Common Percentage Mistakes
The percentage errors that trip up most people — and how to avoid them. Each mistake includes a corrected example.
Reviewed Jan 15, 2025
Quick answer
The most common percentage mistake is assuming a 20% discount followed by a 20% markup returns to the original price. It doesn't — the two percentages are taken from different bases.
The five most common mistakes
1. Reversing percentages
20% off then 20% added back is a net loss, because the second 20% is computed on the discounted price.
2. Confusing 'percent of' with 'percent change'
30% of 200 ≠ a 30% increase from 200.
3. Forgetting to convert to decimal
Multiplying by 20 instead of 0.20 gives an answer 100× too big.
4. Mixing base and result
'A is what percent of B' uses B as the base, not A.
5. Adding percentages from different bases
Two products both up 10% don't combine to 20% on a portfolio unless they're equal-weighted.