Math

Rule of three calculator

Solve a proportion: if A gives B, what does C give?

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How it works

X = B × C / A. Example: 3 tickets cost 4.50, so 7 tickets cost 4.50 × 7 / 3 = 10.50.

Results are for guidance only and do not replace professional advice.

Worked example

With the default values:

  • A3
  • gives B4.50
  • What does C give?7

you get:

  • Result (X)10.5
  • Working4.5 × 7 / 3

Frequently asked questions

How does the “Rule of three calculator” tool work?

X = B × C / A. Example: 3 tickets cost 4.50, so 7 tickets cost 4.50 × 7 / 3 = 10.50.

What do I need to enter?

Just fill in: a, gives b, what does c give?. The result updates as you type.

Can you give an example?

A: 3; gives B: 4.50; What does C give?: 7 → Result (X): 10.5; Working: 4.5 × 7 / 3.

Is this calculator free, and is my data private?

Yes. HOD Calc is completely free with no sign-up. The calculation runs in your browser: nothing you type is sent or stored.