How to Calculate Percentage
"Percentage" questions usually mean one of three different calculations. This guide covers all three, with the formula and a worked example for each.
1. What is X% of a number?
Formula: (X ÷ 100) × number
Example: What is 20% of 50?
(20 ÷ 100) × 50 = 0.2 × 50 = 10
2. What percent is X of Y?
Formula: (part ÷ whole) × 100
Example: What percent is 30 of 120?
(30 ÷ 120) × 100 = 0.25 × 100 = 25%
3. Percentage increase or decrease between two numbers
Formula: ((new value − old value) ÷ old value) × 100
Example (increase): A value goes from 80 to 100.
((100 − 80) ÷ 80) × 100 = (20 ÷ 80) × 100 = +25%
Example (decrease): A value goes from 100 to 80.
((80 − 100) ÷ 100) × 100 = (−20 ÷ 100) × 100 = −20%
Notice the increase and decrease examples use the same two numbers in reverse, but the percentages (25% and 20%) aren't the same size — that's because each one divides by a different starting value.
Which formula do I need?
- Asking "what is X% of this number" → use formula 1.
- Asking "what share or portion is this of the total" → use formula 2.
- Comparing a before and after value (a price change, a score, a measurement over time) → use formula 3.
Calculate your own
The calculator covers all three of these in one tool, so you don't need to remember which formula to use.
Open the percentage calculator
Frequently asked questions
What is the basic percentage formula?
To find X% of a number, divide X by 100 and multiply by the number: (X ÷ 100) × number.
How do I find what percent one number is of another?
Divide the part by the whole, then multiply by 100: (part ÷ whole) × 100.
What is the difference between percentage change and percentage difference?
Percentage change compares a starting value to an ending value and treats the starting value as the base, so it can be positive (increase) or negative (decrease). Percentage difference compares two values without treating either as the "original," so it is always expressed as a positive gap.
Why is my percentage change negative?
A negative result means the value went down. For example, going from 100 to 80 is a -20% change, since 80 is 20% less than 100.