Percentage Calculator
Use any section below to quickly solve common percentage problems.
1) Find What is X% of Y?
2) Find X is what percent of Y?
3) Find percentage increase or decrease
What is a percentage?
A percentage is simply a way of expressing a number as a fraction of 100. The word comes from the Latin phrase per centum, meaning “by the hundred.” So if something is 35%, it means 35 out of every 100.
Percentages are everywhere: discounts in stores, test scores, tax rates, inflation, savings growth, conversion rates, and nutrition labels. Once you understand a few core formulas, percentage problems become easy and fast.
The 3 most common percentage calculations
1) How to find X% of Y
This is the classic “discount/tip/tax” style question.
Example: What is 15% of 240?
- Convert 15% to decimal: 15 ÷ 100 = 0.15
- Multiply: 0.15 × 240 = 36
Answer: 15% of 240 is 36.
2) How to find what percent one number is of another
Use this when comparing two values, like “50 is what percent of 200?”
Example: 50 is what percent of 200?
- Divide: 50 ÷ 200 = 0.25
- Convert to percent: 0.25 × 100 = 25%
Answer: 50 is 25% of 200.
3) How to calculate percentage increase or decrease
Use this for prices, population changes, salary adjustments, and performance metrics.
Example: Price rises from $80 to $100.
- Difference: 100 − 80 = 20
- Divide by old value: 20 ÷ 80 = 0.25
- Convert to percent: 0.25 × 100 = 25%
Answer: 25% increase.
How to reverse percentages (find the original value)
Reverse percentages are useful when you know the final number after a discount or increase and need the starting number.
After a discount
If a shirt costs $68 after a 15% discount, then $68 is 85% of the original price (because 100% − 15% = 85%).
Original price = 68 ÷ 0.85 = 80.
After an increase
If revenue is $126 after a 5% increase, then $126 is 105% of the original.
Original value = 126 ÷ 1.05 = 120.
Real-life percentage examples
Discounts while shopping
For a 30% sale on a $90 item:
- Discount amount = 0.30 × 90 = $27
- Sale price = 90 − 27 = $63
Tips at restaurants
For an 18% tip on a $55 bill:
- Tip = 0.18 × 55 = $9.90
- Total = 55 + 9.90 = $64.90
Grade percentages
If you got 42 correct out of 50 questions:
- 42 ÷ 50 = 0.84
- 0.84 × 100 = 84%
Simple interest understanding
If you earn 4% annually on $2,000 (simple one-year example):
- Interest = 0.04 × 2000 = $80
- New total = $2,080
Quick mental math tricks for percentages
- 10%: Move decimal one place left (10% of 350 is 35).
- 1%: Move decimal two places left (1% of 350 is 3.5).
- 5%: Half of 10% (5% of 350 is 17.5).
- 25%: One quarter of the number.
- 50%: Half of the number.
- 75%: Half plus a quarter.
Common mistakes to avoid
- Forgetting to divide by 100 when turning percent into decimal.
- Using the wrong base value in percentage change (always divide by old/original value).
- Confusing percentage points with percent change. Example: from 10% to 12% is +2 percentage points, but a 20% relative increase.
- Rounding too early. Keep extra decimals until the final step for better accuracy.
Practice problems (with answers)
- What is 12% of 250? Answer: 30
- 45 is what percent of 60? Answer: 75%
- A value drops from 500 to 425. What is the percentage decrease? Answer: 15%
- A jacket after a 20% discount costs $96. What was the original price? Answer: $120
- Your score rises from 72 to 81. What is the percent increase? Answer: 12.5%
Final takeaway
To calculate percentages confidently, remember one core concept: percentages are ratios out of 100. Learn the three key formulas, practice with everyday examples, and use the calculator above whenever you need a quick check. With repetition, percentage math becomes automatic.