How age is counted
Age is counted in completed years, then completed months, then remaining days — the convention almost every legal and administrative system uses. Someone born on 3 June 2000 is 26 years, 2 months and 15 days old on 18 August 2026. They turned 26 on 3 June 2026 and will turn 27 on 3 June 2027.
The alternative — dividing total days by 365.25 — gives a decimal that is close but never exactly matches a birthday, which is why forms never ask for it.
The 29 February problem
Someone born on 29 February has a birthday in about one year in four. For age purposes, most jurisdictions treat 1 March as the day they turn a year older in common years; some treat 28 February. We use 1 March, because it preserves the rule that a person turns a year older after a full year has elapsed, and we say so on the result rather than hiding the choice. If your jurisdiction uses 28 February, subtract a day from the next-birthday date.
What else the result shows
- Total days, weeks, hours and minutes lived.
- The day of the week you were born.
- Days until your next birthday, and the day of the week it falls on.
- How many leap days you have lived through.
Worked example
A date of birth of 14 November 1988, checked on 18 August 2026, gives 37 years, 9 months and 4 days. That is 13,791 days, or 1,970 weeks and 1 day. 14 November 1988 was a Monday. The next birthday, 14 November 2026, is a Saturday, 88 days away. Nine leap days have fallen in that span.
What this tool is not
It calculates elapsed time from a date. It does not estimate life expectancy, biological age, fitness age or anything medical, and it never will — that is a different kind of claim requiring a different kind of evidence.
The convention, stated plainly
Age is completed years first, then completed months, then the remaining days — and the borrow, when the day of the month has not yet been reached, is taken from the length of the preceding month, not from a nominal 30. That single choice is why two age calculators can disagree by a day or two on the same pair of dates. Checked on 1 March 2026, a birth date of 5 February gives 0 months and 24 days, because the borrow comes from February's 28. A tool that borrows a nominal 30 reports 26 days for the same pair — two days out. Run the same check on 1 March 2028 and the answer is 25 days, because 2028 is a leap year and February has 29.
The other rule worth stating: the day of birth is day zero, not day one. Someone born on 18 August is 0 years old on 18 August and 1 year old on the following 18 August. Some traditional reckonings — East Asian age being the widely known one — count differently, and a few legal contexts define the birthday as beginning on the preceding day. This tool uses the common Western administrative convention and says so rather than leaving you to infer it from the answer.
Second worked example — a date of birth in a leap year
A date of birth of 3 June 2000, checked on 18 August 2026, gives 26 years, 2 months and 15 days. That is 9,572 days, or 1,367 weeks and 3 days. 3 June 2000 was a Saturday. The next birthday, 3 June 2027, is a Thursday, 289 days away. Six leap days have fallen inside that span — 2004, 2008, 2012, 2016, 2020 and 2024 — which is the number that makes 9,572 rather than 9,566. Anyone dividing total days by 365 to get an age will be six days out by this point, and the error grows by another day every four years.
Where age calculators disagree
- The month borrow. Borrowing 30 days rather than the actual length of the previous month shifts the day figure by up to three. Ours borrows the real length.
- Decimal age. Dividing elapsed days by 365 or 365.25 gives a number that never coincides with a birthday. It is fine for a rough figure and useless for a form; we give completed units.
- 29 February birthdays. We treat 1 March as the day the person turns a year older in a common year. Some jurisdictions use 28 February, and the result says which we used so you can adjust.
- Time zones. A calculator that computes from a UTC timestamp will report a birthday a day early or late for part of the day for anyone away from Greenwich. This one works on local calendar dates only.
The figures the panel shows, and why each is there
Completed years, months and days is the form every official document asks for. Total days is what you want for anything computed per day. Total weeks matters for anything with a weekly cycle. The weekday of birth is the fact people actually came for about a third of the time. Days to the next birthday, with its weekday, is the planning number. And the leap day count is the explanation for why the total-days figure is not what a mental estimate produced — it is shown so the arithmetic can be checked rather than trusted.
Related calculations
For the gap between two people rather than one person's age, use the age difference calculator. For a raw span between two dates with the inclusive and exclusive counts side by side, use days between dates. To check whether a particular year carried a 29 February, use the leap year calculator. The counting conventions behind all of these are collected on the methodology page.