional labels (hours, workers, km/h) for clarity

How to use this tool

1
Enter the three knowns — For instance: 4 workers ↔ 12 days, now find the days for 6 workers
2
Add units (optional) — Labelling helps avoid mix-ups between the two quantities
3
Calculate — The missing value appears together with the cross-multiplication working
4
Inspect the steps — Expand the solution to see exactly how the answer was derived

Common uses

Workforce planning

If 5 people take 8 days, how long will 10 people need?

Travel estimates

Doubling your speed halves the journey time — confirm the exact figure

Pressure & volume (Boyle's law)

When a gas is compressed its pressure rises inversely

Supply & duration

Food, fuel, or materials lasting fewer days when consumption increases

Frequently asked questions

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Rule of Three (Inverse)

First Relationship

Second Relationship

Features

Inverse-proportion solver

Transparent working

Precision slider

Unit labels

How to use this tool

Common uses

Workforce planning

Travel estimates

Pressure & volume (Boyle's law)

Supply & duration

Frequently asked questions

What makes a proportion 'inverse'?

How is this different from the direct rule of three?

A concrete example?

Will it handle fractional results?

Related Tools

Rule of Three (Inverse)

First Relationship

Second Relationship

Features

Inverse-proportion solver

Transparent working

Precision slider

Unit labels

How to use this tool

Common uses

Workforce planning

Travel estimates

Pressure & volume (Boyle's law)

Supply & duration

Frequently asked questions

What makes a proportion 'inverse'?

How is this different from the direct rule of three?

A concrete example?

Will it handle fractional results?

Related Tools