Calculating age in Excel is a fundamental task for human resources professionals, educators, and data analysts. While it might seem as simple as subtracting the date of birth from the current date, ensuring accuracy across leap years and specific calendar months requires a more sophisticated approach. The most reliable and widely used method involves the DATEDIF function.

To get the age in years immediately, if the date of birth is in cell A2, use this formula: =DATEDIF(A2, TODAY(), "Y")

This formula compares the birth date in A2 with today's date and returns only the number of completed years.

Understanding the Hidden DATEDIF Function

The DATEDIF function is a unique tool in the Excel library. It is often referred to as a "hidden" function because it does not appear in the formula autocomplete dropdown list when you start typing =DATE.... Furthermore, it lacks the standard "Tooltip" assistance that guides you through arguments.

The origin of DATEDIF traces back to compatibility with Lotus 1-2-3. Despite its hidden status, it remains fully functional in all modern versions of Excel, including Microsoft 365, Excel 2021, 2019, and 2016.

The Syntax of DATEDIF

The function requires three specific arguments to work: =DATEDIF(start_date, end_date, unit)

  1. start_date: This is usually the date of birth (DOB).
  2. end_date: The date up to which you want to calculate the age (often TODAY() for current age).
  3. unit: A text code that determines the type of information returned.

Essential Unit Codes for Age Calculation

The "unit" argument is the most critical part of the formula. It must be enclosed in double quotation marks:

  • "Y": Returns the number of complete years in the period.
  • "M": Returns the number of complete months in the period.
  • "D": Returns the number of days in the period.
  • "YM": Returns the difference between the months, ignoring days and years. This is used to find how many months have passed since the last full year.
  • "MD": Returns the difference between the days, ignoring months and years. This shows how many days have passed since the last full month.
  • "YD": Returns the difference between the days, ignoring years.

What Is the Most Accurate Age Formula in Excel?

In our experience with large-scale demographic datasets, many users attempt to calculate age by subtracting dates and dividing by 365. For example: =(TODAY() - A2) / 365.

While this looks logical, it is flawed. This method does not account for leap years (years with 366 days). Over a lifespan of 80 years, this discrepancy can result in an error of nearly 20 days, potentially miscalculating a person's age on the eve of their birthday.

The DATEDIF function is the "gold standard" because it adheres to the Gregorian calendar logic used by Excel's internal engine. It recognizes that some years are longer than others and that months have varying lengths.

How to Calculate Exact Age in Years, Months, and Days

A common requirement in medical or legal documentation is to provide an exact age. A simple number like "25" is often not enough; you may need "25 Years, 4 Months, and 12 Days."

To achieve this, we must concatenate (join) three different DATEDIF functions using the ampersand (&) symbol.

The Full Concatenation Formula

Assuming cell A2 contains the date of birth: