How age is calculated
Age is counted in completed calendar years, then the remaining whole months, then the remaining days. If the day of the month hasn't been reached yet, one month is borrowed and the number of days in the previous month is added.
People born on February 29 are treated as having their birthday on March 1 in non-leap years.
Worked example
Someone born on June 15, 1990, checking their age on September 30, 2026: from June 15, 1990 to June 15, 2026 is 36 full years. From June 15 to September 15 is 3 more months, and September 15 to September 30 is 15 days. Their age is 36 years, 3 months and 15 days — 13,256 days in total.
Age milestones in days
| Milestone | Reached at about |
|---|---|
| 1,000 days old | 2 years, 9 months |
| 5,000 days old | 13 years, 8 months |
| 10,000 days old | 27 years, 4 months |
| 15,000 days old | 41 years, 1 month |
| 20,000 days old | 54 years, 9 months |
| 25,000 days old | 68 years, 5 months |
| 1 billion seconds old | 31 years, 8 months |
To find the exact date of one of your own milestones, add that many days to your birth date with the Date Calculator.
Frequently asked questions
How is age counted in different cultures?
This calculator uses the international standard: you're 0 at birth and your age goes up by one on each birthday. Some East Asian traditions counted a person as 1 at birth and added a year every New Year, which could make someone one or two years "older" than their international age. South Korea officially standardized on the international system in 2023.
When do people born on February 29 celebrate?
In non-leap years most "leaplings" celebrate on February 28 or March 1. The legal rule depends on the country; this calculator treats March 1 as the birthday in those years, the convention used in many places, including for most purposes in the UK.
Why does the number of days in my age look odd?
Because months are different lengths and leap years add extra days, two people who are both "36 years and 3 months" old can differ slightly in total days. The total days figure counts every actual calendar day, including leap days.
Can I calculate age on a past or future date?
Yes. Change the second date to any day — for example, to find how old someone was on their wedding day, or how old you'll be when a loan is paid off or on the day you plan to retire.
Can I use this for things other than people?
Of course. It works for the age of a pet, a car, a business, or how long you've been in a job or a relationship. To count days to an upcoming event instead, try the Day Counter.