The formula
Days alive equals today minus your birth date, exclusive. That is the whole formula. If you were born on January 1, 2000 and today is October 4, 2026, you have been alive 9,773 days. Every leap day between those dates is already included in the subtraction, because the dates themselves carry that information. You do not need to add leap days separately; the formula handles them.
The hand method, for when you want to see the parts: full years times 365, plus the number of leap days lived through, plus the days since your last birthday. For January 1, 2000 to October 4, 2026: 23 full years to January 1, 2023 is 23 times 365 = 8,395, plus 6 leap days (2000, 2004, 2008, 2012, 2016, 2020) = 8,401. Then 2023 (365), 2024 (366, a leap year), 2025 (365), and January 1 to October 4, 2026 (276 days). 8,401 + 365 + 366 + 365 + 276 = 9,773. Both methods agree.
Leap days, counted properly
A leap day counts if you were alive on it. Someone born on March 1, 2000 lived through the February 29, 2000 leap day only if they were born before it, which they were not, so it does not count. Someone born on February 28, 2000 did live through February 29, 2000, so it counts. The rule "one leap day per four years" is a fine estimate, but the exact count depends on where your birthdays fall relative to the leap days. Date subtraction gets this right automatically, which is why it beats the estimate.
Milestone days worth knowing
Round numbers of days make good milestones. Your 10,000th day of life arrives about 27 years and 4 to 5 months after birth, depending on leap days. For example, someone born on May 19, 1999 reaches day 10,000 on October 4, 2026. The 1,000th day lands around age 2 years 9 months, the 5,000th around age 13 years 8 months, the 20,000th around age 54 years 9 months, and the 30,000th around age 82. To find your next milestone, subtract your days alive from the next round number; that is how many days away it is.
Days into hours, minutes, and weeks
Multiply days by 24 for hours and by 1,440 for minutes. 9,773 days is 234,552 hours, or about 14.07 million minutes. Divide by 7 for weeks: 9,773 days is 1,396 weeks and 1 day. These conversions are exact multiplications, but remember the day count itself is exclusive of today, so the hours are "hours lived through so far," not including the current partial day.
Why days are more precise than years
A year is not a fixed unit: it is 365 or 366 days. Two people both "age 30" can differ by up to 364 days of actual lived time, plus leap-day differences. Days are the fixed unit underneath. That precision is why insurance, finance, and legal age thresholds are all computed in exact days, not rounded years. "Age 18" for a contract means 18 full years have elapsed to the day, which is a day-count question.
Doing it in a spreadsheet
With your birth date in A1 and today in B1 (or =TODAY()), =B1-A1 gives days alive, exclusive. =DATEDIF(A1,B1,"d") gives the same. =DATEDIF(A1,B1,"y") gives full years, and =DATEDIF(A1,B1,"ym") gives the leftover months, so =DATEDIF(A1,B1,"y")&" years, "&DATEDIF(A1,B1,"ym")&" months, "&DATEDIF(A1,B1,"md")&" days" renders a full age breakdown. Google Sheets supports all of these.
Day counts by decade of life
It helps to anchor the big numbers. Using the exclusive convention and ignoring leap days for the rough figures: age 1 is 365 days, age 5 is 1,825, age 10 is 3,650, age 18 is 6,570, age 21 is 7,665, age 30 is 10,950, age 40 is 14,600, age 50 is 18,250, age 65 is 23,725, and age 100 is 36,500. Each leap day lived through adds one. A person born January 1, 2000 turns 30 on January 1, 2030 having lived 10,950 + 8 leap days (2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028) = 10,958 days. The decade table is a quick sanity check against any calculator result: if the tool's number is far from the table figure, something is off.
A note on the birthday edge cases
If you were born on February 29, your "birthday" in non-leap years is a convention, not a date: most people pick February 28 or March 1. For day counts it does not matter, because the subtraction uses your actual birth date, which exists. Only the anniversary label is ambiguous. And if you are counting days alive "including today," add 1 to the exclusive result; just say which convention you used.