“Percent” simply means “out of 100”, so 25% is 25 out of 100, or 0.25. Almost every real percentage question is one of three types, and each has a one-line formula. This guide covers all three with worked examples, and the free percentage calculator does any of them instantly.
What a percentage is
A percentage is just a fraction with 100 on the bottom. To turn a percent into a number you can calculate with, divide by 100: 40% = 40 ÷ 100 = 0.40. To turn a decimal back into a percent, multiply by 100: 0.40 → 40%. Everything below builds on that one idea.
1. Percentage of a number
Result = (percent ÷ 100) × number
Example — what is 15% of 240? 15 ÷ 100 = 0.15, then 0.15 × 240 = 36. This is the one you use for a 15% tip, 18% GST, or 20% off a price.
2. What percent one number is of another
Percent = (part ÷ whole) × 100
Example — 37 out of 50 marks is what percent? 37 ÷ 50 = 0.74, × 100 = 74%. Use this for test scores, completion rates, or “how much of my target have I hit?”
3. Percentage increase and decrease
Change % = ((new − old) ÷ old) × 100
Example — a price rises from ₹800 to ₹1,000. (1000 − 800) ÷ 800 = 0.25, × 100 = 25% increase. If it fell from ₹1,000 to ₹800 instead: (800 − 1000) ÷ 1000 = −0.20 = a 20% decrease. Note the same ₹200 gap is a different percentage each way, because the starting number changed.
Quick mental tricks
- 10% — just move the decimal one place: 10% of 340 = 34.
- 5% — half of 10%: 5% of 340 = 17.
- 1% — move the decimal two places: 1% of 340 = 3.4.
- x% of y = y% of x — 18% of 50 is the same as 50% of 18 = 9. Often one side is far easier.
FAQ
How do I find a percentage of a number fast? Divide the percent by 100 and multiply by the number — or use the 10% / 1% tricks above.
How do I calculate percentage change? Subtract old from new, divide by the old value, then multiply by 100.
Is a 20% rise cancelled by a 20% fall? No — because the base changes, they don't cancel. ₹100 up 20% is ₹120; down 20% from ₹120 is ₹96.
Is my data private? Yes — the calculator runs entirely in your browser and stores nothing.