What This Percentage Calculator Does
This Percentage Calculator handles the three most common percentage questions in one place. Instead of remembering different formulas, you pick the type of calculation you need and fill in the relevant numbers. The three modes are:
- Percentage of a number – find a percentage portion of a value (e.g. what is 20% of 150?).
- Number as a percentage of another – work out what proportion one number is of another (e.g. 30 is what percent of 120?).
- Percentage change between two numbers – measure the increase or decrease from a starting value to a new value.
The tool works with plain numbers, so it applies anywhere in the world — prices, test scores, discounts, statistics or growth rates.
How to Use It
First, choose what you want to calculate. The input fields you fill in depend on your choice:
- Percentage of a number: enter the Number and the Percentage.
- Number as a percentage of another: enter the Number (the part) and the Second Number (the whole).
- Percentage change: enter the Number (original value) and the Second Number (new value).
The Formulas Explained
Behind the scenes the calculator uses these exact formulas:
- Percentage of a number: $$\text{Result} = \text{Number} \times \frac{\text{Percentage}}{100}$$
- Number as a percentage of another: $$\text{Result} = \frac{\text{Number}}{\text{Second Number}} \times 100$$
- Percentage change: $$\text{Result} = \frac{\text{Second Number} - \text{Number}}{\text{Number}} \times 100$$
A positive percentage change means an increase; a negative result means a decrease.
Worked Example
Say a product cost 80 last month and now costs 100. Choose Percentage change between two numbers, enter Number = 80 and Second Number = 100. The calculator computes $$\frac{100 - 80}{80} \times 100 = \frac{20}{80} \times 100 = 25\%$$ The price rose by 25%.
For comparison, using Percentage of a number with Number = 80 and Percentage = 25 gives $$80 \times \frac{25}{100} = 20$$
Frequently Asked Questions
What if my second number is zero? The "percentage of another" and "percentage change" modes divide by your second/original number, so a zero there produces an undefined or infinite result. Use a non-zero base value.
How do I find a percentage decrease? Use the percentage change mode. If the new value is smaller than the original, the result will simply be negative, indicating the size of the drop.
Can I reverse a percentage? To find the original whole when you know a part and its percentage, use the "number as a percentage of another" mode, or divide the part by the decimal form of the percentage.
