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 September 4, 1999?

Current age

27 years, 7 days

Total days: 9,869
Total weeks: 1,409
Completed months: 324
Total hours: 236,856
Total minutes: 14,211,360
Total seconds (live): 852,681,600

Next birthday: September 4, 2027 (358 days, turning 28)

Your date of birth

Date of birth

Calculate age as of

Also show Korean age

Results

Total days: 9,869
Total weeks: 1,409
Total months: 324
Total hours: 236,856
Seconds lived: 0

Elapsed time

Unit	As of September 11, 2026
Years, months, days	27 years, 7 days
Total days	9,869
Total weeks (days ÷ 7)	1,409
Total hours	236,856
Total minutes	14,211,360
Total seconds (at page load)	852,681,600

Age milestones

Age	Date reached	Day of week
1	September 4, 2000	Monday
5	September 4, 2004	Saturday
10	September 4, 2009	Friday
13	September 4, 2012	Tuesday
16	September 4, 2015	Friday
18	September 4, 2017	Monday
21	September 4, 2020	Friday
25	September 4, 2024	Wednesday
30	September 4, 2029	Tuesday
40	September 4, 2039	Sunday
50	September 4, 2049	Saturday
60	September 4, 2059	Thursday
65	September 4, 2064	Thursday
70	September 4, 2069	Wednesday
80	September 4, 2079	Monday
90	September 4, 2089	Sunday
100	September 4, 2099	Friday

Reference dates

Date	Exact age
Birth date (Saturday)	0 years, 0 months, 0 days
September 11, 2026	27 years, 7 days
September 4, 2027	Turning 28
+1 mo → October 4, 2026	27 years, 1 month
+3 mo → December 4, 2026	27 years, 3 months
+6 mo → March 4, 2027	27 years, 6 months
+9 mo → June 4, 2027	27 years, 9 months
+12 mo → September 4, 2027	28 years

Week milestones from birth

Weeks after birth	Date
1	September 11, 1999
4	October 2, 1999
8	October 30, 1999
13	December 4, 1999
26	March 4, 2000
52	September 2, 2000

Related birth dates

50 related calculations
Birth date	Age today	Page
1999-09-03	27	View
1999-09-05	27	View
1999-09-02	27	View
1999-09-06	27	View
1999-09-01	27	View
1999-09-07	27	View
1999-08-28	27	View
1999-09-11	27	View
1999-08-21	27	View
1999-09-18	26	View
1999-08-05	27	View
1999-10-04	26	View
1999-07-06	27	View
1999-11-03	26	View
1999-06-06	27	View
1999-12-03	26	View
1989-09-04	37	View
1990-09-04	36	View
1991-09-04	35	View
1992-09-04	34	View
1993-09-04	33	View
1994-09-04	32	View
1995-09-04	31	View
1996-09-04	30	View
1997-09-04	29	View
1998-09-04	28	View
2000-09-04	26	View
2001-09-04	25	View
2002-09-04	24	View
2003-09-04	23	View
2004-09-04	22	View
2005-09-04	21	View
2006-09-04	20	View
2007-09-04	19	View
2008-09-04	18	View
2009-09-04	17	View
1999-09-08	27	View
1999-09-09	27	View
1999-09-10	27	View
1999-09-12	26	View
1999-09-13	26	View
1999-09-14	26	View
1999-09-15	26	View
1999-09-16	26	View
1999-09-17	26	View
1999-09-19	26	View
1999-09-20	26	View
1999-09-21	26	View
1999-09-22	26	View
1999-09-23	26	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).