To calculate your age in days, count the days from your date of birth to today: 365 for each full year, plus one for every February 29 you’ve lived through, plus the days since your last birthday. Someone born on Thursday, March 15, 1990 was 13,355 days old on Wednesday, October 7, 2026.
The idea is simple, but two details trip people up: leap days, and the difference between calendar months and “30-day” months. This guide shows the hand method step by step, converts the same age into weeks, months and hours, explains how birthdays on the 31st and on February 29 are handled, and lists milestone days such as 10,000 days old. If you only want your own number, the age calculator works it out instantly from your date of birth.
The quick way: use the age calculator
Enter your date of birth in the How old am I? calculator and it shows:
- your exact age in years, months and days;
- the weekday you were born;
- the number of days you’ve been alive, also shown as weeks and days;
- the date of your next birthday and how many days away it is.
The calculation runs in your browser, and your date of birth isn’t sent to the site. If you only know a birth year, the year pages give the range instead: the born in 1990 page, for example, shows both possible ages this year, the range of days alive depending on the birthday, and milestone birthdays.
How to calculate age in days by hand
Split the count into three parts, then add them up:
- Full years: multiply your age in whole years by 365.
- Leap days: add 1 for every February 29 between your birth date and your most recent birthday.
- Days since your last birthday: count the days left in your birthday month, add the length of each full month after it, then add today’s day of the month.
Worked example for a birth date of March 15, 1990, counted on October 7, 2026:
| Step | Calculation | Days |
|---|---|---|
| 36 full years (March 15, 1990 to March 15, 2026) | 36 × 365 | 13,140 |
| Leap days in that span | 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020, 2024 | 9 |
| March 15 to October 7, 2026 | 16 (rest of March) + 30 + 31 + 30 + 31 + 31 + 30 + 7 | 206 |
| Total | 13,140 + 9 + 206 | 13,355 |
This is an exclusive count: the day you were born is day 0, and each midnight after it adds one. That’s the convention age calculators use. If you prefer to count your birth day as day 1, add 1. The guide to counting days between dates explains when each convention fits.
Why leap days change the answer
Multiplying your age by 365 always comes up short, by roughly one day for every four years. In the example above, 36 × 365 gives 13,140, nine days fewer than the real 36-year span of 13,149 days. Multiplying by 365.25 gets closer but can still be a day off, because leap days arrive in steps every four years rather than a quarter-day at a time, and century years such as 2100 skip February 29 under the Gregorian rules.
To count your leap days, list the leap years from your birth year onward and keep only the February 29s that fall after your birth date and on or before today. Someone born in March 1992 missed that year’s leap day; someone born in January 1992 lived through it. The leap years guide covers the full rule and lists the next ones; the next February 29 is in 2028.
Age in weeks, months and hours
Once you have the day count, the other units follow. Here’s the March 15, 1990 example, still measured on October 7, 2026:
| Unit | How to get it | Result |
|---|---|---|
| Days | Hand method above | 13,355 days |
| Weeks | Days ÷ 7, remainder in days | 1,907 weeks and 6 days |
| Months | Whole calendar months, then leftover days | 438 months and 22 days |
| Years, months, days | Whole years, whole months, leftover days | 36 years, 6 months and 22 days |
| Hours | Days × 24 | About 320,520 hours |
| Minutes | Hours × 60 | About 19,231,200 minutes |
Hours and minutes are approximate unless you know your time of birth. The day count assumes midnight to midnight, so the real figure can be up to 24 hours either way, and a birth in a different time zone shifts it too. For a visual version of the weeks figure, the life in weeks chart draws every week as a box.
Calendar months vs. “30-day” months
Dividing days by 30 or 30.44 gives an average, not an age. Real ages use calendar months, and those run from a date to the same date in the next month, whatever the month’s length. That causes two edge cases:
- Birthdays on the 29th, 30th or 31st. Some months don’t have that date. A month that starts on January 31 ends on February 28 in 2026, because February has no 31st. Our calculator treats a short month as complete on its last day.
- Birthdays on February 29. In leap years the birthday is February 29. In other years our calculator counts it on March 1, the first day on which you’ve completed another full year; some people celebrate on February 28 instead. Someone born on February 29, 2000 turns 27 on March 1, 2027 by that rule, after 9,862 days.
Other tools may handle these dates differently, which is why two age calculators can disagree by a day around a birthday. The total day count doesn’t depend on this choice; only the years-months-days breakdown does.
Milestone days: when you turn 10,000 days old
Day milestones don’t line up with birthdays, so they’re easy to miss. For someone born on March 15, 1990:
| Milestone | Date reached |
|---|---|
| 1,000 days old | Wednesday, December 9, 1992 |
| 5,000 days old | Saturday, November 22, 2003 |
| 1,000 weeks old | Thursday, May 14, 2009 |
| 10,000 days old | Monday, July 31, 2017 |
| About 1 billion seconds (11,574 days) | Sunday, November 21, 2021 |
| 2,000 weeks old | Thursday, July 13, 2028 |
| 15,000 days old | Wednesday, April 9, 2031 |
| 20,000 days old | Friday, December 16, 2044 |
| 25,000 days old | Sunday, August 25, 2058 |
10,000 days is a little over 27 years and 4 months. To find your own milestone, put your date of birth and the number of days into the date calculator; to count up from a past date, use the days since calculator. A billion seconds is 11,574 days plus about 1 hour 47 minutes, so the exact moment depends on your birth time.
Age in days in a spreadsheet
Excel and Google Sheets store dates as day numbers, so with a date of birth in cell A1, =TODAY()-A1 returns your age in days. Format the result as a number, not a date. Divide by 7 for weeks. For whole years, months and days, the Excel date formulas guide covers DATEDIF and its quirks.
Frequently asked questions
How do I calculate my age in days?
Multiply your age in whole years by 365, add one day for each February 29 you’ve lived through, then add the days since your last birthday. The age calculator does it automatically.
How many days old is a 30-year-old?
Between 10,957 and 10,958 days on their 30th birthday, depending on whether the span contains seven or eight February 29s. Add the days since that birthday for an exact count.
When am I 10,000 days old?
About 27 years and 4 months after you were born. Add 10,000 days to your date of birth in the date calculator for the exact date; for a March 15, 1990 birthday it was July 31, 2017.
Does my age in days include the day I was born?
Not usually. Calculators count the day of birth as day 0, the same way you’re 0 years old until your first birthday. Add 1 if you want to count it.
How do I convert my age in days to years?
Divide by 365.25 for an approximate answer. For an exact age, count whole years from your birthday instead, because leap days don’t fall evenly.