Common percentage mistakes (and how to avoid them)
The five percent errors that show up in school, in spreadsheets, and on the news every week.
4 min readReviewed Apr 1, 2026
Quick answer
Most percentage mistakes come from mixing up the base, double-counting changes, or confusing percentage points with percent.
Five mistakes that cost real money
- Reversing a discount with a markup of the same percent and expecting to be back where you started.
- Treating 'percentage points' and 'percent' as the same thing. Going from 5% to 7% is 2 percentage points, but a 40% increase.
- Comparing percentages from different bases. 10% of 1,000 is much more than 10% of 100, even though the percent is identical.
- Stacking percent changes by adding them. A 10% gain followed by a 10% loss is a 1% loss, not breakeven.
- Forgetting to convert percent to a decimal before multiplying.