How to Calculate Percentages: Formula & Real-World Examples

The basic percentage formula

Percentage = (Part / Whole) × 100. If you scored 85 points out of 100, that's (85/100) × 100 = 85%. If a store sells 60 phones out of 500 in stock, that's (60/500) × 100 = 12%.

Rearranged, you can solve for any piece: if you want to know what number is 15% of 80, rearrange to Part = Percentage × Whole / 100, so 15 × 80 / 100 = 12. All percentage problems reduce to these three variables.

Percentage increase and decrease

Percentage change = (New Value − Old Value) / Old Value × 100. A price that rises from $50 to $65 is a ($65−$50)/$50 × 100 = 30% increase. A stock that falls from $100 to $80 is a (80−100)/100 × 100 = −20% decrease (negative means loss).

A common trap: a 50% increase followed by a 50% decrease does NOT return you to your starting point. $100 + 50% = $150, then $150 − 50% = $75. The base you're calculating from changes each time, so the effects are asymmetric.

Mental math tricks

1% is easy — move the decimal point two places left. For $456, 1% is $4.56. Now any percentage is just a multiple: 5% = 5 × 1%, 25% = 25 × 1%, and so on. A 20% tip on $45 is 1% ($0.45) × 20 = $9.

For common splits, learn the shortcuts: 10% (move decimal left), 25% (divide by 4), 50% (divide by 2). Percentages between them are often a quick add or subtract: 15% is 10% + half of 10%.

Percentage points vs. percentages

A subtle but critical distinction: if interest rates rise from 2% to 3%, they've risen by 1 percentage point, not 1%. (A 1% increase would be 2% × 1.01 = 2.02%.) In politics, 'approval up 5 percentage points' is very different from 'approval up 5%', yet headlines often conflate them.