5 Percentage Mistakes Every Student Makes (And How to Fix Them)

Percentages are one of those topics that seem simple on the surface — but in exams, marks slip away surprisingly fast. The good news is that most percentage mistakes come down to the same handful of habits. Fix these five and you'll find percentage questions much more manageable.

1. Confusing Percentage Of and Percentage Change

The mistake: Using the wrong method — calculating a simple percentage of a number when the question is actually asking for a percentage change, or vice versa.

Example:

A jacket costs £80 and goes on sale for £60. What is the percentage decrease?

Some students calculate 60/80 × 100 = 75% instead of finding the actual change first.

The fix: For percentage change, always use this formula:

(Change ÷ Original) × 100

So: (80 − 60) ÷ 80 × 100 = 25% decrease

Always read the question carefully — "percentage of" and "percentage change" need different methods.

 

2. Not Converting Percentages to Decimals Correctly

Mistake: Making errors when converting between percentages and decimals, especially with small percentages.

Example:

5% converted incorrectly as 0.5 instead of 0.05

15% converted incorrectly as 1.5 instead of 0.15

The fix: To convert a percentage to a decimal, always divide by 100.

5% ÷ 100 = 0.05

15% ÷ 100 = 0.15

A quick check — 50% should be 0.5, so anything less than 50% must be less than 0.5.

3. Finding the Wrong Original Value

The mistake: When given a value after a percentage increase or decrease, working with the wrong starting number.

Example:

A price after a 20% increase is £120. What was the original price?

Many students calculate 20% of £120 and subtract, getting £96 — which is wrong.

The fix: The £120 represents 120% of the original price (100% + 20% increase). So:

Original = 120 ÷ 1.20 = £100

Always identify what percentage the given value represents before calculating.

4. Mixing Up Percentage Increase and Decrease Multipliers

The mistake: Using the wrong multiplier for percentage increase and decrease calculations.

Example:

For a 30% increase, some students multiply by 0.30 instead of 1.30

For a 30% decrease, some students multiply by 0.30 instead of 0.70

The fix:

Percentage increase → add to 1: 30% increase = × 1.30

Percentage decrease → subtract from 1: 30% decrease = × 0.70

Think of it this way — if something increases by 30%, you still have the original 100% plus the extra 30%, giving you 130% = 1.30.

5. Rounding Too Early

The mistake: Rounding numbers in the middle of a calculation instead of at the very end, which leads to inaccurate final answers.

Example:

Calculating 17.5% of £63:

Wrong: 0.175 × 63 = 11.025 → rounded to 11 mid-calculation → used in next step

Right: Keep the full value (11.025) throughout and only round the final answer

The fix: Never round until you reach the final answer. Keep all decimal places throughout the calculation and round only once at the very end — or when the question specifically tells you to.

A Final Word

Percentage mistakes almost always come down to rushing, using the wrong formula, or rounding too early. Slow down, identify exactly what the question is asking, and double check your multipliers — and percentages will stop costing you easy marks.

Want a complete step-by-step guide to percentages? 'Percentages for Nervous Students' walks through every key topic clearly and simply, with worked examples and practice questions designed to build real confidence — perfect for students aged 11–14. Check it out on Amazon →