7 Percentage Mistakes People Make Without Realizing

Percentage math looks simple, which is exactly why small errors slip through unnoticed. Here are 7 mistakes that show up constantly, from shopping to statistics.

1. Using the wrong base number for percentage change

Percentage change should always divide by the original (starting) value, not the new one. Going from 80 to 100 is a 25% increase, but going from 100 back to 80 is only a 20% decrease, not 25%, because the base number changed.

2. Confusing percentage points with percent

Moving from 20% to 25% is a 5 percentage-point increase, but a 25% relative increase (since 5 ÷ 20 = 0.25). Headlines often blur this distinction, which can make a change look bigger or smaller than it really is.

3. Adding stacked percentages instead of multiplying

20% off plus an extra 10% off isn’t 30% off, it’s 28%, because the second discount applies to the already-reduced price. See our full breakdown of why stacked discounts don’t add up the way you’d expect.

4. Averaging percentages incorrectly

A simple average of two percentage rates is only valid if the underlying sample sizes are equal. If a class of 10 students scores 80% and a class of 40 scores 60%, the overall average isn’t 70%, it’s weighted toward the larger group.

5. Forgetting to convert to a decimal before multiplying

Using 15 instead of 0.15 in a calculation inflates the result by a factor of 100, a simple but common slip that produces an obviously wrong number if you don’t sanity-check it.

6. Assuming a percentage increase and decrease cancel out

A 20% increase followed by a 20% decrease doesn’t return you to the original number. $100 increased by 20% is $120; decreased by 20% from there is $96, not $100.

7. Misreading “percent of” versus “percent off”

“30% of $80” and “30% off $80” describe two different, related-but-opposite numbers ($24 versus $56 remaining), and mixing them up in a quick mental calculation is an easy way to land on the wrong figure.

For calculations you need to get exactly right, use our percentage calculator rather than doing the mental math under pressure.

Why doesn’t a 20% increase followed by a 20% decrease return the original number?

Because each percentage is calculated on a different base number. The decrease applies to the already-increased amount, which is larger than the original, so the two percentages don’t cancel out evenly.

What’s the correct way to calculate percentage change?

Subtract the original value from the new value, divide by the original value, then multiply by 100. Always use the starting number as the base, not the ending number.

Can I average two percentages together directly?

Only if they represent equal-sized groups or samples. Otherwise, a weighted average based on the actual sample sizes is needed for an accurate combined figure.