The core algorithm
Age calculators use a three-pass algorithm:
- Pass 1 — Years: Subtract the birth year from the current year. Adjust down by 1 if the current month/day is before the birth month/day.
- Pass 2 — Months: Count the calendar months from the last birthday to today. Each month boundary adds 1 to the count.
- Pass 3 — Days: Count the days from the last complete month boundary to today.
This approach works because it mirrors how humans think about age: whole years first, then leftover months, then leftover days.
Why not just divide by 365?
Dividing total days by 365 gives an approximate answer but fails in several ways:
- Leap years add extra days that accumulate over decades.
- Month boundaries matter — being born on Jan 31 vs Feb 1 produces different month counts on the same day.
- The answer "36.3 years" is less useful than "36 years, 4 months, 20 days."
Our calculators track each day individually, respecting real calendar boundaries.
Handling edge cases
Month-end births
Someone born on January 31 advances to February 28 (or 29 in a leap year) after one month. The algorithm then counts the remaining days from that clamped anchor date to the target date. This prevents impossible dates like February 31 from appearing.
Leap day births
February 29 exists only in leap years. Our algorithm uses March 1 as the birthday in non-leap years, following a common observance convention.
Same-day calculation
If today is your birthday, the age is exact: 0 extra months and 0 extra days. You are exactly N years old.
Total counts
After computing years, months and days, the calculator also provides:
- Total days: the exact number of calendar days from birth to today.
- Total weeks: total days divided by 7, rounded down.
- Total months: (years × 12) + extra months.
Try the Age Calculator or Days Since Birth Calculator to see these counts for your own birth date.