Percentages: Common Mistakes, Tips and FAQs

Introduction

Most maths mistakes come from mixing up similar-looking methods, like percentage of versus percentage change. Knowing which one applies solves most problems.

This article covers the most common mistakes people make with percentages, what really moves the result, and answers to the questions we hear most often.

Why it matters

Getting the method right means you can check prices, understand statistics in the news and help children with GCSE-style questions confidently.

How it's calculated

X% of Y = Y × X ÷ 100. To find what percentage A is of B, divide A by B and multiply by 100.

What changes the result most

We raised each input by 10% on its own, starting from a typical example where percentage of number is 30. Here's what happened:

• Raising percentage from 15 % to 16.5 % changes percentage of number to 33 (+10.0%).
• Raising of the number from 200 to 220 changes percentage of number to 33 (+10.0%).

Focus first on percentage, which has the biggest effect in this example.

Common mistakes to avoid

1. Reversing a percentage increase by subtracting the same percentage.
2. Mixing units, such as centimetres and metres, in one calculation.
3. Rounding too early and carrying the error forward.
4. Using the population formula for sample data.

A quick example

With the inputs below, percentage of number comes out at 30.

Tips

• Write down which number is the 'whole' before working out any percentage.
• Round only at the end of a calculation, not in the middle.
• Check answers roughly in your head – does the result look sensible?
• For statistics, note whether your data is a full population or a sample.

Frequently asked questions

What's the quickest way to work out percentages?

Use our free percentage Calculator. It applies this method automatically: X% of Y = Y × X ÷ 100. To find what percentage A is of B, divide A by B and multiply by 100.

Are these suitable for GCSE revision?

Yes. The methods match those taught in UK schools, and each page explains the formula used.

How many decimal places are shown?

Most results show up to two decimal places. Use the rounding in your working as your teacher or exam board requires.

Can I use negative numbers?

Yes, where they make sense mathematically, such as percentage change.

Related calculators

• Percentage Change Calculator
• Reverse Percentage Calculator
• Ratio Calculator
• Average Calculator (Mean, Median, Mode)
• Standard Deviation Calculator
• Circle Calculator

For the full method, read How to Calculate Percentages in the UK: Step-by-Step Guide.