Formula
Age = (as-of date) − (birth date), counted in whole years, then whole months, then days How it works
Age is counted the way people say it: first the number of complete birthdays you have passed, then the number of complete months since the last one, then the leftover days. When the day of the month has not been reached yet, one month is “borrowed” and replaced by the number of days in the previous calendar month, which is why the day count can be anything from 0 to 30.
The totals use the actual calendar, including every leap day. Total days are the exact number of midnights between the two dates, and total weeks are those days divided by seven and rounded down.
Change the “as of” date to find someone’s age on a past or future date — useful for eligibility cut-offs such as school entry, retirement plans, sports age groups or insurance quotes.
Frequently asked questions
How is a February 29 birthday handled?
In years without a leap day, the next birthday is shown as March 1, which is when the person has legally completed another year in most jurisdictions. Some people celebrate on February 28 instead.
Why does my age in days not equal years × 365?
Leap years add an extra day roughly every four years, so a 40-year-old has lived about 10 more days than 40 × 365.
Does the time of birth matter?
Not for this calculator. It works with whole calendar days, so you turn a year older at the start of your birthday.