To calculate your age in days, multiply your age in full years by 365, add one day for every February 29 you have lived through, then add the days since your last birthday. Someone born on March 15, 1990 is 13,359 days old on October 11, 2026: 36 × 365 = 13,140, plus 9 leap days, plus 210 days since the March 15 birthday. Divide by 7 for weeks or multiply by 24 for hours.
The formula
Each piece is easy on its own:
- Full years is your age as normally stated.
- Leap days are the February 29s between your birth date and your most recent birthday.
- Days since last birthday is a short count across a few months.
Or skip the arithmetic: the age calculator shows your age in years, months, weeks and days, and the date difference calculator counts days between any two dates.
Worked example
Born March 15, 1990; today is October 11, 2026.
Step 1: Full years. The most recent birthday was March 15, 2026, so the person is 36. 36 × 365 = 13,140 days.
Step 2: Leap days. Count February 29s between March 15, 1990 and March 15, 2026: 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020 and 2024, so 9 days. (2000 counts: a century year is a leap year when it is divisible by 400.)
Step 3: Days since the last birthday. From March 15 to October 11, 2026:
| Span | Days |
|---|---|
| March 15 → March 31 | 16 |
| April | 30 |
| May | 31 |
| June | 30 |
| July | 31 |
| August | 31 |
| September | 30 |
| October 1 → October 11 | 11 |
| Total | 210 |
Step 4: Add. 13,140 + 9 + 210 = 13,359 days.
From there:
- Weeks: 13,359 ÷ 7 = 1,908 weeks and 3 days
- Hours: 13,359 × 24 = about 320,616 hours
- Minutes: about 19,236,960 minutes
A second method: day-of-year numbers
If counting months feels error-prone, use each date’s day of the year (January 1 = 1, December 31 = 365 or 366):
For the example, March 15, 1990 is day 74 (31 + 28 + 15) and October 11, 2026 is day 284. From January 1, 1990 to January 1, 2026 is 36 × 365 + 9 leap days = 13,149 days. So the age is 13,149 + 284 − 74 = 13,359 days, the same answer. This version is handy when you already know day-of-year numbers, for example from a planner or the date difference calculator. Remember that day-of-year numbers after February shift by one in a leap year.
Counting leap days quickly
Leap years under the Gregorian calendar are years divisible by 4, except century years not divisible by 400. So 1900 and 2100 are not leap years, but 2000 is. A shortcut for the count between two years:
Over any 36-year span you will usually find 9 leap days, but check the ends: if you were born in January or February of a leap year, that year’s February 29 counts; if your last birthday was before March 1 of a leap year, that year’s does not count yet. When in doubt, list them as in Step 2, or check with the leap year calculator. For why the rule works this way, see how leap years work.
Days per year of age
| Age (on the birthday) | Days, with typical leap days |
|---|---|
| 1 | 365 or 366 |
| 10 | 3,652 or 3,653 |
| 18 | 6,574 or 6,575 |
| 21 | 7,670 or 7,671 |
| 30 | 10,957 or 10,958 |
| 40 | 14,610 |
| 50 | 18,262 or 18,263 |
| 65 | 23,741 or 23,742 |
The one-day range reflects whether your span contains one more or one fewer leap day; any 40-year span between 1901 and 2099 contains exactly 10. On average, a year lasts 365.2425 days in the Gregorian calendar.
Milestone days
Day counts make fun milestones. For someone born March 15, 1990:
| Milestone | Date reached | Age at the time |
|---|---|---|
| 10,000 days | July 31, 2017 | 27 years, 4 months |
| 15,000 days | April 9, 2031 | 41 years |
| 20,000 days | December 16, 2044 | 54 years, 9 months |
| 25,000 days | August 25, 2058 | 68 years, 5 months |
| 1 billion seconds (about 11,574 days) | November 21, 2021 | 31 years, 8 months |
To find your own, add the number of days to your birth date with the date calculator, or see how many days remain until a milestone with the days until calculator.
Doing it in a spreadsheet
With the birth date in cell A1:
| Result | Formula |
|---|---|
| Age in days | =TODAY()-A1 |
| Age in days (alternative) | =DATEDIF(A1,TODAY(),"d") |
| Age in weeks | =INT((TODAY()-A1)/7) |
| Full years | =DATEDIF(A1,TODAY(),"y") |
| Days since last birthday | =DATEDIF(A1,TODAY(),"yd") |
| Date of 10,000th day | =A1+10000 |
Format the date results as dates and the counts as numbers. Spreadsheets store dates as serial day numbers, so subtraction gives elapsed days directly and handles leap years for you.
Common questions about the method
Is the birth date day 0 or day 1?
Elapsed-day counting, used by calculators and spreadsheets, treats the birth date as day 0. If you prefer to count the day of birth as your first day of life, add 1. The same choice comes up whenever you count days between dates; how to count days between dates explains inclusive versus exclusive counting.
What about time zones and the time of birth?
For whole days, they do not matter. For hours or minutes, start from your actual time of birth and the time zone you were born in. If you were born at 11 PM, your 24-hour mark falls late the next evening.
Born on February 29?
Your age in days is calculated the same way. Only the “birthday” in common years is ambiguous; legally, many jurisdictions treat March 1 as the date you turn a year older in a non-leap year, though some use February 28. The birthday calculator shows the weekday of past and future birthdays.
Frequently asked questions
How many days old am I if I am 30?
Somewhere between 10,957 and 11,322 days, depending on how many leap days you have lived through and how far you are past your 30th birthday. On your 30th birthday itself, it is 30 × 365 plus 7 or 8 leap days: 10,957 or 10,958 days.
When is my 10,000th day?
About 27 years and 4.5 months after you were born. Add 10,000 days to your birth date; for someone born March 15, 1990, day 10,000 was July 31, 2017.
Do I count the day I was born?
The usual convention counts elapsed days, so the day you were born is day 0 and you are 1 day old the following day. If you want to count the birth date as day 1, add one to the result.
How do I calculate age in days in Excel or Google Sheets?
Put the birth date in A1 and use =TODAY()-A1, formatted as a number. =DATEDIF(A1,TODAY(),"d") gives the same result. Both count elapsed days and handle leap years automatically.
How many hours old am I?
Multiply your age in days by 24 for an approximate figure, then adjust for the time of day you were born and the current time if you want precision. A 13,359-day-old person is about 320,616 hours old.