Calculating someone’s age sounds trivial — subtract the birth year from the current year and you’re done. But that shortcut is often wrong by a year, and it ignores months and days entirely. This guide explains the method properly, shows where people slip up, and gives you a reliable way to do it by hand or instantly online.
The basic idea
Age is the amount of completed time between a date of birth and a later reference date — usually today. “Completed” is the key word: you only count a year once the birthday has actually passed. That’s why the year-subtraction trick fails. Someone born in December 2000 is not 26 in January 2026; they’re still 25 until their December birthday.
Step by step, by hand
Write the two dates as day, month and year, then subtract each part, borrowing when a value goes negative — exactly like long subtraction.
- Subtract the days. If the result is negative, borrow days from the previous month (using that month’s actual length) and reduce the months by one.
- Subtract the months. If that’s now negative, borrow 12 months from the years and reduce the years by one.
- Subtract the years. What remains is the age in whole years, with the leftover months and days from the steps above.
Days: 1 − 15 = −4 → borrow May’s 31 days → 27 days, months drop by one.
Months: (6 − 1) − 3 = 2 months.
Years: 2026 − 2000 = 26.
Result: 26 years, 2 months, 27 days.
The two things people get wrong
1. Month lengths
When you borrow days, you must use the length of the previous month, not a flat 30. Borrowing into March means using February’s 28 (or 29) days; borrowing into June means using May’s 31. Getting this wrong throws the day count off.
2. Leap years
Every four years February gains a 29th day (with a fine-print exception for century years not divisible by 400). Over a long span those extra days add up. If you ever convert age into a total number of days, you cannot just multiply years by 365 — you’ll be several days short. Counting the real calendar dates is the only exact method.
The fast way
For an instant, error-free result, let a tool do the borrowing for you. Our age calculator takes a date of birth and a reference date and returns the exact years, months and days, along with totals in weeks, days, hours and seconds — all calculated from the real calendar, so leap years and month lengths are handled automatically.
When the reference date isn’t today
The same method works for any reference date. To find out how old someone was at a past event, use that event’s date as the reference. To find a future age — say, how old you’ll be at a milestone birthday or a planned event — use the future date instead. The calculator lets you change the second date for exactly this reason.