Calculate any percentage in four ways: find what percent one number is of another, find a percentage of a number, calculate percentage increase or decrease between two values, and reverse-calculate the original value from a known percentage. No formula memorization required.
The four core percentage calculations each use a simple formula. Percent of total: (Part ÷ Whole) × 100. If 45 out of 180 students passed, that's (45 ÷ 180) × 100 = 25%. Percentage of a number: (Percent ÷ 100) × Number. 18% of $240 = (18 ÷ 100) × 240 = $43.20. These two cover the vast majority of everyday percentage questions.
Percentage change — increase or decrease — uses: ((New − Old) ÷ Old) × 100. A price rising from $80 to $96 is ((96 − 80) ÷ 80) × 100 = 20% increase. A salary cut from $75,000 to $68,000 is ((68,000 − 75,000) ÷ 75,000) × 100 = −9.3%. The sign tells you the direction: positive is an increase, negative is a decrease.
Reverse percentage works backwards from a result. If $34 is 17% of something, the original is 34 ÷ (17 ÷ 100) = $200. This is useful when a price shown is already discounted or marked up and you need the original figure — for example, reconstructing a pre-tax or pre-discount price.
What's the difference between percentage point and percent change?
A percentage point is an absolute difference between two percentages. If interest rates rise from 4% to 6%, that's a 2 percentage point increase. Percent change measures the relative shift: (6 − 4) ÷ 4 × 100 = 50% increase. Politicians and advertisers frequently conflate these — "rates rose 50%" (percent change) sounds more dramatic than "rates rose 2 points" (percentage points), even though they describe the same event.
How do I calculate a tip?
Multiply the bill by the tip percentage divided by 100. A 20% tip on a $47 bill is 47 × 0.20 = $9.40, for a total of $56.40. A fast mental shortcut: 10% of any number is just moving the decimal left one place ($47 → $4.70). Double it for 20% ($9.40). Add half again for 15% ($4.70 + $2.35 = $7.05).
How do I calculate a discount?
Multiply the original price by (1 − discount ÷ 100). A 30% off sale on a $120 item: 120 × (1 − 0.30) = 120 × 0.70 = $84. Alternatively, find 30% of $120 ($36) and subtract: $120 − $36 = $84. Both methods work — pick whichever feels more natural.
How do I calculate percentage increase in Excel or a spreadsheet?
Use the formula =(New-Old)/Old and format the cell as a percentage. For example, =(B2-A2)/A2 where A2 is the old value and B2 is the new value. Multiply by 100 if you want the result as a plain number rather than a formatted percentage. For a running column of changes, anchor the old value with an absolute reference: =(B2-$A$2)/$A$2.