Why does 10% off plus 10% off not equal 20% off?

Because the second 10% discount applies to the already-reduced price, not the original. $100 × 0.90 × 0.90 = $81, which is 19% off, not 20%.

Why do you need more than 10% to recover a 10% loss?

Because the recovery percentage is calculated on the smaller, post-loss amount. Losing 10% of $100 gives $90. Recovering requires $10/$90 = 11.1% gain, not 10%.

What is the difference between markup and margin?

Markup is profit divided by cost. Margin is profit divided by selling price. A 20% markup equals only a 16.7% margin. A 20% margin equals a 25% markup.

5 Percentage Mistakes Everyone Makes

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What Most People Think

$80.00

What Actually Happens

$81.00

Mistake #1: Stacking Percentages

10% off + 10% off = 19% off, not 20%. The second discount applies to the already-reduced price. Try our stacking calculator →

Mistake #2: Recovering a Loss

Lose 10%, and you need 11.1% to get back to even — not 10%. Lose 50%, and you need 100% to recover. The math is asymmetric. See the recovery calculator →

Recovery % needed = Loss% / (100% − Loss%)

Mistake #3: Markup vs Margin

20% markup on a $100 item = $120 selling price. But the margin is $20/$120 = 16.7%, not 20%. Markup and margin are calculated from different bases. See the difference →

Mistake #4: Percentage OF vs OFF

"10% of $100" = $10 (what you find). "10% off $100" = $90 (what you pay). They sound similar but answer completely different questions.

Mistake #5: Reverse Percentages

If a sale price is $90 after 10% off, the original was $90 / 0.90 = $100, not $90 × 1.10 = $99. You must divide by (1 − discount), not multiply by (1 + discount).

Original Price = Sale Price ÷ (1 − Discount%/100)

Calculate any price with our 10% Off Calculator →

Frequently Asked Questions

Because the second 10% is applied to the already-reduced amount. $100 × 0.90 = $90, then $90 × 0.90 = $81. That's 19% off total ($19 saved), not 20% ($20). The "missing" 1% is 10% of the first 10% discount.

Because the gain percentage is calculated on a smaller base. Lose 10% of $100 = $90. To get back to $100, you need $10/$90 = 11.1% gain. The bigger the loss, the worse it gets: 50% loss needs 100% gain.

Markup is profit as a percentage of COST (how much you add to your cost). Margin is profit as a percentage of SELLING PRICE (how much of the selling price is profit). 20% markup = 16.7% margin. 20% margin = 25% markup.

Divide the sale price by (1 minus the discount as a decimal). If the sale price is $90 after 10% off: $90 ÷ 0.90 = $100. Do NOT add 10% to $90 — that gives $99, which is wrong.

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5 Percentage Mistakes Everyone Makes

Mistake #1: Stacking Percentages

Mistake #2: Recovering a Loss

Mistake #3: Markup vs Margin

Mistake #4: Percentage OF vs OFF

Mistake #5: Reverse Percentages

Frequently Asked Questions

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