In Excel: put the birth date in A2 and use =DATEDIF(A2,TODAY(),"Y") to return the completed age in whole years.
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Put the date of birth in A2.
In B2 enter =DATEDIF(A2,TODAY(),"Y").
Format the result cell as a plain number, not a date.
Use "M" for months or "D" for days if you need finer detail.
What this does
DATEDIF measures the gap between a start date and an end date in the unit you ask for. With the birth date as the start, TODAY() as the end and "Y" as the unit, it returns the number of full years lived — the person's current age. Swap the unit to "M" for whole months or "D" for total days. DATEDIF is hidden from the function list but works in every Excel version and in Google Sheets. The same idea underpins a lot of everyday Excel work, so the few minutes spent getting it right here pay back across every sheet you build afterwards. Treat it as a pattern, not a one-off, and it stops being something you look up and starts being something you reach for. The difference between a quick fix and a sheet you can trust is the extra minute you spend validating “convert date of birth to age in excel”. Start on a copy or a tiny sample, keep the affected cells visible, and compare the result with the outcome you expected before you touch the real workbook. When a formula is involved, keep the inputs labelled beside it, reference cells instead of typing values, and apply number formatting only after the result checks out. The point is a data step that keeps your analysis trustworthy, but the practical win is that someone else can open the file and understand what happened without asking you.
A worked example
A2 holds the birth date 1990-06-15. In B2 type =DATEDIF(A2,TODAY(),"Y"). With today around 2026 the result is 35, because the person has completed 35 birthdays. You reach for DATEDIF whenever a report needs a live age from a birth date — staff records, customer profiles, eligibility checks — that recalculates as time passes. Check the referenced cells, argument order and result type in your workbook. Start with the worked example before changing an input, then compare the changed result with the calculation you expected. An error value or a result of a different type is a reason to inspect the inputs and arguments.
In Google Sheets
Google Sheets support for this task has not been verified. Check the function, arguments and workflow in the target product before transferring the example. Matching formula notation does not establish identical behaviour. Keep this page bookmarked for the next time the same question comes up. Better still, repeat the steps once in your own workbook — doing it yourself is what turns a copied answer into something you remember. For “convert date of birth to age in excel”, keep the input cells and the result visible while you check the formula in Excel. Follow the worked example and change one input at a time. The steps explain the workbook calculation; this page does not run a live calculator for that task.
Common mistakes
- Formatting the result as a date, so 35 shows as a date instead of a number.
- Using "Y" when you actually wanted months or days.
- Putting the dates in the wrong order — start must be earlier than end or
DATEDIFerrors. - Hard-coding today's date instead of
TODAY(), so the age never updates.
Frequently asked questions
Why is DATEDIF not in the function list?
It is a Lotus 1-2-3 compatibility function, so Excel gives it no entry and no autocomplete — type it in full. Microsoft does document it, and advises against the "md" unit in particular.
How do I get age in months or days?
Replace "Y" with "M" for whole months or "D" for total days.
Is there an alternative?
Yes. =YEARFRAC(A2,TODAY()) gives a decimal age you can round down with INT.
Other ways people ask this
This is also commonly searched as “convert birth date to age in excel” and “how to convert date of birth to age in excel”. They describe the identical operation, so you are in the right place no matter how you phrased it.
Why do people search for this in so many different ways?
Because the same task has many names. “convert birth date to age in excel”, “how to convert date of birth to age in excel” all point at the one operation explained on this page, which is why they all lead here.