Math
•
Apr 2, 2026
•
4 min de lecture
Percentage Calculations: 7 Common Mistakes and How to Avoid Them
Percentages look easy but the language traps even smart people. Here are the mistakes that show up in everyday math.
Mistake 1: Stacking discounts additively. Two 20% coupons do not give 40% off — they give 36% (1-0.8*0.8). Mistake 2: Confusing "X% of" with "X% off". 30% of $100 is $30; 30% off $100 is $70. Mistake 3: Reverse engineering. To find original price from sale price: original = sale / (1 - discount/100). Mistake 4: Percentage points vs percent. Going from 5% to 7% is "2 percentage points" or "40% increase" — both correct, very different statements. Use our Percentage Calculator to check.