Age Calcualation in Excel

There are many ways to calculate a person's age in Excel. However, most formulas are only valid for birthdates after 1/1/1900, because they rely on serial numbers.

For Birthdates After 1900

To calculate the age of a person on date since their birthdate:

=INT((date-birthdate)/365.25)

To figure their age as of today, date can be replaced with a function like TODAY() or NOW(). The INT() function is used to return only the number of complete years.

To calculate age in terms of years, months, and days, DATEDIF is a handy function. The following formula is similar to the one above.

=DATEDIF(birthdate,date,"y")

For Birthdates Prior to 1900

Although it is possible to create a really long formula to do the necessary text-to-date conversions necessary for dates prior to 1900, the simplest solution is to use a custom Excel function. The function AGE() below takes advantage of the Visual Basic DateDiff() function, which can handle all Gregorian dates. See the VBA help for details on DateDiff().

Function AGE(birthdate As Variant, asofdate As Variant, _
             Optional interval As Variant) As Variant    If IsMissing(interval) Then        interval = "yyyy"
    End If    AGE = DateDiff(interval, birthdate, asofdate)End Function

How Excel Works with Dates

Excel considers dates as numbers. Each date is assigned a unique serial number. For example, the 27th September 1999 was date serial 36430. Fortunately, you don't need to know this but the fact that all dates have numerical values can be very useful. Windows uses the 1900 date system in which 1st January 1900 is date serial 1, 2nd January 1900 is date serial 2 and so on.
When you type a date into a cell, Excel shows you a date but is thinking of a number.
To find out the serial number of a date, select the cell containing the date then go to Format > Cells. Go to the Number tab and click General in the Category list. The date's serial number will appear in the Sample box on the right.
You can make use of these numbers in all sorts of ways. You can add a number to a date to give a date that number of days later (or subtract a number to get a date before), you can take one date from another to find out how many days in between. There are lots of ready-made date functions too.

Working Out a Person's Age

A person's age is the amount of time since they were born (I know you know that but the computer doesn't, and we have to start thinking like the computer). So, all we have to do is put today's date in one cell and the person's date of birth in another cell, then take their date of birth away from today and you get their age - right? Well, sort of... you get a number. Because you took a date serial from another date serial you get the number of days in between*[note]. It looks like this...
In this example the formula in cell A3 is:
=A1-A2


We need to convert this number of days into a number of years. Most years have 365 days but every fourth year has 366 days. So the average number of years is 365.25. Let's modify our formula...
In this example the formula in cell A3 is:
=(A1-A2)/365.25


Note the brackets around the first part of the formula. Brackets mean "Work out this bit first...". I've used them here to stop Excel trying to divide A2 by 365.25 before taking it away from A1. Excel formulas work do any multiplying and dividing before it does adding and subtracting, but anything in brackets gets done first.
Now we can see a number of years, but it's still not quite right. We are getting an accurate result but we don't really want to see the fraction. As a last refinement we'll wrap the whole thing inside an INT() function to give us a whole number (an integer). This is better than changing the number of decimal places displayed, which would risk some numbers being rounded up and giving an incorrect result. Here's the finished result...
In this example the formula in cell A3 is:
=INT((A1-A2)/365.25)


*Note: In fact, to start with, you get another date. Confused? Don't be... Excel is trying to help but has misunderstood what we need. In date calculations, the result cell gets automatically formatted the same way as the first cell in the formula. Because the first cell was formatted as a date Excel showed you the result as a date, although you wanted to see a number. Just reformat the cell manually by going to Format > Cells > General. [back]

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Inserting Today's Date Automatically

You can save yourself the effort of entering today's date manually. Excel has a function, TODAY(), that creates the current date. All you need to do is place this function into the age calculation formula in place of the reference of the cell that had today's date in it...
In this example the formula in cell A2 is:
=INT((TODAY()-A1)/365.25)

How accurate do you need to be?

This formula yields pretty accurate results but it isn't infallible. Dividing by the average number of days in a year works for most people most of the time, but sometimes it gets it wrong. Supposing the person in question is a child, who hasn't yet lived through a 366 day year, you should be dividing by 365 and not 365.25. So how can we get an exact, guaranteed correct figure? Read on...

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Using Nested IF Statements to Tell It Like It Is!

Supposing it's August and you need to know old someone is...

• The person was born in 1975. How old are they? You can't say.
• The person was born in August 1975. How old are they? You still can't say.
• A person was born on 23rd August 1975. How old are they? At last! You have enough information. You can say for certain.

In order to calculate someone's age precisely you need to know the year in which they were born, and whether or not hey have had their birthday.
If the have had their birthday you subtract their birth year from the current year. If they have not yet had their birthday you subtract their birth year from the current year, and then subtract 1. Easy! We do it all the time without thinking about it. But explaining the rules to Excel is a bit more complicated. Here goes...
=IF(MONTH(TODAY())>MONTH(A1),YEAR(TODAY())-YEAR(A1),
 IF(AND(MONTH(TODAY())=MONTH(A1),DAY(TODAY())>=DAY(A1)),
 YEAR(TODAY())-YEAR(A1),(YEAR(TODAY())-YEAR(A1))-1))
I've written this calculation on three lines for clarity but you should write is as a single expression without spaces. It assumes that cell A1 contains the person's date of birth. Here's what it says...

• IF(MONTH(TODAY())>MONTH(A1)
  If this month is later than the month of the persons birthday...
• YEAR(TODAY())-YEAR(A1)
  ...subtract the year in which they were born from this year because they must have had their birthday.

But what if we haven't passed the month in which they were born. We might be in that month, or we might not have reached it yet. Let's find out...

• IF(AND(MONTH(TODAY())=MONTH(A1),DAY(TODAY())>=DAY(A1))
  If we are currently in the month of the person's birthday and it is either their birthday today or we have passed it...
• YEAR(TODAY())-YEAR(A1)
  ...subtract the year in which they were born from this year because they must have had their birthday.

But what if this isn't the month in which they were born. We know we haven't passed their birthday so...

• (YEAR(TODAY())-YEAR(A1))-1
  ...subtract the year in which they were born from this year then subtract 1, because they haven't had their birthday yet.

Phew!