Percentage Calculator
Use this quick tool to calculate percentages in common real-life situations such as discounts, score comparisons, and growth rates.
What Is a Percentage?
A percentage is simply a way of expressing a number as a fraction of 100. The word comes from Latin: per centum, which means “for each hundred.” So when you see 25%, it means 25 out of 100.
Percentages are used everywhere: school grades, discounts, taxes, interest rates, surveys, business growth, and financial planning. If you understand a few formulas, you can quickly solve most percentage problems.
Core Percentage Formulas
1) Find A% of B
Formula: (A / 100) × B
Example: What is 15% of 240?
(15 / 100) × 240 = 0.15 × 240 = 36
2) Find what percent A is of B
Formula: (A / B) × 100
Example: 18 is what percent of 60?
(18 / 60) × 100 = 30%
3) Percentage change from old value to new value
Formula: ((New − Old) / Old) × 100
Example: Price rises from 80 to 100.
((100 − 80) / 80) × 100 = 25% increase
4) Percentage difference between two values
Formula: |A − B| / ((A + B) / 2) × 100
This is useful when comparing two numbers equally, without treating one as the “original.”
How to Calculate Percentage Step by Step
Step 1: Identify what you are solving
- Are you finding a portion? (“What is 20% of 150?”)
- Are you finding a ratio? (“30 is what percent of 120?”)
- Are you measuring growth or decline? (“From 400 to 350?”)
Step 2: Write the correct formula
Most errors happen because people choose the wrong equation. Always decide which value is the base and which is the percentage.
Step 3: Convert percent to decimal when needed
To convert a percent to decimal, divide by 100: 5% = 0.05, 35% = 0.35, 120% = 1.20.
Step 4: Compute and check for reasonableness
If you calculate 10% of 50 and get 500, something is off. Quick estimation helps confirm your answer.
Real-Life Examples
Shopping discounts
A shirt costs $80 and is on sale for 25% off.
Discount = 25% of 80 = 20. Final price = 80 − 20 = 60.
Tax and tip
Bill is $45, tip is 18%.
Tip = 0.18 × 45 = 8.10. Total = 53.10.
Exam scores
You scored 42 out of 50:
(42 / 50) × 100 = 84%.
Savings growth
Investment grows from $1,000 to $1,150:
((1150 − 1000) / 1000) × 100 = 15%.
Common Mistakes to Avoid
- Mixing up base and part: “A is what percent of B?” means divide A by B, not B by A.
- Forgetting to multiply by 100: 0.2 is 20%, not 0.2%.
- Using wrong old/new value: percentage change must divide by the original (old) value.
- Ignoring negative results: a negative percentage change means a decrease.
Mental Math Shortcuts for Percentages
- 10% = move decimal one place left (10% of 250 = 25).
- 1% = move decimal two places left (1% of 250 = 2.5).
- 5% = half of 10%.
- 15% = 10% + 5%.
- 50% = half, 25% = quarter, 75% = three quarters.
Quick Reference Table
- Find part:
Part = Percent × Whole(use decimal form) - Find percent:
Percent = (Part / Whole) × 100 - Find whole:
Whole = Part / Percent(decimal form) - Percent change:
((New − Old) / Old) × 100
Final Thoughts
Learning how to calculate percentage is one of the most practical math skills you can build. It helps with budgeting, decision-making, analyzing trends, and understanding data in everyday life. If you remember just a few formulas and practice with real scenarios, percentage math becomes fast and intuitive.
Use the calculator above whenever you want a quick answer, and use the formulas below it to strengthen your understanding over time.