Percentage Calculator

Percent math without the mental asterisks

Percentages show up on sale tags, pay stubs, exam scores, and investment summaries, and each context hides a slightly different question. Sometimes you need “what is 18% of 240.” Sometimes you need “45 is what percent of 180.” Sometimes you need the change from last quarter to this one. This percentage calculator answers those three questions in one place.

The tool runs locally. Students, shoppers, and analysts can mash numbers without creating an account. Results update as you type so you can try a 15% discount and a 20% discount in the same breath.

The three formulas

A percent of a number is value × percent ÷ 100. That is discounts, tips, commissions, and “15% down.” What percent X is of Y is X ÷ Y × 100. That is test scores, win rates, and “what share of the budget did we spend.” Percent change is (new − old) ÷ old × 100. Positive means increase, negative means decrease.

The dangerous habit is swapping the base. “Up 25% then down 25%” does not return to the start. Markups and margins are also different words: a 25% margin on a $100 sale is $25 profit, while a 25% markup on a $75 cost is also $18.75 on top of cost—not the same as 25% of the selling price. This calculator does the arithmetic you enter; it will not rename a margin as a markup for you.

How to use each mode

Use “What is X% of Y” for discounts and tips. Enter 20 and 85 to see $17 off an $85 item. Use “X is what % of Y” for scores: 46 of 50 is 92%. Use increase/decrease for before-and-after numbers: a price that moved from $40 to $52 is a 30% increase.

Keep units consistent. Do not mix monthly and annual figures in a percent-change field unless you intend to. A 2% monthly change is not a 2% annual change; compounding would make the annual figure much larger.

Everyday examples

A jacket listed at $120 with 30% off is $36 off and $84 to pay before tax. A $2,400 rent that rises to $2,520 is a 5% increase. A team that won 14 of 22 games has a 63.64% win rate. An employee whose salary moves from $68,000 to $71,400 received a 5% raise.

On a $1,500 invoice, 8.25% sales tax is $123.75. If you already know the tax amount and want the rate, switch to “X is what % of Y” with tax as X and the pre-tax amount as Y.

Mistakes this page is designed to prevent

People add percents that apply to different bases (“the stock was +10% and −10%, so I broke even”). They also divide by the new number instead of the old number when measuring change. The increase/decrease mode always uses the starting value as the denominator so the language matches “percent change from.”

If you are comparing pay, convert both sides to the same period first with the salary calculator. If you are splitting a restaurant check, the tip calculator applies a percent and a headcount in one step.

Typical examples

Frequently asked questions

How do I calculate a percentage of a number?

Multiply the number by the percent, then divide by 100. 18% of 240 is 240 × 18 ÷ 100 = 43.2. This is the same math behind sales tax, tips, and discounts.

How do I find what percent one number is of another?

Divide the part by the whole and multiply by 100. 45 ÷ 180 × 100 = 25%. If the whole is zero, a percent is undefined.

What is the percent increase formula?

((new − old) ÷ old) × 100. Going from 80 to 100 is a 25% increase. Going from 100 to 80 is a 20% decrease—the two are not mirrors because the base changed.

Why is a 50% decrease then 50% increase not a break-even?

Percents apply to the current base. 100 minus 50% is 50; 50 plus 50% is 75. Always check which number is the denominator.

Can I use this for grades or tax?

Yes for the raw percent. Grade weights belong in the GPA calculator, and tax withholding belongs in the take-home tool. This page is the general-purpose arithmetic.