How to Calculate Any Percentage with the Rule of Three

One method instead of three

Percentage questions come in a few shapes, and most people learn a separate trick for each one. What is 15% of 2000? What percentage is 45 of 60? If 30% is 90, what is the whole?

You do not need three tricks. All three are the same proportion, with 100 sitting in one of the four positions. Once you see that, the only thing you have to remember is where the 100 goes.

The word "percent" is literally "per hundred", so a percentage is a comparison against a total of 100. That is why the number 100 always turns up somewhere.

Type 1 — what is 15% of 2000?

The whole thing (2000) corresponds to 100%. The part you want corresponds to 15%.

• 100% → 2000
• 15% → x

Working type 1 through

x = (2000 × 15) ÷ 100 = 30000 ÷ 100 = 300.

That is the calculation loaded into the calculator on this page. Change the 15 to any other percentage and the answer updates, with the working shown underneath.

Sanity check: 15% is a bit less than a sixth, and a sixth of 2000 is about 333. So 300 is the right size.

Type 2 — what percentage is 45 out of 60?

Here you know both actual amounts and want the percentage, so the unknown moves to the other column.

Set 60 (the whole) against 100%, and 45 against x%:

• 60 → 100%
• 45 → x%

Working type 2 through

x = (100 × 45) ÷ 60 = 4500 ÷ 60 = 75%.

Notice that the layout did not change at all — only which of the four positions held the unknown. That is the advantage of writing the two rows out rather than trying to recall a formula for each variety of question.

Type 3 — 30% of something is 90. What is the whole?

This is the one people find hardest, and it is exactly as easy as the other two once it is laid out. You know that 30% corresponds to 90, and you want what 100% corresponds to.

30% → 90, and 100% → x, so x = (90 × 100) ÷ 30 = 300.

Reverse percentage questions like this show up constantly in real life — working out a pre-discount price, or a pre-tax total. The trap is to add 30% back onto 90, which gives 117 and is wrong. Laying it out as a proportion avoids that entirely.

Percentage increases and decreases

For an increase or decrease, the neat move is to change what corresponds to 100%.

A 20% discount means you pay 80% of the original, so set 100 → original price and 80 → x. A 15% tip means you pay 115%, so use 115 in that position.

Doing it this way is one calculation rather than two, and it removes the most common percentage mistake of all: finding the 20% and then forgetting to subtract it.