How age is counted
The calculator uses the same method as official documents and everyday speech: it counts completed years first (from birthday to birthday), then the completed months since your last birthday, then the leftover days. You only "turn" a year older once you've fully reached that birthday — so someone one day short of turning 40 is still 39, not 40. This is different from simply dividing total days by 365, which drifts out of step because of leap years.
Worked example
Say you were born on 15 March 2000 and today is 27 July 2026. That's 26 complete years (you've passed your 2026 birthday), then 4 complete months (March to July), then 12 days (15th to 27th) — so 26 years, 4 months, 12 days. The calculator also totals the raw days lived and counts down to your next birthday, which in this case would be 15 March 2027.
Why months and days aren't always tidy
Months have different lengths, so the "days" portion is measured against the length of the relevant month. When the day-of-month you're calculating to is earlier than your birth day-of-month, the tool borrows a full month and adds that month's number of days — exactly how you'd work it out on paper. This keeps the result honest across February, 30-day months, and 31-day months alike.
Leap years and February 29 birthdays
Leap years (an extra day, 29 February, roughly every four years) are handled automatically in the day totals. If you were born on 29 February, most of the time you'll only have a "real" birthday every four years. By common convention — and in this calculator — your birthday is treated as falling on 1 March in non-leap years for the purpose of counting your age, so you still age by one year annually.
Frequently asked questions
Can I calculate age on a past or future date?
Yes. Leave the second field blank to use today, or set an "age on" date to find how old someone was (or will be) on any specific day — handy for eligibility cut-offs, anniversaries, or historical dates.
Is my birth date sent anywhere?
No. The whole calculation runs in your browser. Nothing you type is uploaded, saved, or tracked — close the tab and it's gone.
How many days old am I?
The result shows your exact total days lived (and the equivalent in weeks). It's a fun figure, and also useful for milestone birthdays like a 10,000-day celebration, which lands a little after your 27th birthday.
Related: the Unit Converter and Percentage Calculator for other everyday number work.