Eight percentage mistakes.

Mistake 1: Stacking discounts by adding the percentages

A 20% off coupon plus a 10% loyalty discount does NOT give 30% off. The second percentage applies to the already-reduced price:

$100 × 0.80 = $80   (after 20% off)
$80 × 0.90 = $72   (after another 10% off)

Total savings: $28, or 28% off — not 30%. The smaller the second discount, the closer it gets to the sum, but it’s never quite equal.

Mistake 2: Assuming percent change is symmetric

A 50% increase followed by a 50% decrease does NOT return to the start:

100 → 150  (after +50%)
150 → 75   (after −50% from 150)

To return from 150 back to 100, you need only a 33.3% decrease. Reversing a percentage requires using the new value as the base, not the original.

Mistake 3: Confusing percent change with percentage points

If interest rates go from 4% to 6%, that’s:

• A 2 percentage point increase
• A 50% relative increase

Headlines that say “rates rose 2%” in this context are ambiguous and frequently wrong.

Mistake 4: Markup and margin used interchangeably

A 50% markup is NOT a 50% margin:

• $40 cost with 50% markup → sell at $60 → profit is $20 → 50% markup
• $20 profit out of $60 sell price → 33.3% margin

Retailers and accountants need to specify which they mean.

Mistake 5: Adding percentages from different bases

If 60% of class A and 40% of class B passed an exam, the “average pass rate” is NOT 50% unless both classes were the same size:

Class A: 30 students, 60% pass = 18 passed
Class B: 70 students, 40% pass = 28 passed
Total: 46 of 100 = 46% pass rate

Percentages from different denominators must be weighted before they can be combined.

Mistake 6: “200% more” vs “200% of”

These phrasings mean different things:

• “Sales are 200% of last year’s” → new = 2 × old (doubled)
• “Sales are 200% more than last year’s” → new = 3 × old (tripled)

The word “more” flips the interpretation. Native English speakers often write one and mean the other. When ambiguous, ask.

Mistake 7: Percentage of zero

You cannot divide by zero. If a metric starts at 0 and ends at 50, there is no percent change — the change is “undefined”, not “infinite%” or “5000%”. Just describe the absolute change.

Mistake 8: Reading a percentage as a count

“The drug reduces risk by 50%” sounds dramatic. But 50% reduction of what? If your baseline risk was 2 in 1,000, a 50% reduction is now 1 in 1,000 — an absolute reduction of 0.1 percentage points. The relative reduction (50%) is technically true but obscures the small absolute effect. In medicine and statistics, always check both relative and absolute changes.