How to Calculate Percentages (the Easy Way)
Published June 7, 2026
Percentages show up everywhere: a store tag, a pay stub, a test score. Once you know the three core setups, you can solve any of them in seconds.
- "Percent" means per hundred. 40% is just 40 out of 100, or 0.40 as a decimal.
- Three setups cover almost every problem: find a percent of a number, find percentage change, and find what percent one number is of another.
- The 10% trick lets you build any percentage mentally without a calculator.
- Reverse percentages (finding the original price before a discount) require one extra step most people skip.
What percent of a number is
This is the most common setup: “What is 35% of $80?”
Multiply the number by the decimal version of the percent.
So 35% of $80 is:
That’s it. Convert the percent to a decimal (move the decimal point two places left), then multiply. The Percentage Calculator does this instantly for any numbers you throw at it.
Percentage increase and decrease
When a price, grade, or measurement changes, you want to know by what percent.
New value minus old value. A positive result is an increase; a negative result is a decrease.
Divide that difference by the original (old) value.
Convert the decimal to a percent. Done.
Example: A shirt was $50, now costs $35. Change = (35 − 50) ÷ 50 × 100 = −30%. A 30% discount. Use the Discount Calculator to check any sale price in one tap.
X is what percent of Y
You have two numbers and want to know their relationship. “My score was 47 out of 60. What percent is that?”
47 ÷ 60 × 100 = 78.3%. Same formula, different unknown. You just divide the part by the whole and multiply by 100.
What is 20% of $150?
$150 × 0.20 = $30
$30 is what percent of $150?
($30 ÷ $150) × 100 = 20%
Same numbers, same relationship, just a different question. Recognizing which value is missing is the whole skill.
Reverse percentages (finding the original)
If a price already includes tax or a discount has already been applied, working backwards trips people up.
Wrong approach: “$65 after a 20% discount. Add 20% back: $65 × 1.20 = $78.” That gives the wrong answer.
Right approach: The $65 is 80% of the original (100% minus 20%). So:
The same logic applies to tax-inclusive prices. If a receipt shows $54.08 and your tax rate is 8%, the pre-tax amount is $54.08 ÷ 1.08 = $50.07. Use the Sales Tax Calculator for the reverse-tax version.
Mental-math tricks
You don’t always have a calculator handy. These two tricks handle 90% of real-world situations.
The 10% trick. 10% of $340 is $34 (one decimal shift). Half that is $17 (5%). Add them for 15%: $51. Need 30%? Triple the $34. Need 18%? 10% + 8%, or 10% + 5% + 3%. Build any percentage out of 10% chunks and halves.
The swap trick. Percentages are commutative: A% of B equals B% of A. So “4% of 75” feels hard, but “75% of 4” is instantly $3. This works because both expressions reduce to (A × B) ÷ 100.
Tipping shortcut. For a 20% tip, find 10% (move the decimal), then double it. A $47 bill: 10% = $4.70, doubled = $9.40. The Tip Calculator splits it by person too.