Home Rule of three calculator
Rule of three calculator
Direct and inverse proportion on one page.
If 3 matches 12, the page finds what 7 matches. Inverse means one side grows while the other shrinks.
Matching amount
28
12 × 7 ÷ 3 = 28
Direct multiplies the match by the new amount and divides by the known amount. Inverse multiplies the two known amounts and divides by the new amount. A zero divisor leaves the result empty. The sixth decimal rounds half up.
Why this exists
A shop price and a school proportion are the same division. Two known amounts and one new amount are enough.
What percent something is belongs on the percent page. This page only finds the matching amount.
-
01How to use it
Choose direct or inverse.
Type the three amounts. The large number underneath is the match.
-
02What you get
The large number is the match. The line under it is the multiply-and-divide the page actually did.
The link keeps the three amounts and whether the proportion is direct or inverse.
Entered3 with 12, then 7
Known 3 Matches 12 New 7 Result 28 -
03Worth knowing
Where a country groups thousands or writes a decimal comma, the page follows that. The division stays the same.
The result stays in this browser.
Questions
- QWhat do 3, 12 and 7 make?
- A28. Multiply 12 by 7 and divide by 3.
- QWhat about inverse 6, 4 and 8?
- A3. Multiply 6 by 4 and divide by 8.
- QWhat if the divisor is zero?
- AThe result is a dash. The zero stays in the field.
- QIs this a percent?
- ANo. Percent change has its own page.
How this page works it out
Direct proportion multiplies the matching amount by the new amount and divides by the known amount. When 3 matches 12 and the new amount is 7, 12 times 7 is 84, and 84 divided by 3 is 28. A shop price and a school proportion are this same division.
Inverse proportion multiplies the two known amounts and divides by the new amount. Six people finishing in four days means eight people finish in three days, because 6 times 4 is 24 and 24 divided by 8 is 3. Use that order only when one side grows and the other shrinks.
A zero divisor leaves the result empty. A remainder rounds half up at the sixth decimal, so 2, 5 and 3 make 7.5. Percent change is a different page.