How age is calculated, and why it's trickier than subtraction
Most people think of age as simple subtraction: current year minus birth year. But that ignores months, days, and the awkward fact that months have different lengths. A precise age calculation requires accounting for all three.
The exact algorithm
Age in years = floor((today − birthdate) in complete calendar years). Then the remaining months and days fill in the rest.
Concretely: if you were born on 15 March 1990 and today is 9 May 2026:
- Years: 2026 − 1990 = 36, but your 2026 birthday (15 March) has already passed → 36 years
- Remaining months: May is month 5, March is month 3 → 5 − 3 = 2 months
- Remaining days: 9 − 15 = negative, so borrow from April (30 days) → 9 + 30 − 15 = 24 days
Result: 36 years, 2 months, 24 days.
Leap years and age
If you were born on 29 February, most years your birthday technically doesn't exist. Convention varies: some systems treat 28 February as your legal birthday in non-leap years; others use 1 March. This tool uses 28 February in non-leap years, which is the most common legal interpretation.
Total days lived
Total days = floor((today − birthdate) ÷ 86,400,000 ms). This is exact regardless of leap years because we measure milliseconds, not calendar arithmetic.
A 30-year-old has lived approximately 10,958 days, closer to 11,000 if they were born in a decade with extra leap years.
Life percentage and what it means
The life percentage compares your age to the global average life expectancy of 73 years (World Health Organization, 2024 data). It is not a prediction; individual lifespans depend on genetics, lifestyle, and access to healthcare. India's average is 70.2 years; Japan's is 84.3 years.
Think of it as a reference frame, not a countdown. At 36, you have lived roughly 49% of the global average, still well within the majority of your statistical life.
Why "days until next birthday" matters
Many people use this for countdown planning: anniversary gifts, party preparations, visa applications that require "at least 6 months validity." The tool calculates this as the calendar distance to your next birthday date, not to a fixed anniversary number.