Percentage Calculator
Free online percentage calculator with 5 modes. Calculate X% of Y, reverse percentages, percent change, and more — with step-by-step formulas. Instant, browser-based, private.
What is 15% of 200?
📊 What This Tool Does
A free online percentage calculator that handles the five most common percentage questions: finding a percent of a number, calculating what percent one number is of another, finding the whole from a part and a percent, and calculating percent increase or decrease between two values. Each calculation shows the step-by-step formula so you can see exactly how the answer is derived.
Perfect for students, shoppers (discounts and sales tax), financial planning (interest rates, tips), statistics, grades, or any situation where you need to work with percentages. Everything runs in your browser — no sign-up, no ads, no data sent anywhere.
🧮 The Five Percentage Modes
What is X% of Y?
The most common. Find a percentage of a number.
result = (X ÷ 100) × YExample: 15% of 200 = 30
X is what percent of Y?
Given a part and a whole, find the percentage.
result = (X ÷ Y) × 100Example: 30 is 15% of 200
X is Y% of what?
Given a part and a percent, find the whole.
result = (X × 100) ÷ YExample: 30 is 15% of 200
Percent Increase / Decrease
Compare two values and get the percent change.
((B − A) ÷ A) × 100Example: 50 → 75 is a 50% increase
🎯 Common Uses
🛍️ Shopping discounts
"30% off $89" — use "What is X% of Y?" with X=30, Y=89 to get the discount amount ($26.70). Subtract from the original for the final price ($62.30).
🧾 Tip calculations
Calculate a 15%, 18%, or 20% tip on any restaurant bill. Use "What is X% of Y?" with X = tip percent, Y = bill amount.
📈 Investment returns
"My stock went from $50 to $65, what's the return?" Use Percent Increase: A=50, B=65 → 30% return.
🎓 Grades & stats
"I scored 42 out of 50 on a test" — use "X is what percent of Y?" with X=42, Y=50 to get 84%.
💡 Mental Math Shortcuts
- 10% — move the decimal one place left. 10% of 45 = 4.5
- 5% — half of 10%. 5% of 45 = 2.25
- 15% — 10% + half of 10%. 15% of 45 = 4.5 + 2.25 = 6.75
- 20% — 10% × 2. 20% of 45 = 9
- 25% — divide by 4. 25% of 80 = 20
- 50% — divide by 2. 50% of 80 = 40
- 1% — move the decimal two places left. 1% of 450 = 4.5
- Percent flip trick: X% of Y = Y% of X. So 8% of 50 = 50% of 8 = 4. Much easier!
🔒 Privacy
- ✓ All calculations run in your browser using JavaScript
- ✓ Your numbers are never sent, stored, or logged anywhere
- ✓ Works offline after the page first loads
❓ Frequently Asked Questions
How do I calculate a percentage of a number?
To find X percent of Y, divide X by 100 and multiply by Y. For example, 15% of 200 = (15 ÷ 100) × 200 = 0.15 × 200 = 30. You can also think of it as moving the decimal point two places left on X (15% becomes 0.15), then multiplying by Y.
How do I calculate a percentage increase?
Subtract the old value from the new value, divide the result by the old value, then multiply by 100. Formula: ((new − old) ÷ old) × 100. For example, going from 50 to 75 is a (75 − 50) ÷ 50 × 100 = 50% increase.
How do I calculate a percentage decrease?
Subtract the new value from the old value, divide by the old value, then multiply by 100. Formula: ((old − new) ÷ old) × 100. For example, going from 100 to 75 is a (100 − 75) ÷ 100 × 100 = 25% decrease. Use this for discount and sale calculations.
How do I calculate a discount?
If an item costs $80 and is 25% off, the discount is 25% of $80 = $20, so the sale price is $80 − $20 = $60. Use "What is X% of Y?" mode with X = discount percent and Y = original price. The answer is how much you save.
How do I calculate sales tax?
Multiply the price by the tax rate (as a decimal). For example, 8% sales tax on $50 = $50 × 0.08 = $4. Total = $50 + $4 = $54. In this calculator, use "What is X% of Y?" with X = tax rate and Y = price.
How do I calculate a tip?
Multiply the bill by the tip percent. For a 15% tip on $42, calculate 15% of $42 = $6.30. For quick mental math, find 10% (move decimal one left: $4.20) and add half of that ($2.10) for 15%.
What's the difference between "percent of" and "percent change"?
"Percent of" asks what fraction of a whole something is — e.g., 25 out of 100 is 25%. "Percent change" asks how much something has grown or shrunk between two values — e.g., going from 100 to 125 is a 25% increase. They use different formulas and answer different questions.
How do I convert a decimal to a percentage?
Multiply the decimal by 100 and add the % symbol. For example, 0.75 becomes 75%, 0.125 becomes 12.5%, and 1.5 becomes 150%. To go the other way (percent to decimal), divide by 100.
Is my calculation private?
Yes. All calculations happen in your browser using JavaScript. Your numbers are never sent, stored, or logged. Works offline after the page first loads.
How accurate is this calculator?
The calculator uses double-precision floating-point arithmetic, accurate to about 15 significant digits. For typical percentage problems (money, statistics, grades), this is more than enough precision. Results are displayed rounded to 6 decimal places for readability but computed at full precision.