How to Calculate a Percentage

What "calculate a percentage" actually means

A percentage is just a fraction out of 100. The word breaks down to "per cent" — per hundred. So 25% is the same as 25/100, or 0.25. Almost every percentage question is one of three shapes: finding part of a whole (what is 25% of 200?), finding what share one number is of another (35 is what percent of 200?), or finding how much something changed (it went from 100 to 130 — by what percent?).

Once you know which shape you're dealing with, the math is a single line. Below you'll find the formulas so you can do it on paper, plus the fastest path: typing the numbers into a tool that shows the worked steps. This is handy for school homework, splitting a bill, checking a sale price, or working out a tax line on an invoice.

How to calculate a percentage

1. Decide which question you're answering. "X% of Y" finds a part. "X is what % of Y" finds a share. "From old to new" finds a change. Knowing the shape tells you which formula to use.

2. For "X% of Y," divide the percent by 100 and multiply. The formula is (X ÷ 100) × Y. Example: 25% of 200 → 0.25 × 200 = 50. Open the percentage calculator on the Basic tab, enter 25 and 200, and the answer appears with a step-by-step breakdown so you can check each line.

3. For "X is what % of Y," divide the part by the whole, then times 100. The formula is (X ÷ Y) × 100. Example: 35 out of 200 → 35 ÷ 200 = 0.175, then × 100 = 17.5%. Switch to the Of Total tab, type the part (35) and the whole (200), and read the percentage.

4. For a percentage change, subtract, divide by the original, then times 100. The formula is ((New − Old) ÷ Old) × 100. Example: 100 → 130 is (30 ÷ 100) × 100 = 30% increase. The Change tab labels the result as an increase or decrease and shows the unit difference too.

5. To work backwards, use the Reverse tab. If you know that 75 is 25% of some number, the original is (75 ÷ 25) × 100 = 300. This is the one most people get wrong by hand, so let the tool rearrange it for you.

6. Copy or save the result. Each tab has a copy button and a Save action that drops the calculation into a History list, so you can run several and compare them without losing earlier answers.

Tips and common problems

• Convert percent to decimal first. The most common mistake is forgetting the ÷ 100 step. 8% is 0.08, not 8. If your answer looks 100× too big or too small, that's usually why.

• "Percent of" is not the same as "percent change." "20% of 50" is 10. "50 increased by 20%" is 60 (it's 50 + 10). Use the Basic tab for the first and the Change tab for the second.

• Adding then removing the same percent doesn't return the start. A 50 price marked up 20% becomes 60; take 20% off 60 and you get 48, not 50. Discounts and markups work on different bases, which is exactly why a discount calculator handles the "final price" math for you.

• Watch the divide-by-zero case. Percentage change from an original value of 0 is undefined — there's no baseline to grow from. The tool returns a dash rather than a wrong number.

• Everything runs in your browser. The numbers you type stay on your device, nothing is uploaded, and there's no sign-up. You can use it offline once the page has loaded.

FAQ

How do I find what percentage one number is of another? Divide the smaller part by the total and multiply by 100. For 18 out of 24: 18 ÷ 24 = 0.75, then × 100 = 75%. The Of Total tab does this and shows the "18 out of 24" phrasing under the result so it's easy to sanity-check.

What's the difference between markup and margin? Both describe profit, but against different bases. Markup is profit over cost: (Profit ÷ Cost) × 100. Margin is profit over selling price: (Profit ÷ Selling Price) × 100. A 50 cost sold for 75 is a 50% markup but a 33.3% margin. The Margin tab shows both side by side from the same two inputs.

How do I add a tax or tip percentage to a total? Multiply the base by the rate, then add it back. For an 8.5% tax on 250: 250 × 0.085 = 21.25, total 271.25. The percentage calculator has dedicated Tax and Tip tabs, and for splitting a restaurant bill across people the tip calculator shows the per-person amount.

Can I calculate several percentages at once? Yes. The Batch tab lets you add rows of "X% of Y" and gives each result plus a running total, which is useful when you're checking a list of figures rather than a single number.

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Working with prices and invoices? Pair this with the VAT calculator for tax-inclusive totals, or the scientific calculator for longer expressions.