Find the fourth proportional value

When values need comparing or a one-off calculation needs checking, this helps students, individuals and professionals. Find the fourth proportional value.

Find the fourth proportional value. The result appears instantly in your browser, ready to copy, with no account, upload or storage involved.

How to use

Enter the first value A in the matching field.
Type the value B that corresponds to A.
Add the value C for which you want to find X.
Run the calculation to get the fourth proportional value.
Review the result and check that the units make sense.

Example

With an input such as 2, 4 and 6 on three lines, the tool handles a simple case where 2 corresponds to 4. It then calculates the value X linked to 6. Look at the displayed result to confirm that the proportion is preserved and the relationship matches your situation.

Input

2
4
6

Output

X = 12

Limits and formats

The rule of three assumes direct proportionality; it does not fit inverse or non-linear relationships.
Units must remain consistent between A and B, then between C and X, or the result has no meaning.
Very large or very small numbers may be rounded in the display; check precision if it matters.

Where processing happens

The calculation runs entirely in your browser. The values you enter stay on your device and are not sent to any external service. You can try several sets of numbers without an account or storage.

Frequently asked questions

When should I use the rule of three?

It helps find a fourth value when three values form a direct proportion. For example, if one quantity matches one price, you can estimate the price of another quantity. Always confirm that the relationship is proportional before relying on the answer.

What if the result looks strange?

Check the three values you entered and their order. Swapping A and B or B and C changes the outcome. Also review the units and do a simple manual calculation to confirm that the proportion is set up correctly.

Can I use it with percentages?

Yes, as long as you treat percentages as numbers and keep the same comparison base. If you mix a percentage with a quantity, the result can be misleading. Convert the data into a common unit first when needed.