Rule of Three

What Rule of Three does

Given a:b as c:x, returns x = (b×c)/a. Unlike Percentage Change this is a proportion, not a percent delta. Unlike Recipe Scaler this is three numbers, not a recipe list. Zero a fails.

The interactive controls for Rule of Three load after this article so crawlers still read the instructions. Path /rule-of-three is the canonical address. Work happens on this device; Tool-You does not take a copy of the file or pasted text for Rule of Three.

How to use Rule of Three

1. Confirm you need Rule of Three, not Percentage Calculator (Percent of a number, X is what % of Y, or change %.). Address: /rule-of-three.
2. Enter the numbers or dates Rule of Three uses. Solve a/b = c/x for x.
3. Read the result as Rule of Three defines it — not as Percentage Change (Percent change and delta; optional rounding.). Given a:b as c:x, returns x = (b×c)/a. Unlike Percentage Change this is a proportion, not a percent delta. Unlike Recipe Scaler this is three numbers, not a recipe list. Zero a fails.
4. Rule of Three is an estimate in the browser, not professional advice. a:b = c:x. Zero a fails.
5. Need a different formula? Try Percentage Change (Percent change and delta; optional rounding.) instead of stretching Rule of Three.

When not to use Rule of Three

Skip Rule of Three when a neighbor tool matches better — start with Percentage Calculator or Percentage Change — or when the description already warns the job will fail. In the tool’s own words: Given a:b as c:x, returns x = (b×c)/a. Unlike Percentage Change this is a proportion, not a percent delta. Unlike Recipe Scaler this is three numbers, not a recipe list. Zero a fails.

Questions about Rule of Three

What does Rule of Three do that other Calculate pages on this site do not?

Given a:b as c:x, returns x = (b×c)/a. Unlike Percentage Change this is a proportion, not a percent delta. Unlike Recipe Scaler this is three numbers, not a recipe list. Zero a fails. Short version: Solve a/b = c/x for x.

Does Rule of Three upload my file or text to Tool-You servers?

No. Rule of Three runs in this browser tab. Files stay on the device. Limit note: a:b = c:x. Zero a fails.

Should I use Percentage Calculator instead of Rule of Three?

Use Rule of Three when you need Solve a/b = c/x for x. Use Percentage Calculator when you need Percent of a number, X is what % of Y, or change %. They share a category (Calculate) but not the same job.

What are the limits of Rule of Three?

a:b = c:x. Zero a fails. Also: Given a:b as c:x, returns x = (b×c)/a. Unlike Percentage Change this is a proportion, not a percent delta. Unlike Recipe Scaler this is three numbers, not a recipe list. Zero a fails.