Method 1: Calculate Age Manually
Follow these simple steps to calculate someone's age by hand:
- Write down the current date in day-month-year format.
- Write down the birth date in the same format.
- Subtract the birth year from the current year to get the initial age.
- Check the months: If the current month is before the birth month, subtract 1 from the year result.
- If months are equal but the current day is before the birth day, subtract 1 year.
- Calculate remaining months by counting forward from the birth month to the current month.
- Calculate remaining days by counting forward from the birth day.
Example: Born March 15, 1990. Today is June 10, 2026.
- Years: 2026 − 1990 = 36 years
- Months: June (6) − March (3) = 3 months
- Days: 10 − 15 = −5 → borrow 1 month → 2 months, 25 days
- Final Result: 36 years, 2 months, 25 days
Method 2: Calculate Age in Excel
Use the DATEDIF function for accurate age calculation in Microsoft Excel or Google Sheets:
Put the birth date in cell A1. This formula accounts for leap years automatically.
Method 3: Calculate Age in Google Sheets
Google Sheets uses the same DATEDIF formula as Excel:
This returns just the full years. Use the extended formula above for years, months, and days.
Method 4: Calculate Age in Python
For developers, here's a simple Python script:
birth = date(1990, 3, 15)
today = date.today()
age = today.year - birth.year - ((today.month, today.day) < (birth.month, birth.day))
print(age)
Handling Leap Years
If someone is born on February 29 (leap day), their legal birthday is usually considered March 1 in non-leap years (or February 28 in some jurisdictions). Our DOB calculator handles this automatically.
A leap year occurs every 4 years, except for years divisible by 100 but not by 400. For example, 2000 was a leap year, but 1900 was not.
Chronological Age vs Other Types
- Chronological age: Time since birth (what most calculators show).
- Biological age: How old your body seems based on health markers.
- Mental age: Your cognitive ability level compared to average.
- Korean age: 1 at birth, +1 every January 1st (phased out legally in 2023).