How do I calculate my age from my date of birth?

Subtract your birth year from the current year. If your birthday has not happened yet this year, subtract one more year from that total. Then count the months and days separately using the same subtract-and-borrow method. This tool does all of that automatically. You just get the finished answer.

Can I calculate my age on a specific date?

Yes. Instead of leaving the calculate-as-of field on today, pick any date, past or future, and the tool works out your age as of that day. Use this for school enrollment cutoffs, immigration age requirements, or figuring out how old you will be on a future date like a wedding or retirement.

How does this handle leap years?

Leap years are built into the calendar math, not added as an afterthought. If someone's birthday is February 29, this tool treats it as February 28 in non-leap years for counting elapsed years, the same convention most legal and administrative systems use, while still calculating the exact day count correctly underneath.

What is chronological age?

Chronological age is the time that has passed since birth, measured in the calendar: years, months, and days. It is different from biological age, which estimates how old a body seems rather than how long a person has actually been alive. When a form or a doctor asks for your age, chronological age is what they mean.

How is Korean age different from international age?

Korean age starts at one, not zero, on the day someone is born, and everyone's age increases together on New Year's Day instead of on individual birthdays. Depending on birth month, that can put Korean age one or two years ahead of the international age most of the world uses. Toggle Also show Korean age to see both numbers at once.

How do I calculate age difference between two people?

Switch to the Age Difference tab, enter both birth dates, and the calculator returns the gap as years, months, and days, plus a flat day count if you need one number instead of three. It works the same way whether the gap is a few months or several decades.

How accurate is this online tool?

It is accurate to the day, using real calendar date math rather than a simplified days-divided-by-365 estimate. Leap years, varying month lengths, and the borrow-a-month logic are all handled correctly. The one thing it cannot account for is a wrong birth date typed in.

How do I calculate age in Excel?

Put the birth date in a cell, then wrap it in Excel's DATEDIF function with "Y", "YM", and "MD" as the unit arguments to pull out years, months, and days separately. The full formula is in the Excel section above. It is the same calendar-based approach this tool uses, just typed out manually.

Yes. Everything runs in your browser using JavaScript. Your birth date never leaves your device. There are no servers processing your information and no account required. You can disconnect from the internet after loading the page and the calculator still works.

Can I use this for my pet?

Yes. Click the Pet Age tab, choose dog or cat, and enter how many years old your pet is. The tool converts to human-equivalent age using the standard veterinary formula, not the old seven dog years equals one human year shortcut.

How Old Is Someone Born on December 21, 2009?

Current age

16 years, 8 months, 21 days

Total days: 6,108
Total weeks: 872
Completed months: 200
Total hours: 146,592
Total minutes: 8,795,520
Total seconds (live): 527,731,200

Next birthday: December 21, 2026 (101 days, turning 17)

Your date of birth

Date of birth

Calculate age as of

Also show Korean age

Results

Total days: 6,108
Total weeks: 872
Total months: 200
Total hours: 146,592
Seconds lived: 0

Elapsed time

Unit	As of September 11, 2026
Years, months, days	16 years, 8 months, 21 days
Total days	6,108
Total weeks (days ÷ 7)	872
Total hours	146,592
Total minutes	8,795,520
Total seconds (at page load)	527,731,200

Age milestones

Age	Date reached	Day of week
1	December 21, 2010	Tuesday
5	December 21, 2014	Sunday
10	December 21, 2019	Saturday
13	December 21, 2022	Wednesday
16	December 21, 2025	Sunday
18	December 21, 2027	Tuesday
21	December 21, 2030	Saturday
25	December 21, 2034	Thursday
30	December 21, 2039	Wednesday
40	December 21, 2049	Tuesday
50	December 21, 2059	Sunday
60	December 21, 2069	Saturday
65	December 21, 2074	Friday
70	December 21, 2079	Thursday
80	December 21, 2089	Wednesday
90	December 21, 2099	Monday
100	December 21, 2109	Saturday

Reference dates

Date	Exact age
Birth date (Monday)	0 years, 0 months, 0 days
September 11, 2026	16 years, 8 months, 21 days
December 21, 2026	Turning 17
+1 mo → October 21, 2026	16 years, 10 months
+3 mo → December 21, 2026	17 years
+6 mo → March 21, 2027	17 years, 3 months
+9 mo → June 21, 2027	17 years, 6 months
+12 mo → September 21, 2027	17 years, 9 months

Week milestones from birth

Weeks after birth	Date
1	December 28, 2009
4	January 18, 2010
8	February 15, 2010
13	March 22, 2010
26	June 21, 2010
52	December 20, 2010

Related birth dates

50 related calculations
Birth date	Age today	Page
2009-12-20	16	View
2009-12-22	16	View
2009-12-19	16	View
2009-12-23	16	View
2009-12-18	16	View
2009-12-24	16	View
2009-12-14	16	View
2009-12-28	16	View
2009-12-07	16	View
2010-01-04	16	View
2009-11-21	16	View
2010-01-20	16	View
2009-10-22	16	View
2010-02-19	16	View
2009-09-22	16	View
2010-03-21	16	View
1999-12-21	26	View
2000-12-21	25	View
2001-12-21	24	View
2002-12-21	23	View
2003-12-21	22	View
2004-12-21	21	View
2005-12-21	20	View
2006-12-21	19	View
2007-12-21	18	View
2008-12-21	17	View
2010-12-21	15	View
2011-12-21	14	View
2012-12-21	13	View
2013-12-21	12	View
2014-12-21	11	View
2015-12-21	10	View
2016-12-21	9	View
2017-12-21	8	View
2018-12-21	7	View
2019-12-21	6	View
2009-12-01	16	View
2009-12-02	16	View
2009-12-03	16	View
2009-12-04	16	View
2009-12-05	16	View
2009-12-06	16	View
2009-12-08	16	View
2009-12-09	16	View
2009-12-10	16	View
2009-12-11	16	View
2009-12-12	16	View
2009-12-13	16	View
2009-12-15	16	View
2009-12-16	16	View

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How this is calculated

Exact age counts completed calendar years, months, and days between the birth date and the reference date.
Total days is the count of calendar days elapsed between those dates (including leap days).

Age = Reference date − Birth date

Weeks ≈ total days ÷ 7. Hours = total days × 24. Minutes = total hours × 60 (exact for whole-day spans).