Age Calculator

How to Use the Age Calculator

Age looks like the simplest calculation there is, and it is the one people most often get slightly wrong — because months are not a fixed length and because subtracting years without checking the day gives an answer that is right for most of the year and wrong for the rest of it.

1. Enter the date of birth. Any date from the year 1 onwards. The calculator rejects impossible dates such as 31 February rather than quietly rolling them forward, which is what naive date handling does.
2. Leave the second field blank for today, or set a date to find an age on a particular day. That is the version you want for a passport deadline, an eligibility cut-off or a school entry date.
3. Read the years, months and days figure first. It is how age is actually stated, and it is not the same as dividing total days by 365.25.
4. Use the totals for anything else. Total days, weeks, months and hours are all exact, and the day count is the one milestone birthdays are built on.

Every date is handled at midnight UTC, so daylight saving cannot shift a result by a day. Nothing you enter leaves your browser.

How Age Is Actually Calculated

There is no formula for age in the algebraic sense — there is a procedure, and following it in the right order is what separates a correct answer from an off-by-one.

The clamping rule in the footnote is the subtle part. Somebody born on 31 August is one month old on 30 September, not on 1 October — the month simply ends sooner. Software that adds a month by incrementing the month number and leaving the day alone produces 31 September, which JavaScript silently converts to 1 October, and the age is a day out.

The other trap is dividing by 365.25. It gives a plausible decimal and it never gives the right calendar age, because your birthday does not arrive after a fixed number of days — it arrives on a date.

Example

Worked example one: born 15 March 1990, measured on 28 August 2026. The year difference is 2026 − 1990 = 36, and since 15 March has already passed in 2026 no adjustment is needed: the age is 36 years. The thirty-sixth birthday fell on 15 March 2026.

From 15 March to 28 August is 5 complete months, taking us to 15 August, and from 15 August to 28 August is 13 days. So the exact age is 36 years, 5 months and 13 days.

The totals: 13,315 days lived, which is 1,902 weeks and 1 day, 437 months plus 13 days, and 319,560 hours. Dividing 13,315 by 365.25 gives 36.4544 years — close to the calendar answer and not identical to it, which is exactly why both are shown.

Worked example two: the birthday itself. The thirty-seventh birthday falls on 15 March 2027, which is 199 days after 28 August 2026 — a Monday. The birth date itself was a Thursday.

A useful cross-check: 13,315 + 199 = 13,514 days from birth to the thirty-seventh birthday. Divide by 37 and you get 365.24 days a year, which is the length of the tropical year almost exactly. That is not a coincidence — it is the leap year rule doing its job across nine leap days.

Calendar Age and Decimal Age

Use the years-months-days figure for anything a person will read and the total-day figure for anything a computer will use. The first matches how age is spoken about; the second is unambiguous, orderable and immune to arguments about how long a month is.

The 'age at this date' field is the one worth remembering. Eligibility rules — for school entry, driving, pensions, competition age groups, visa categories — are almost always written as an age on a particular date, and working that out by hand in January for a September cut-off is exactly where mistakes happen.

The decimal-years figure is deliberately shown alongside the calendar age rather than instead of it. They differ by a fraction of a year and both appear in real documents: actuarial and medical contexts often want a decimal age, while everything else wants the calendar one.

A note on people born on 29 February. They have a birthday on the calendar once every four years, and the two common conventions place the non-leap-year anniversary on 28 February or 1 March. This page uses 28 February, which is the more widespread of the two and the one most legal systems follow.

For the gap between two arbitrary dates rather than an age, the date difference calculator does the same arithmetic without assuming one of them is a birth date, and the birthday calculator adds the weekday, the star sign and the day-count milestones.

Five Things That Make Dates Hard

Five things that make date arithmetic harder than it looks.

Months have no fixed length. Adding one month to 31 January is genuinely ambiguous, and every system resolves it by convention rather than by logic. Clamping to the end of the target month is the least surprising choice and the one used here.

Leap years are not every four years. A year divisible by 100 is not a leap year unless it is also divisible by 400. 1900 was not a leap year and 2000 was, which is why an average year is 365.2425 days rather than 365.25.

Time zones can shift a date. Somebody born late on 14 March in one time zone was born on the 15th in another, and their legal birth date follows the place of birth. This calculator works entirely in UTC so its arithmetic is consistent, but the input date is whatever the certificate says.

East Asian age reckoning differs. Traditional systems in several countries count a person as one at birth and add a year at the new year rather than at the birthday, making someone up to two years older than their international age. South Korea moved to international reckoning in 2023.

Historical dates cross a calendar change. The switch from the Julian to the Gregorian calendar happened in different years in different countries — 1582 in much of Catholic Europe, 1752 in Britain and its colonies — and eleven days were simply skipped. Dates before those changes need care that no calculator supplies.