The simple version
In the most straightforward case, your age in years is just the current year minus your birth year.
If someone was born in 1990 and the current year is 2026:
2026 − 1990 = 36 years old
But this only works if the birthday has already occurred this year. Most of the time, the birthday matters.
The birthday adjustment
If your birthday hasn't occurred yet this year, you subtract one more year.
Example: Born on November 15, 1990. Today is September 23, 2026.
November 15 hasn't happened yet in 2026, so:
2026 − 1990 − 1 = 35 years old
Once November 15 arrives, the person turns 36.
More precisely:
Age = Current year − Birth year − (1 if birthday hasn't occurred yet this year, else 0)
Age in months and days
For a more precise figure — useful for a baby's age, legal contexts, or medical records — you calculate years, then remaining months, then remaining days.
Example: Born March 5, 2023. Today is September 23, 2026.
Years: From March 5, 2023 to March 5, 2026 = 3 complete years.
Remaining months: From March 5, 2026 to September 5, 2026 = 6 complete months.
Remaining days: From September 5, 2026 to September 23, 2026 = 18 days.
Age: 3 years, 6 months, 18 days.
Why months are tricky
Months have different lengths — 28, 29, 30, or 31 days. This makes month arithmetic non-trivial.
Consider someone born on January 31. What is their "one month" anniversary? February has only 28 or 29 days. Different systems handle this differently:
- Some treat February 28 as the one-month mark.
- Others treat March 2 or 3 (31 days after January 31) as the one-month mark.
There is no universally agreed answer. Most age calculators use the convention that "one month later" is the same day number in the next month, or the last day of the month if that day doesn't exist.
Leap years
Leap years add a day to February (February 29). This affects two groups of people:
Born on February 29 ("leaplings"): Their actual birthday only occurs every four years. Convention varies — some countries legally recognize February 28 as the birthday in non-leap years, others use March 1.
Ages that span February in a leap year vs non-leap year: The day count differs by one. An age calculator accounts for this automatically, but manual calculation needs to check whether February in the range is 28 or 29 days.
Years divisible by 4 are leap years, with two exceptions:
- Divisible by 100: not a leap year (e.g. 1900)
- Divisible by 400: is a leap year (e.g. 2000)
Age for legal and official purposes
Age in legal contexts is almost always measured in complete years. A person turns a given age on their birthday — not the day before, not a fraction of the way through the year.
Voting age: You must be 18 on or before election day — the day matters precisely.
Retirement age: Calculated to the day in many pension systems.
Insurance: Age is often rounded in specific ways depending on the policy — sometimes to the nearest birthday, sometimes always down.
Medical age: Paediatric dosing and growth charts often use age in months for children under 2, where the difference between 18 and 21 months is medically significant.
Using the Age Calculator
Rather than working through the arithmetic manually, use the Age Calculator — enter your date of birth and today's date, and it returns your exact age in years, months, and days.
The Date Calculator is useful for related tasks: calculating how many days until an event, or finding the date a certain number of days in the past or future.