Percentage Calculator

How to Calculate Percentages: A Complete Guide

Percentages are one of the most useful mathematical concepts in everyday life — from working out a discount at the shops, to calculating GST, to understanding interest rates on your mortgage. Here's how each type of percentage calculation works.

1. What is X% of Y?

This is the most common percentage calculation. To find X% of Y, multiply Y by X and divide by 100.

Example: What is 15% of $200? → (15 ÷ 100) × 200 = $30

2. X is What Percentage of Y?

To find what percentage X is of Y, divide X by Y and multiply by 100.

Example: 30 is what % of 200? → (30 ÷ 200) × 100 = 15%

3. Percentage Increase

To calculate the percentage increase from one value to another, subtract the original from the new value, divide by the original, and multiply by 100.

Example: Price went from $80 to $100 → ((100 − 80) ÷ 80) × 100 = 25% increase

4. Percentage Decrease

The same formula applies for a decrease — the result will be negative, indicating a reduction.

Example: Price dropped from $100 to $75 → ((100 − 75) ÷ 100) × 100 = 25% decrease

5. Percentage Difference

The percentage difference between two values is calculated by dividing the absolute difference by the average of the two values.

Common Percentage Uses in Australia

• GST: 10% added to most goods and services
• Superannuation: 11.5% of gross salary (2024–25)
• Mortgage interest rates: Typically expressed as % per annum
• Retail discounts: "20% off" sales
• Investment returns: % return on shares, property, super
• Tax rates: 19%, 32.5%, 37%, 45% marginal income tax rates