How it works
Age is counted in whole years, then the months since the last birthday, then the days since the last "month-day". If the day of the month hasn't been reached yet, the calculator borrows the days of the previous month, the same way you'd do it on paper.
Total days is the exact number of calendar days between the two dates, which automatically includes leap days.
For people born on February 29, the birthday in non-leap years is counted as February 28 when working out the next birthday.
Worked example
Someone born on May 15, 1990, on October 21, 2026:
Years: 2026 − 1990 = 36 (the May birthday has passed). Months: October − May = 5. Days: 21 − 15 = 6.
Age = 36 years, 5 months, 6 days, or 13,308 days in total. Their next birthday is 206 days away.
Frequently asked questions
How is age calculated?
Count the full years since birth, then the full months since the last birthday, then the remaining days. That's the legal and everyday way of stating age in the US.
Does this count leap years?
Yes. Total days and weeks come from the real calendar, so every February 29 in between is included.
Can I find someone's age on a past or future date?
Yes. Change the "Age on this date" field to any date, for example to check age on a school cutoff date or a retirement date.
How do I calculate age in Excel?
Use =DATEDIF(birth_date, TODAY(), "Y") for whole years. Use "YM" and "MD" for the leftover months and days.