How to Calculate Percentages: The Complete Guide

Percentages come up everywhere: shopping discounts, test scores, cricket strike rates, the chance of rain tomorrow. If you can work confidently with percentages, a surprising amount of everyday maths becomes straightforward.

This guide walks through the core percentage skills step by step, with worked examples you can follow along with on paper.

What does "percent" actually mean?

"Per cent" literally means "per hundred." So 25% means 25 out of every 100. That is it. A percentage is just a way of expressing a number as a fraction of 100.

This means:

50% = 50/100 = 1/2
10% = 10/100 = 1/10
75% = 75/100 = 3/4

Once you see percentages as fractions with 100 on the bottom, the conversions and calculations below will feel much more natural.

Converting between fractions, decimals and percentages

These three forms all describe the same thing, so you need to be able to move between them freely.

Fraction to percentage

Multiply the fraction by 100.

Example: Convert 3/5 to a percentage.
3/5 × 100 = 300/5 = 60%

Decimal to percentage

Multiply by 100 (which just shifts the decimal point two places to the right).

Example: Convert 0.35 to a percentage.
0.35 × 100 = 35%

Percentage to decimal

Divide by 100 (shift the decimal point two places to the left).

Example: Convert 8% to a decimal.
8 / 100 = 0.08

Percentage to fraction

Write the percentage over 100, then simplify.

Example: Convert 45% to a fraction.
45/100 = 9/20

Finding a percentage of a number

This is the most common percentage calculation. "What is 15% of 80?" really means "15/100 of 80."

Method: Convert the percentage to a decimal, then multiply.

Example 1: What is 15% of 80?
15% = 0.15
0.15 × 80 = 12

Example 2: What is 7.5% of 200?
7.5% = 0.075
0.075 × 200 = 15

The 10% shortcut

Finding 10% of any number is easy: just divide by 10. From there you can build up to other percentages quickly.

Example: Find 35% of 240.

10% of 240 = 24
30% = 24 × 3 = 72
5% = half of 10% = 12
35% = 72 + 12 = 84

This mental method is faster than reaching for a calculator in many situations.

Expressing one quantity as a percentage of another

The question here is: "What percentage is 18 out of 45?"

Method: Divide the part by the whole, then multiply by 100.

Example: A student scores 18 out of 25 on a quiz. What is their percentage?
18 / 25 = 0.72
0.72 × 100 = 72%

Percentage increase and decrease

These crop up constantly: price increases, population growth, sale discounts.

Percentage increase

Example: A shirt costs $60 and the price rises by 20%. What is the new price?

Find 20% of $60: 0.20 × 60 = $12
Add it to the original: $60 + $12 = $72

Shortcut: Multiply by 1.20 directly. $60 × 1.20 = $72. The "1" keeps the original amount and the "0.20" adds the increase.

Percentage decrease

Example: A $90 pair of shoes is on sale for 30% off. What is the sale price?

Find 30% of $90: 0.30 × 90 = $27
Subtract from the original: $90 - $27 = $63

Shortcut: Multiply by 0.70 (since you are keeping 70%). $90 × 0.70 = $63.

Finding the percentage change

If a value goes from 50 to 65, what is the percentage increase?

Method: (Change / Original) × 100

Change = 65 - 50 = 15
15 / 50 × 100 = 30% increase

Common mistakes to watch out for

Dividing instead of multiplying (or vice versa). "Find 20% of 80" means multiply. "What percentage is 20 of 80?" means divide. Read the question carefully.
Percentage change using the wrong base. Always divide by the original value, not the new value.
Forgetting that percentages can exceed 100%. If something doubles, that is a 100% increase. If it triples, that is a 200% increase.

Quick reference table

Percentage	Fraction	Decimal
10%	1/10	0.1
25%	1/4	0.25
33.3%	1/3	0.333...
50%	1/2	0.5
75%	3/4	0.75
100%	1	1.0

Practise with purpose

The best way to get comfortable with percentages is to use them in context. Work out the tip at a restaurant, figure out how much you save on a sale item, or calculate your score on a quiz as a percentage. Real problems stick better than textbook drills.

If you want structured practice that walks you through percentages step by step, imSteyn covers percentage calculations as part of the Year 7 Australian Curriculum. It guides you through each problem rather than just giving the answer, so you actually build the skill. You can try it free and see how it works.

Percentages are one of those topics where a solid understanding pays off well beyond the classroom. Get the basics right and everything that builds on them, from ratios to probability to financial maths, becomes much more manageable.