Age Calculator

How age is calculated

Age is a calendar difference, not a number of days divided by 365. The calculator subtracts the birth year, month and day from the target date's year, month and day. If the day of the month has not yet been reached, it borrows a month, adding the days left in the month before the target date (none, if the birth day is past that month's end); if the month has not yet been reached, it borrows a year. The result is the familiar "years, months and days" that a person would count on a calendar.

years = Y₂ − Y₁, months = M₂ − M₁, days = D₂ − D₁
if days < 0: months − 1, days = max(0, length of previous month − D₁) + D₂
if months < 0: years − 1, months + 12

Month-end start dates. When the starting day number does not exist in the borrowed month, the whole of that month counts and the remaining days are those of the target month. So from 31 January to 1 March the result is 1 month and 1 day: 31 January to the end of February is one full month (February has no 31st), and one more day reaches 1 March. The same rule gives 1 month and 1 day from 31 March to 1 May.

Total days lived is computed separately and exactly, by converting both dates to serial day numbers and subtracting, so leap days are always counted. Total weeks is that number divided by seven and rounded down, and total months is twelve times the years plus the months.

Worked examples

Example 1: an everyday birthday

Born 1990-08-15, age on 2026-10-20. Years: 2026 − 1990 = 36. Months: 10 − 8 = 2. Days: 20 − 15 = 5. Age: 36 years, 2 months and 5 days, which is 13,215 days in total.

Example 2: borrowing a month

Born 2015-01-31, age on 2026-03-01. The day 1 is earlier than 31, so a month is borrowed: February 2026, which has only 28 days. A start day beyond the end of the borrowed month counts from that month's last day, so the borrowed month adds nothing and the 1 day of March remains, giving 11 years, 1 month and 1 day. Counting on a calendar agrees: 31 January to 28 February is one month (the last day of February), and one more day reaches 1 March.

Example 3: a leap-day birthday

Born 2004-02-29. In common years the anniversary is 1 March, and months and days are counted from the most recent anniversary. On 2027-02-28 the last anniversary was 1 March 2026, so the calculator gives 22 years, 11 months and 27 days; the 23rd birthday arrives the next day, 1 March 2027, when the result is exactly 23 years, 0 months and 0 days. On 2028-02-29 the person is exactly 24, and the result wishes them a happy birthday. Likewise, someone born 2000-02-29 is exactly 1 year, 0 months and 0 days old on 2001-03-01.

How to read the result

The headline gives completed years — the age you would write on a form. The line below adds the months and days since the last birthday. The bar shows how far through the current year of age you are, from the last birthday to the next. The table lists total months, weeks and days lived, the weekday of birth and the date of the next birthday with a countdown in days and the age about to be reached.

Leap-day birthdays

People born on 29 February have a real birthday only once every four years (or eight across a century year such as 2100). For the years in between, this calculator treats 1 March as the birthday: on 28 February the person is still one day short, on 1 March they reach the new age exactly, and the months and days that follow are counted from that 1 March anniversary. That convention is used by many legal systems for reaching an age of majority, but not all. Some jurisdictions and organisations treat 28 February as the birthday in common years. If an exact legal age matters — for a driving licence, retirement benefit or alcohol purchase — check the rule that applies where you are.

Common uses

Parents check school-entry eligibility, where cut-off dates such as "must be five by 1 September" make a single day decisive. Sports leagues group players by age on a fixed date. Employers and pension schemes verify eligibility by age on a given date. Genealogists compute ages at marriage or death from records. Doctors and nurses record infant ages in weeks and months because development milestones are tracked that way. And plenty of people simply want to know how many days old they are — the total-days figure answers that, and milestones such as 10,000 days (about 27 years and 4 months) are fun to plan for.

Limits and assumptions

The calculator uses dates only, not birth times or time zones, so someone born late in the evening in one country is treated as born on the date printed on the birth certificate. It applies the Gregorian calendar to all dates between years 1 and 9999. Some cultures count age differently — for example, the traditional East Asian system in which a person is one at birth and gains a year at the new year — and those systems are not modelled here. Ages before birth are not defined, so a target date earlier than the birth date produces an error message. For durations between any two events that are not birthdays, the days between dates calculator gives the same exact count with weekday detail, and the add days calculator finds the date a given number of days or years after a starting point.