Years, then whole months, then days — the way a person says it. The one genuinely contested case, a 29 February birthday, is offered both ways rather than decided for you.
Why an age has more than one right answer
For most people it does not: you are a year older on your birthday, and the arithmetic in between is a matter of counting whole months and then the days left over. The complication is people born on 29 February, who have a real birthday only once every four years, and jurisdictions genuinely differ on what happens in the other three.
Much of the United States treats the birthday as falling on 28 February in a common year, which never delays a coming-of-age. England and Wales, among others, take 1 March, on the reasoning that a year is not complete until the corresponding month has run out. Both are defensible, both are in force somewhere, and the difference is a full day of legal age on exactly the date it matters most. This page offers both and names the one it used.
Why months are counted rather than divided
The tempting way to compute an age in months is to divide the day count by 30.44 and round. It is also wrong, because months are not equal and the error moves with the calendar rather than staying still. This page counts whole months by stepping through them one at a time from the last birthday, then reports the remaining days exactly. It is slower and it is right, which on a page whose only product is being right about dates is the correct trade.
The day before your birthday
You turn a year older on your birthday, not the day before it, and a calculator that gets this wrong looks correct for three hundred and sixty-four days a year. That case is asserted directly on every build: someone born on the 25th is one age on the 24th and the next age on the 25th, with nothing in between.
Questions
How old am I exactly?
Enter your date of birth and the date you want the age on — today, by default — and the answer above gives years, whole months and remaining days, along with the total number of days you have been alive. All three parts are exact: the months are counted one at a time rather than estimated from an average month length, which is the usual source of an answer that is a day out.
What happens if I was born on 29 February?
You get to choose, because the law does not agree with itself. Much of the United States treats your birthday as 28 February in a common year, which means you reach each legal age at the earliest defensible moment. England and Wales use 1 March instead, reasoning that a full year has not passed until the month has. In a leap year both rules agree it is the 29th. The control above sets which reading you want, and the answer names it.
Does it count the day I was born?
Your age is zero on the day you are born and one on your first birthday, which is the ordinary human reading and the one used here. The separate \u201ctotal days\u201d figure is a plain count of days elapsed since birth, so on the day you are born it is zero rather than one. Some medical and legal contexts count differently; where that matters, the number you want is usually the total-days figure rather than the years.
Why does my age in months not match my age in years times twelve?
Because the months figure is the number of whole months since your last birthday, not since your birth — it is always between zero and eleven. Thirty-six years and five months means thirty-six full years, and then five more whole months on top. If you want the total number of months you have been alive, multiply the years by twelve and add the months.
Is anything I type here sent anywhere?
No, and not as a matter of policy. This page is a static file and a script your browser downloads once; there is no server behind it to receive anything. Your date of birth is read by the script in your own browser and the answer is written back into the same page. Nothing is sent, nothing is stored, and no analytics or third-party script runs here.
Dates and Days