The method, honestly
You can do this with a calendar and patience. Count full years first, then the leftover days. The only part that needs care is leap days: a year has a 29 February if it divides by 4, except century years, which need to divide by 400.
2000 was a leap year. 1900 was not. 2100 will not be. People get this wrong all the time, and it is the most common reason two age calculations disagree by a day.
Worked example: 1 January 2000 to 1 January 2025
Twenty-five years is 25 × 365 = 9,125 ordinary days. Then add the leap days in that span: 29 February in 2000, 2004, 2008, 2012, 2016, 2020, and 2024. That is 7 leap days. Total: 9,132 days.
I checked this against our Age Calculator. Same answer. If your hand count lands one day off, you almost certainly missed a leap day or counted both endpoints. Count the days between the dates, not the dates themselves.
A second example: 15 June 1990 to 1 January 2025
Thirty-four full years, from 15 June 1990 to 15 June 2024, come to 12,419 days. That is 34 × 365 = 12,410 ordinary days plus 9 leap days: 29 February in 1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020, and 2024.
Then the leftover stretch, 15 June 2024 to 1 January 2025, is 200 days. Add them: 12,419 + 200 = 12,619 days. The 200 days at the end are where most people slip. Count them month by month instead of guessing. June gives 15, July 31, August 31, September 30, October 31, November 30, December 31, and the step into 1 January adds the last one.
Why dividing by 365.25 fails
The shortcut everyone tries is 9,132 ÷ 365.25. It gives 25.0021, not 25. Close, but wrong. The average smears leap days across years that never had them, so it cannot recover exact calendar dates, and it drifts near birthdays.
Averages are fine for trivia. They are useless for exact dates. If the number has to be right, count real days on a real calendar.
The 29 February problem
Born on 29 February? In non-leap years there is no birthday to count to. Our calculator rolls the date to 1 March and says so openly. There is no universal law here. Some families celebrate on 28 February. Either choice is a convention, and the honest move is to state which one you used.
A milestone worth knowing: 10,000 days
People love round numbers, and 10,000 days is a good one. If you were born on 1 January 2000, your 10,000th day lands on 19 May 2027. If you were born on 15 June 1990, it already passed, on 31 October 2017. Milestones like this are the one place where the exact count feels fun instead of bureaucratic. The Age Calculator will tell you how close you are.
When days actually matter
Most of the time, years and months are enough. Days matter when a form asks for them, when you are tracking a baby's age week by week, or when you are settling an age-difference argument properly. For all of those, run the dates through the Age Calculator instead of trusting a rough division.
There is one more everyday use: streaks. People tracking a habit, a fitness run, or days since an event usually want the exact count, not an average. A streak of 100 days means 100 real days. The same counting discipline applies: real calendar, real leap days, no shortcuts. And if your ages live in a spreadsheet rather than on paper, Calculate Age in Excel shows the three formulas that do this counting for you.