How to Calculate Percentages, Discounts, and Percentage Change
Percentage math shows up in sales, tax, tips, raises, grades, finance, and everyday decisions. The fastest way to avoid mistakes is to know which formula matches the question.
Need the answer right now?
Open the Percentage Calculator1. Find a Percentage of a Number
Use this when a question asks things like “What is 15% of 200?” or “How much is 8% sales tax on $75?”
Formula: percentage / 100 x number
Example: 15% of 200 = 15 / 100 x 200 = 30. If sales tax is 8% on a $75 purchase, the tax is 8 / 100 x 75 = $6.
2. Calculate a Discount
Discount math uses the same percentage formula, then subtracts the discount amount from the original price.
Formula: final price = original price - (discount percent / 100 x original price)
Example: A $120 item with 25% off saves $30, so the final price is $90. Useful keywords for this type of search include discount calculator, sale price calculator, percent off calculator, and final price calculator.
3. Percentage Increase or Decrease
Use percentage change when comparing an old value and a new value, such as a price increase, salary raise, follower growth, or weight change.
Formula: ((new value - old value) / old value) x 100
If rent goes from $1,500 to $1,650, the change is (($1,650 - $1,500) / $1,500) x 100 = 10% increase. If a price drops from $80 to $60, the result is -25%, meaning a 25% decrease.
Common Mistakes
- Confusing percentage points with percent change.
- Using the new value instead of the old value as the base.
- Adding tax before applying a store discount when the store actually discounts first.
FAQ
What is the easiest way to calculate 10%?
Move the decimal one place left. Ten percent of 250 is 25.
How do I calculate 20% off?
Find 20% of the original price, then subtract that amount from the original price.
Is a 50% increase followed by a 50% decrease back to the same number?
No. Starting at 100, a 50% increase gives 150. A 50% decrease from 150 gives 75.