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Proportion calculator (Rule of three)

Calculate a missing value in a direct or inverse proportion. Choose the relationship, enter A, B and C, and get the result with the rule-of-three formula.

  • 100% free
  • No sign-up
  • Files stay on your device

Use the rule of three to solve a direct or inverse proportion. Enter three known values to calculate X.

Direct: X = (B × C) ÷ A. Use when the values increase or decrease together.

Result12

Why people use QRSmith

Private by design

Your files and text never leave your device. Nothing is uploaded or stored.

Instant results

No queues and no waiting for uploads. Everything is calculated on your own device.

Free, no limits

No account, no watermark and no daily limit. Use it as often as you need.

How it works

  1. 1

    Enter your values or paste your content.

  2. 2

    Choose the operation you need.

  3. 3

    Get the result instantly in your browser.

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Frequently asked questions

How do I calculate a proportion?

For direct proportion, X = (B × C) ÷ A. For inverse proportion, X = (A × B) ÷ C. The divisor cannot be zero; choose direct or inverse based on how the two quantities relate.

Is it really free?

Yes. There is no sign-up, no watermark and no limit on how many times you use it.

Are my files or text uploaded anywhere?

No. Everything is processed inside your browser on your own device, so your data never reaches our servers.