Chronological age calculator — years, months, days
Enter a date of birth and read the exact chronological age: whole years, remaining months and days, the total in days and weeks, and how long until the next birthday. Change the "as of" date to ask how old someone was on any date — the arithmetic is the same.
The same calculation, three questions
"How old am I" is the everyday form, but the identical arithmetic answers two more: "how old was she on the date of the contract" (set the as-of date to the contract date) and "what is my age in days" (read the total-days line). Forms that ask for age at a specific event — school enrolment cutoffs, visa applications, insurance quotes — all mean the as-of version, and getting it right matters more there than anywhere.
The months-and-days remainder is where hand calculations go wrong, because months are unequal. From 31 January to 1 March is one month and one day by one convention, and 29 days by another, and neither is wrong — they borrow differently. This calculator borrows the real length of the month before the as-of date, the convention used by most official age computations.
How to use this
- Enter the date of birth — the exact age appears immediately.
- Leave "as of" at today for current age, or set it to any date for age-on-that-date.
- Read the total days and weeks for the age-in-days version of the question.
Worth knowing: Ages here are calendar arithmetic in UTC. A birth time and timezone can shift the first and last day by one; no calculator without both can resolve that.
FAQ
How is chronological age calculated?
Count whole calendar years from the birthday, then whole months from the last birthday, then leftover days. The fiddly part is borrowing: born on the 31st, measured in a 30-day month, the day count borrows the actual length of the previous month rather than a flat 30 — which is why hand calculations drift by a day or two.
What about people born on 29 February?
The age in years still increments once a year. This calculator observes the birthday on 1 March in common years, which is the convention most legal systems use; some (New Zealand, for instance) use 28 February. Either way the total-days figure is exact.
Why does my age in days not divide evenly into years?
Because years are not equal: a span of 25 years contains six or seven leap days depending on where it falls. The days figure is the count of real days lived, not 365 × years.