Percentage Calculator
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If you've ever wondered, "How do you calculate the percentage?" you're not alone. Percentages show up everywhere: school grades, sale discounts, restaurant tips, tax rates, loan interest, investment returns, and even battery charge levels.
The good news is that percentage math is straightforward once you know the core formulas. In this guide, you'll learn exactly how to calculate percentages in different real-life situations, with plain-language examples you can copy immediately.
What Is a Percentage?
A percentage means "per hundred." So 1% means 1 out of 100, 25% means 25 out of 100, and 100% means the entire amount.
That single idea powers almost every percentage question you'll ever solve.
The 3 Most Common Percentage Calculations
1) Find X% of Y
Use this when someone asks: "What is 20% of 150?"
Example: What is 20% of 150?
- 20 ÷ 100 = 0.20
- 0.20 × 150 = 30
Answer: 20% of 150 is 30.
2) Find What Percent One Number Is of Another
Use this when someone asks: "50 is what percent of 200?"
Example: 50 is what percent of 200?
- 50 ÷ 200 = 0.25
- 0.25 × 100 = 25%
Answer: 50 is 25% of 200.
3) Find the Original Number from a Percentage
Use this when you know a percentage result and need to reverse it.
Example: 30 is 15% of what number?
- 15% = 0.15
- 30 ÷ 0.15 = 200
Answer: The original number is 200.
How to Calculate Percentage Increase and Decrease
When values change over time (prices, salaries, traffic, revenue), you'll usually need percentage change.
Percentage Increase
Example: Price rises from $80 to $100:
- Change = 100 - 80 = 20
- 20 ÷ 80 = 0.25
- 0.25 × 100 = 25%
Increase: 25%
Percentage Decrease
Example: Value drops from 120 to 90:
- Change = 120 - 90 = 30
- 30 ÷ 120 = 0.25
- 0.25 × 100 = 25%
Decrease: 25%
Practical Everyday Examples
Discounts While Shopping
A jacket costs $120 and is 35% off.
- Discount amount = 35% of 120 = 0.35 × 120 = $42
- Final price = 120 - 42 = $78
Restaurant Tip
Your bill is $64. You want to leave 18% tip.
- Tip = 0.18 × 64 = $11.52
- Total = 64 + 11.52 = $75.52
Exam Score
You got 42 correct answers out of 50 questions.
- Percentage = (42 ÷ 50) × 100 = 84%
Sales Tax
An item costs $250, tax is 8%.
- Tax = 0.08 × 250 = $20
- Total price = 250 + 20 = $270
Common Percentage Mistakes (and How to Avoid Them)
- Forgetting to divide by 100: 25% is 0.25, not 25.
- Using the wrong base number: In change calculations, divide by the original value, not the new value.
- Confusing percentage points with percent change: Going from 10% to 12% is a 2 percentage-point increase, but a 20% relative increase.
- Rounding too early: Keep extra decimals until your final answer.
Mental Math Shortcuts for Percentages
- 10%: move decimal one place left (10% of 350 = 35)
- 1%: move decimal two places left (1% of 350 = 3.5)
- 5%: half of 10% (5% of 350 = 17.5)
- 15%: 10% + 5%
- 20%: double 10%
- 25%: one quarter of the number
- 50%: half the number
Step-by-Step Method You Can Use Every Time
- Identify the question type: "percent of," "what percent," or "percent change."
- Write down the matching formula.
- Convert any percent to decimal by dividing by 100.
- Do the arithmetic carefully.
- Add units and context (%, dollars, points, etc.).
Fast FAQ
How do I calculate 30% of a number quickly?
Find 10% first, then multiply by 3. Example: 30% of 90 = 9 × 3 = 27.
How do I add a percentage to a number?
Multiply by (1 + percentage as decimal). Example: add 8% to 250 → 250 × 1.08 = 270.
How do I subtract a percentage from a number?
Multiply by (1 - percentage as decimal). Example: subtract 20% from 80 → 80 × 0.80 = 64.
Final Thoughts
To calculate percentages, remember one core principle: percentages are just parts of 100. Once you can switch between percent and decimal, you can solve discounts, tips, taxes, grades, business growth, and more with confidence.
If you want a quick answer right now, use the calculator above. If you want long-term skill, practice a few examples daily using the formulas in this article.