RunToolRunAge Calculator
Calculate your age from your birth date.
How to Calculate Exact Age
Enter the birth date
Pick the day, month and year from the date field.
Calculation runs instantly
Years, months and days are computed precisely, accounting for leap years.
Read the exact result
See age broken down to the day — plus total days lived.
Why age is not simple subtraction
Age in years, months and days cannot be computed by subtracting two numbers, because months are not equal. Going from 31 January to 28 February is one month and zero days by the calendar, but only 28 elapsed days. Going from 1 March to 1 April is also one month, and 31 days.
The convention almost everyone expects is calendar-based: count whole years first, then whole months, then the remaining days. That is what makes 'two years, three months and four days' mean the same thing to everyone, even though the number of days it represents varies depending on when it started.
The end-of-month problem
Some date pairs have no unambiguous answer. From 31 January to 30 April: is that three months exactly, since April has no 31st, or two months and thirty days? Different systems answer differently, and both answers are defensible.
The usual rule is to clamp to the last valid day of the target month, which makes 31 January plus one month equal 28 February. It is worth knowing this rule exists rather than assuming a discrepancy between two calculators means one of them is broken — frequently both are right under different conventions.
Leap years and 29 February birthdays
A leap year is any year divisible by four, except centuries, except centuries divisible by 400. 1900 was not a leap year; 2000 was. This matters for any calculation spanning a century boundary, and it is a classic source of off-by-one-day errors in software.
People born on 29 February have a birthday that exists roughly once in four years, and different jurisdictions resolve their legal birthday differently — some treat it as 28 February in common years, some as 1 March. If the answer has legal weight, the local rule decides, not the arithmetic.
Where the exact number actually matters
Most of the time an approximate age is fine. But eligibility thresholds are not approximate: school entry cut-offs, pension dates, insurance bands, medical dosing by age and legal majority all turn on a specific day, and being one day out changes the answer entirely.
In those cases, note that age is normally counted in completed years — you are 17 until the moment your eighteenth birthday arrives, not on the day you enter your eighteenth year. That distinction accounts for a surprising share of confusion.
Time zones and how this runs
Dates are calculated from the calendar rather than from a timestamp, which avoids the classic bug where a birthday shifts by a day depending on the viewer's time zone. A date of birth is a calendar fact, not a moment on a global clock.
The calculation happens in your browser, so a date of birth typed here is not transmitted anywhere. That is worth something: a date of birth is one of the more useful pieces of information an identity thief can collect, and it is routinely typed into websites that keep it.