How to Calculate How Many Days Old Someone Is

Calculating someone's age in days is ordinary date arithmetic. Spreadsheet programs make this calculation simple.

Calculating someone's age in days is a form of date arithmetic. The calculation is fairly easy for a newborn, for whom you can count days in your head. For older people, though, calculating the count of days becomes a matter of bookkeeping: you have to track the number of days in all the months and years since birth, and correct for leap years.

Making the calculation is simple arithmetic, but it does get tedious. The use of spreadsheets such as Microsoft Excel and Google Documents makes performing this arithmetic easy.

Things You'll Need

• Microsoft Excel (optional)

Instructions

Excel Spreadsheet

1. Open a new Excel spreadsheet and format the first column (column A) as a date. Click on the column header and choose "Home" from the toolbar. Click on the format drop-down list and change the column format from "General" to "Date."

2. Type today's date (e.g., 11/7/2010) in cell A1. Type the person's birth date (example: 6/15/1982) in cell A2.

3. Create a formula in cell B2 to perform the date subtraction. Type = A1 -- A2 and press "Enter." The person's age in days will appear automatically.

Google Documents Spreadsheet

1. Create a new Google spreadsheet (see Resources for link.) Sign into any Google account (such as gmail or igoogle) or create a new account.

2. Locate the button for "Create New" near the upper left corner. Click this button and choose "Spreadsheet" from the drop-down list. Depending on your browser and settings, Google will open a blank spreadsheet in a new window or tab.

3. Click the column A header to highlight the column. Click on the down arrow beside the "123" in the toolbar and choose "Date" from the drop-down menu.

4. Type today's date (e.g., 11/7/2010) in cell A1. Type the person's birth date (example: 6/15/1982) in cell A2.

5. Create a formula in cell B2 to perform the date subtraction. Type = A1 -- A2 and press "Enter." The person's age in days will appear automatically.

Hand Calculation

1. Subtract the two years. If the person hasn't had a birthday yet this year, subtract 1 from the difference. Multiply the difference by 365 and add 1 for each leap year in between the 2 years. A leap year occurs in every year divisible by 4, except years divisible by 100 that are not divisible by 400. Example: 2010 -- 1982 = 28 years * 365 = 10,220 plus 7 (for the leap years 1984, 1988, 1992, 1996, 2000, 2004, 2008) = 10,227.

2. Sum the days in the whole months between the two dates and add to the previous number. For example, from June, 1982, to November, 2010, add 31 days for July, 31 for August, 30 for September and 31 for October. Summed to the previous number, that is 10,227 + 123 = 10,350.

3. Add the days remaining in the earlier month and the days past in the more recent month to the previous total. For example, from June 15, 1982, to November 7, 2010, add 15 days of February and 7 days of May. Summed to the previous number, that is 10,350 + 22 = 10,372.

Tips & Warnings

• If either spreadsheet returns a date instead of a number, correct this by setting the format of the cell to "number."

• An Excel spreadsheet will not perform the calculation if the person was born before January 1, 1900; however, the Google Documents spreadsheet will.

References

• Google Documents Help: Formatting Your Data
• Google Documents Help: Adding Formulas

Resources

• Google: Docs