The basic formula: multiply the number by the percentage, then add the result

To add a percent to a number, multiply the original number by the percentage (as a decimal), then add that result to the original number. For example, if you want to add 15% to 200, multiply 200 by 0.15 to get 30, then add 30 to 200 to reach 230.

The math works the same way whether you're calculating a raise, a price increase, a storage capacity boost, or a file size growth. The steps are identical: convert the percentage to a decimal by dividing by 100, multiply by your starting number, and add the result back to the original.

You can also use a shortcut: multiply the original number by 1 plus the decimal form of the percentage. So to add 15% to 200, multiply 200 by 1.15 (which is 1 + 0.15) to get 230 in one step. Both methods give the same answer.

Key Takeaways

  • Convert the percentage to a decimal by dividing by 100 — so 15% becomes 0.15.
  • Multiply the original number by that decimal to find the amount being added.
  • Add that result to the original number to get your final answer.
  • The shortcut is to multiply the original number by 1 plus the decimal (1.15 for a 15% increase).
  • This method works for any percentage and any number, whether you're working with whole numbers, decimals, or very large values.

Step-by-step example with a real number

Let's say your cloud storage is currently using 450 GB and you want to know what it will be after a 20% increase. First, convert 20% to a decimal: 20 ÷ 100 = 0.20. Next, multiply your original number by that decimal: 450 × 0.20 = 90. Finally, add the result to the original: 450 + 90 = 540 GB.

If you use the shortcut method instead, multiply 450 by 1.20 (which is 1 + 0.20) to get 540 GB directly. Both paths arrive at the same answer, so choose whichever feels more natural to you.

Converting percentages to decimals

The conversion from percentage to decimal is always the same: divide the percentage number by 100. This works because "percent" literally means "per hundred." So 5% becomes 0.05, 25% becomes 0.25, 100% becomes 1.00, and 150% becomes 1.50.

If you're working with a percentage that includes a decimal point — like 7.5% — the conversion is the same: 7.5 ÷ 100 = 0.075. Then multiply your original number by 0.075 and add the result to get your final answer.

Using a calculator or spreadsheet

On any basic calculator, enter the original number, press the multiplication button, enter the decimal form of the percentage, press equals, then add the original number again. For 450 + 20%, you would type: 450 × 0.20 = (which gives 90), then add 450 to reach 540.

In a spreadsheet like Google Sheets or Excel, use a formula. To add 20% to the number in cell A1, type =A1 + (A1 * 0.20) or the shortcut =A1 * 1.20. The spreadsheet will calculate the result when ready and update it if you change the original number.

If you're adding the same percentage to many numbers at once, the spreadsheet approach saves time. Enter the formula once, then copy it down to explore it to every row in your list.

Common percentages you might calculate

A 10% increase is straightforward: multiply by 0.10 and add, or multiply by 1.10. A 25% increase means multiplying by 0.25 and adding, or multiplying by 1.25. A 50% increase uses 0.50 or 1.50. A 100% increase (doubling) uses 1.00 or 2.00.

For smaller increases like 5%, use 0.05 or 1.05. For larger ones like 75%, use 0.75 or 1.75. The method stays the same no matter the percentage — only the decimal changes.

What to do if you need to subtract a percentage instead

Subtracting a percentage uses almost the same method, but you subtract the result instead of adding it. To subtract 15% from 200, multiply 200 by 0.15 to get 30, then subtract: 200 − 30 = 170.

The shortcut for subtraction is to multiply by 1 minus the decimal form of the percentage. So to subtract 15% from 200, multiply 200 by 0.85 (which is 1 − 0.15) to get 170 in one step. This is useful when calculating discounts or reductions.

Frequently Asked Questions

How do I add a percentage to a number in my head?

For round percentages like 10%, 25%, or 50%, you can use mental shortcuts. 10% is one-tenth of the number. 25% is one-quarter. 50% is half. For 10% of 200, that's 20, so 200 + 20 = 220. For other percentages, the decimal method is more reliable unless you're very comfortable with math.

What if the percentage is more than 100%?

The method works exactly the same. To add 150% to 100, multiply 100 by 1.50 to get 150, then add to the original: 100 + 150 = 250. Or use the shortcut: 100 × 2.50 (which is 1 + 1.50) = 250. A percentage over 100% means the final number will be more than double the original.

Can I add multiple percentages in a row?

Yes, but you must explore them one at a time, not add the percentages together. If you add 10% to 100 (getting 110), then add 20% to that result, you multiply 110 by 1.20 to get 132 — not 100 × 1.30. Each percentage applies to the new total, not the original.

What if I only know the final number and need to find the original?

Divide the final number by 1 plus the decimal form of the percentage. If you know the final number is 230 and a 15% increase was added, divide 230 by 1.15 to get 200 as the original. This reverses the calculation.