Working Out Exact Age From a Date of Birth

The most common way: completed years

In most everyday use around the world, your age is the number of birthdays you have already had. You are 25 until the day you turn 26, no matter how close you are to that day. This is often called counting in "completed years" or international age, and it is the method most calculators use.

To find someone's age on a given day, follow these steps.

Subtract the birth year from the year of the date you are interested in.
Check whether the birthday has happened yet in that year.
If the birthday has not yet arrived, subtract one.

Example: born on 10 May 2000, age on 3 March 2026. 2026 − 2000 = 26, but 10 May has not arrived by 3 March, so the age is 25. On 10 May 2026 the person turns 26.

Second example: born on 15 July 1990, age on 14 July 2026. 2026 − 1990 = 36, but the birthday is one day away, so the age is 35. One day later, on 15 July, the person turns 36. This "one day before" case is exactly where simple year subtraction goes wrong.

Be careful when comparing ages. Two people aged 30 and 28 are between one and three years apart, not exactly two, because their birthdays fall at different points in the year. Always compare birth dates to find the true gap.

Exact age in years, months and days

Often you want more than a whole number, for example for a form, a baby's progress, or a project's age. The idea is to count full years first, then full months after that, and finally the leftover days.

Take the same example, 10 May 2000 to 3 March 2026.

Full years: 10 May 2000 plus 25 years is 10 May 2025. One more year would pass 3 March 2026, so that is 25 years.
Full months: from 10 May 2025, nine months later is 10 February 2026. A tenth month would overshoot, so that is 9 months.
Days left: from 10 February to 3 March 2026 is 18 days to the end of February (which has 28 days in 2026) plus 3 more, so 21 days.

The exact age is 25 years, 9 months and 21 days. The age calculator does these steps automatically for any dates, and it also shows the total days lived.

The same method gives a baby's age in months. A baby born on 20 January 2026 is exactly 6 months old on 20 July 2026. One day earlier, on 19 July, the baby is 5 months and 29 days old, because 5 months from 20 January is 20 June and 29 days follow.

Total days lived

Counting days is simpler to verify. Someone born on 1 January 2000 has lived, on 1 January 2026, exactly 26 × 365 plus the number of leap days in between. The leap years in that range are 2000, 2004, 2008, 2012, 2016, 2020 and 2024: seven in all. So the total is 26 × 365 + 7 = 9,490 + 7 = 9,497 days.

For the same reason, you cannot get an exact age by multiplying years by 365, because years do not all contain the same number of days. Read more about how months and leap years affect counting in the date calculation guide.

Born on 29 February

A person born on 29 February has a "real" birthday only in leap years. Someone born on 29 February 2000 has birthdays on 29 February 2004, 2008, and so on. In the years in between, the date does not exist, so people and systems choose a convention: some treat 28 February as the birthday, others treat 1 March.

The difference only matters for one day a year. On 28 February 2026, a person born on 29 February 2000 is either already 26 or still 25, depending on which convention is used. Rules for official purposes differ by country, so if an exact legal birthday date matters, check the rules that apply to you. For everyday use, either choice is fine as long as you are consistent.

Different age traditions

Not every culture counts age the same way. In some East Asian traditions, a baby is considered one year old at birth and everyone adds a year together at New Year rather than on their own birthday. Under that system, a baby born in December can be "two years old" when only a few weeks old. The two systems can differ by one or two years for the same person.

Counting method	Starts at	Age goes up
Completed years (international)	0 at birth	On each birthday
Traditional New Year counting	1 at birth	At New Year for everyone

Which method an organisation uses depends on the country and the purpose, and rules can change over time. When filling in a form, read what it asks for. This site's calculator uses completed years.

Handy uses beyond birthdays

How old a pet, building or company is: use its start date in place of a birth date.
How long ago an event happened, in years, months and days.
Your age on a future date, such as the day of an exam or a trip.

FAQ

How many days until my next birthday?
Count from today to the next occurrence of your birth date, as described in the date guide. The age calculator shows this number too.

Why do two calculators sometimes give different months and days?
Month lengths vary. Counting from 31 January, "one month later" has no obvious answer in February, so tools choose slightly different rules for end-of-month dates.

Is a baby's age in months just days divided by 30?
No, that is only a rough guide. Months have 28 to 31 days, so count calendar months from the birth date instead.

Open the calculator: Age Calculator →