Percentage Calculator
Use the tools below to solve the three most common percentage questions.
1) What is X% of Y?
2) A is what percent of B?
3) Increase or decrease an amount by a percentage
Understanding percentage in plain language
A percentage means “per hundred.” So if you say 25%, you are really saying 25 out of 100. Percentages help you compare values quickly, whether you are calculating discounts, interest, tax, salary raises, exam scores, or profit margins.
When people search for “how to calculate percentage for amount,” they usually need one of three things:
- Find a percentage of a number (for example, 18% of 250)
- Find what percentage one number is of another (for example, 30 is what percent of 120)
- Increase or decrease an amount by a percentage (for example, reduce 800 by 15%)
The calculator above handles all three, and the sections below explain the exact formulas step by step.
Formula 1: how to find X% of an amount
Formula: Percentage value = (Percentage ÷ 100) × Amount
This is the most common use case. You turn the percent into a decimal, then multiply by the base amount.
Example: 20% of 350
Step 1: Convert 20% into decimal form: 20 ÷ 100 = 0.20
Step 2: Multiply by the amount: 0.20 × 350 = 70
Answer: 20% of 350 is 70.
Where this is useful
- Finding sales tax on a bill
- Calculating tip in a restaurant
- Computing commission or service fee
- Calculating a discount amount during shopping
Formula 2: how to find what percentage one amount is of another
Formula: Percentage = (Part ÷ Whole) × 100
Use this when you already have two numbers and want the percent relationship between them.
Example: 45 is what percent of 180?
Step 1: Divide part by whole: 45 ÷ 180 = 0.25
Step 2: Convert to percent: 0.25 × 100 = 25%
Answer: 45 is 25% of 180.
Common scenarios
- Exam score percentages (marks obtained out of total)
- Business conversion rates (buyers out of visitors)
- Project completion tracking
- Savings progress toward a goal
Formula 3: how to increase or decrease an amount by percentage
Sometimes you need the final amount after adjustment, not just the percentage value itself.
Increase formula: Final amount = Amount × (1 + Percentage ÷ 100)
Decrease formula: Final amount = Amount × (1 - Percentage ÷ 100)
Example A: increase 1,200 by 10%
Final = 1,200 × (1 + 10/100) = 1,200 × 1.10 = 1,320
Example B: decrease 1,200 by 10%
Final = 1,200 × (1 - 10/100) = 1,200 × 0.90 = 1,080
How to calculate discount and sale price quickly
If you see an item priced at 80 with a 25% discount:
- Discount amount = 25% of 80 = (25/100) × 80 = 20
- Sale price = 80 - 20 = 60
You can also do it in one line: 80 × (1 - 0.25) = 80 × 0.75 = 60.
How to find the original amount before discount or increase
This is a reverse percentage problem. For example, if a price after a 20% discount is 200, what was the original price?
Original = Final ÷ (1 - Percentage ÷ 100)
Original = 200 ÷ (1 - 0.20) = 200 ÷ 0.80 = 250.
If the final number is after an increase, use:
Original = Final ÷ (1 + Percentage ÷ 100)
Percentage increase vs percentage point increase
These are not the same, and mixing them is a common mistake:
- Percentage points: from 10% to 15% is an increase of 5 percentage points.
- Percentage increase: from 10% to 15% is a 50% increase because (5 ÷ 10) × 100 = 50%.
Mental math shortcuts for everyday percentage calculations
- 10% = move decimal one place left (10% of 250 = 25)
- 5% = half of 10% (5% of 250 = 12.5)
- 1% = divide by 100
- 15% = 10% + 5%
- 25% = one quarter
- 50% = half
These shortcuts are useful when calculating tips, taxes, or quick discounts without a calculator.
Common errors to avoid
- Forgetting to divide the percent by 100
- Using the wrong base amount (especially in chained discounts)
- Confusing “percent of” with “percent change”
- Dividing by zero when finding “A is what percent of B” and B is 0
Practical examples from daily life
Restaurant tip
Bill = 64, Tip rate = 15%
Tip = 64 × 0.15 = 9.6, Total = 73.6
Salary raise
Current salary = 4,000, Raise = 8%
New salary = 4,000 × 1.08 = 4,320
Exam score
Scored = 72 out of 90
Percentage = (72 ÷ 90) × 100 = 80%
Quick recap
- To find percentage of amount: (p/100) × amount
- To find what percent one value is of another: (part/whole) × 100
- To increase/decrease: amount × (1 ± p/100)
If you want a fast answer, use the calculator above. If you want to understand the math deeply, use the formulas and examples in this guide and practice with your own numbers.