How your age is actually calculated
Years, months and days sounds like simple subtraction, and it very nearly is, except that months are not all the same length. Subtract year from year, month from month and day from day, and the days column occasionally goes negative, because the day of the month you are counting from can fall later than the day you are counting to.
That is when you borrow, the same move used for column subtraction at school: take a month from the years side, then add back however many days sit in that borrowed month. It is also why dividing total days by 365.25 gets the headline figure wrong: an average year length never asks how long the actual months involved were.
Take someone born on 15 March 1990, checked here as of 10 July 2026.
The answer, 36 years, 3 months, 25 days, is exactly what walking the calendar one step at a time produces, and it is how every figure on this page is worked out.
Leap year birthdays, explained properly
Around one birth in 1,461 lands on 29 February, a date missing from the calendar three years out of four. So which day counts as the birthday: 28 February, or 1 March?
UK common law, and the law in most other places, treats 1 March as the anniversary in a common year: a person reaches a new age at the first moment of their birthday, and since 29 February is not available, the date rounds forward rather than back. This page follows that rule for the next-birthday countdown above. New Zealand is the well-known exception, its interpretation legislation using 28 February instead, a genuine quirk rather than a mistake.
That convention only governs when the countdown resets. The precise years-months-days total at the top comes from the calendar walk described above, which treats 28 February as that year's stand-in for the 29th, the same clamping any date arithmetic applies whenever a month runs a day short.
Why two sites can give you different day totals
Ask two calculators how many days you have been alive and you can get two different, equally defensible answers. The usual reason is inclusive versus exclusive counting: does the total include the day you were born, the day you are asking on, both, or neither? This page counts whole days elapsed between midnight on your date of birth and midnight on the date you are checking, the exclusive method calendar arithmetic normally uses. Add one and you get the inclusive version: the total you see if a site counts your birth day as day one of your life, not day zero.
Time zones cause a smaller version of the same wrinkle. A total worked out against a server clock on the other side of the world can tick over to the next day a few hours before or after it does where you are, which only matters in the hour either side of midnight. This page works from the dates you type in, not a server clock, so it will not drift depending on where that server happens to sit.
Age is counted differently elsewhere
The years-months-days system here is the international standard, though not the only way a life has been counted. South Korea's traditional reckoning treated a newborn as already one year old at birth, counting time in the womb as time lived, then added a year to everyone together on New Year's Day rather than on individual birthdays, a custom once shared across East Asia. South Korea officially retired that system for most legal and everyday purposes in June 2023, adopting the method used on this page, though a few contexts, such as the age for buying alcohol, still count differently. It is a reminder that "how old are you" only has one obvious answer once everyone agrees what is being counted.
What the milestone table is for
A birthday ending in a zero is not actually more significant than the ones either side of it; base ten is a habit of having ten fingers, not a fact about biology. The milestone table further up offers a few dates worth noticing, chosen for being genuinely uncommon rather than merely divisible by ten. Ten thousand days old, for instance, lands around 27.4 years, a Wednesday as easily as a Saturday, and a rather better excuse for a party than the 27th birthday it falls inside. Each row is worked out from your date of birth the moment you enter it, marked passed once it is behind you and dated in full while still ahead.
Questions people ask
Am I a different age in another time zone?
No. The moment you were born is fixed; only the calendar wrapped around it shifts as you cross time zones. Your age in years, months and days does not change when you travel, though a whole-day total can tick over a day earlier or later than it would at home right around midnight.
How old am I in days, exactly?
The totals table above gives the exact figure, worked out from the same two dates as the headline number rather than an average year length, and it updates the moment you change either one.
When is my golden birthday?
Your golden birthday is the year your age matches the day of the month you were born, turning 8 on the 8th, or 21 on the 21st. It happens exactly once in a lifetime, is not on the milestone list above since it is different for everyone, and only around one birthday in thirty gets to claim it.
Is a 29 February birthday legally a different date?
In the UK and most other places, yes, in a narrow sense: the law treats 1 March as the day a leap-day baby reaches a new age in a common year, even though the birth date itself stays 29 February. New Zealand is the well-known exception, using 28 February instead. It has occasionally decided the exact date someone became old enough to vote, drive or buy a drink.
This page works out age from calendar dates alone. For anything resting on a legal or statutory age, voting, licensing, contracts or benefits, check the rule that applies where you live, since the precise moment an age is reached occasionally differs from plain calendar arithmetic, as the leap-year note above shows.