Age Calculator
Calculate exact age in years, months, and days from a birth date, plus total days and weeks lived.
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About the Age Calculator
Age is more than a single number of years — this calculator gives you exact years, months, and days between a birth date and any reference date, plus the total number of days and weeks lived, useful for legal forms, milestone tracking, or simple curiosity.
How It Works
The calculator compares the birth date to the reference date (today, by default), first finding the difference in years, then adjusting for months and days by borrowing from the previous month or year whenever the day-of-month or month component would otherwise be negative — the same logic used for calendar-accurate date subtraction. It separately counts total elapsed days to compute total weeks lived.
Age = (Years, Months, Days) between Birth Date and Reference Date, adjusted so each component stays non-negative using calendar month lengths.
Worked example
Scenario: Someone born on June 15, 1990, measured on September 26, 2026.
- Whole years: from June 15, 1990 to June 15, 2026 = 36 years.
- Whole months: June 15 → September 15, 2026 = 3 months.
- Remaining days: September 15 → September 26 = 11 days.
- Total days lived: 13,252 (1,893 full weeks).
Result: 36 years, 3 months, 11 days old on September 26, 2026.
Tips and common mistakes
- Leave the as-of date blank to use today's date on your device.
- Birthdays on the 29th–31st are measured to the last day of shorter months, so someone born January 31 turns one month older on February 28.
- People born on February 29 are counted a year older on February 28 in non-leap years.
Assumptions & Limitations
- Uses calendar months and years, so month lengths vary (28-31 days) exactly as they do on a real calendar, including leap years.
- Defaults the reference date to today if no 'as of' date is provided.
- Does not account for time zones — the calculation uses calendar dates only, not exact time-of-day.
How age calculations handle dates
Age is based on completed calendar years, not simply the number of days divided by 365. The calculation first compares the birth month and day with the target date, then reports the remaining months and days. This avoids errors caused by leap years and months of different lengths.
Practical example
If someone was born on 15 March 1990, they do not turn the next age until 15 March of the target year. On 14 March, one day before the birthday, the completed-year age is still one year lower.