The basic formula: multiply by (1 + the percentage)

To add a percentage to a number in a spreadsheet, multiply that number by 1 plus the percentage as a decimal. If you want to add 20% to the number 100, you multiply 100 by 1.20, which gives you 120.

The reason you use 1 plus the percentage is that you're keeping the original amount and adding more to it. If you only multiplied by 0.20, you'd get just the 20% portion (20), not the original plus the increase (120).

In a spreadsheet cell, this looks like: =A1*1.20 if the number 100 is in cell A1 and you want to add 20%. You can replace 1.20 with whatever percentage you need — 1.15 for 15%, 1.05 for 5%, 1.50 for 50%.

Key Takeaways

  • Multiply the original number by 1 plus the percentage as a decimal: if you want to add 25%, multiply by 1.25.
  • The formula =A1*1.25 adds 25% to whatever number is in cell A1.
  • You can also write the percentage in a separate cell and reference it: =A1*(1+B1) if B1 contains 0.25.
  • This method works the same way in Excel, Google Sheets, and most other spreadsheet programs.

Using a separate cell for the percentage

If you're adding the same percentage to many numbers, put the percentage in its own cell so you only have to type it once. Put 0.20 in cell B1 (which represents 20%), then use the formula =A1*(1+B1) in the cell where you want the result.

This approach is faster when you're working with a list. If you have ten prices in column A and want to add 20% to each one, you can write the formula once in C1, then copy it down to C10. The spreadsheet automatically adjusts the row numbers (A1 becomes A2, A3, and so on) while keeping B1 the same.

Make sure the percentage cell shows the decimal form: 0.20 for 20%, 0.15 for 15%. If you type "20%" directly into the cell, most spreadsheets will read it correctly, but using the decimal form is clearer and less likely to cause mistakes.

Converting percentages to decimals

A percentage is just a number divided by 100. So 20% becomes 0.20, 5% becomes 0.05, and 150% becomes 1.50. When you add 1 to the decimal, you're saying "keep the original amount (the 1) and add this percentage to it."

If you see a percentage written as a whole number in your spreadsheet — like the cell just says "20" instead of "0.20" — you need to divide it by 100 first. The formula would be =A1*(1+(B1/100)) if B1 contains 20 as a whole number.

Working with negative percentages (discounts)

To subtract a percentage instead of adding it, use 1 minus the percentage. If you want to reduce a price by 15%, multiply by 0.85 (which is 1 minus 0.15). The formula =A1*0.85 takes 15% off the original number.

You can also use a negative percentage in the separate-cell method: =A1*(1-B1) if B1 contains 0.15. This subtracts 15% instead of adding it, which is useful when you're calculating discounts or reductions.

Rounding the result

When you add a percentage to a number, you might end up with many decimal places. If you're working with money, you usually want to round to two decimal places. Use the ROUND function: =ROUND(A1*1.20, 2) rounds the result to two decimal places.

The number after the comma tells the spreadsheet how many decimal places to keep. Use 0 if you want a whole number, 2 for money, or 3 if you need more precision. This doesn't change the actual calculation — it just makes the result easier to read.

Copying the formula down a column

If you have a list of numbers and want to add the same percentage to all of them, write the formula in the first row, then copy it down. Click the cell with your formula, then drag the small square in the bottom-right corner of the cell down to the last row you need.

Most spreadsheets also let you double-click that small square, and the program will automatically copy the formula down as far as your data goes. This saves time when you're working with long lists and makes sure you don't miss any rows.

Frequently Asked Questions

What's the difference between adding a percentage and multiplying by a percentage?

Adding 20% to 100 gives you 120 (the original plus the increase). Multiplying 100 by 20% gives you 20 (just the percentage portion). Use multiplication by 1.20 when you want to add; use multiplication by 0.20 only if you want just the percentage amount itself.

Can I use this method in Google Sheets the same way as Excel?

Yes. The formulas =A1*1.20 and =ROUND(A1*1.20, 2) work identically in Google Sheets, Excel, and most other spreadsheet programs. The syntax is the same across all of them.

What if I want to add different percentages to different numbers?

Put each percentage in its own cell next to the number it applies to, then use =A1*(1+B1) where A1 is the number and B1 is the percentage. You can copy this formula down, and each row will use its own percentage.

How do I add a percentage that's already written as "20%" in a cell?

Most spreadsheets read "20%" as the decimal 0.20 automatically, so =A1*(1+B1) will work even if B1 displays as "20%". If it doesn't work, try =A1*(1+(B1/100)) instead, which treats the cell as a whole number.