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 do I calculate exact age 40?

Enter the full date of birth in the calculator above. Exact age uses calendar years, months, and days—not year subtraction alone.

What Year Were 40-Year-Olds Born? Age Calculator

Reference date: September 10, 2026

Your date of birth

Date of birth

Calculate age as of

Also show Korean age

Results

Total days: 14,610
Total weeks: 2,087
Total months: 480
Total hours: 350,640
Seconds lived: 0

Birth year range

Birth year	Jan 1	Dec 31
1985	41	40
1986	40	39

Example dates

Birth date	Current age	Turned 40	Next birthday
January 1, 1985	41	January 1, 2025	January 1, 2027 (113d)
June 15, 1986	40	June 15, 2026	June 15, 2027 (278d)
September 10, 1986	40	September 10, 2026	September 10, 2027 (365d)
December 31, 1986	39	December 31, 2026	December 31, 2026 (112d)

How this is calculated

Birth year ≈ reference year − age. If the birthday has not occurred yet in the reference year, subtract one more year from the birth year.

Age = Reference date − Birth date

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

Calculate exact age Age calculator hub Age 41 Age 39

Related ages

Age 39 Age 40 Age 41

Nearby birth years

1985 1986 2003 2004 2005 2010 2015 2019

FAQ

What year were 40-year-olds born?: Typical birth years for age 40 today: 1985 and 1986.
How do I calculate exact age 40?: Enter the full date of birth in the calculator above. Exact age uses calendar years, months, and days—not year subtraction alone.

What Year Were 40-Year-Olds Born?