Direct Proportion Rule
Solve direct proportion problems using the rule of three. Enter three known values to calculate the missing fourth value.
a / b = c / d
Enter 3 values to find the 4th
Why use this tool?
The direct proportion rule (also known as the rule of three) is essential for solving problems where two ratios are equal. This tool helps you quickly find the missing value.
- Simple interface — enter three values and get the fourth instantly.
- Shows the formula — understand how the calculation works.
- Privacy-focused — all calculations happen in your browser. No data is sent to servers.
- Fast and free — no sign-up required, instant results.
How to use
- Enter three of the four values in the proportion a/b = c/d.
- Leave the field blank for the value you want to calculate.
- Click Calculate to see the result.
- View the calculated value and the formula used.
- Click Copy Result to copy the answer to your clipboard.
Frequently Asked Questions
Direct proportion occurs when two quantities increase or decrease at the same rate. Mathematically, if a/b = c/d, then a is directly proportional to c, and b is directly proportional to d.
Yes, the MushTools Direct Proportion calculator is completely free with no sign-up required. All calculations happen in your browser.
No. Everything runs client-side in your browser. Your data is not stored or sent to any server.