Age calculation from date of birth
Age is the amount of time that has passed since a person's birth. A calendar age expresses that time in completed years, additional months and remaining days. To calculate it, compare the date of birth with a reference date, which may be today, a past date or a future date.
An exact age calculation follows the calendar rather than assuming that every year has 365 days or every month has 30 days. This distinction matters near birthdays, at the end of a month and when February 29 falls within the date range.
How to calculate age in years, months and days
Start by counting completed birthdays between the birth date and the reference date. Subtract the birth year from the reference year, then subtract one more year if the birthday has not yet occurred in the reference year.
Next, count complete calendar months after the most recent birthday. The remaining days are the days between the end of those complete months and the reference date. The result describes three parts of the same elapsed period; the month and day values are additional to the completed years.
- Write down the date of birth and the date for which you need the age.
- Count completed years up to the most recent birthday.
- Count complete months after that birthday.
- Count the remaining calendar days.
For month-end dates, decide how to handle a month that does not contain the original day number. The calculation above uses the last valid day of that month. For example, one calendar month after January 31 is February 28 in a common year.
Age calculation example
Someone born on January 1, 2001 is 25 years, 9 months and 9 days old on October 10, 2026. They completed 25 years on January 1, 2026, another nine months on October 1, and another nine days on October 10.
A birthday later in the year changes the year count. Someone born on December 15, 2001 is still 24 years old on October 10, 2026, because their 25th birthday has not yet arrived.
How to calculate age on a specific date
Age on a specific date uses that date as the reference point. It answers questions such as how old someone was on their first day of school, how old they will be on a future birthday, or how many completed years they had reached on a historical date.
A reference date earlier than the birth date does not describe an elapsed age. When comparing ages for an application or a dated record, use the stated reference date rather than today's date. A person's completed age increases on their birthday, so dates just before and just after a birthday can produce different year counts.
How leap years and month-end birthdays affect age
A Gregorian leap year has 366 days, including February 29. Years divisible by four are leap years, except century years must also be divisible by 400. For example, 2000 was a leap year, while 1900 was not.
For a February 29 birth date, a common year has no matching calendar date. The calculation above treats February 28 as the anniversary in a common year. Under this convention, someone born on February 29, 2000 reaches 25 years on February 28, 2025. Using March 1 as the anniversary would give a different answer at that boundary.
Month lengths also affect the remaining days. A birth date on the 29th, 30th or 31st may not exist in every later month. Counting complete months with a defined month-end convention avoids negative day values and makes the breakdown consistent.
Age in total months, weeks, days, hours and minutes
Calendar age and total elapsed days describe the same interval in different ways. Total days count the actual calendar days between the birth date and the reference date, including leap days. Dividing that number by seven gives complete weeks and any remaining days.
Complete calendar months are the completed years multiplied by 12, plus the additional complete months. They are not obtained by dividing elapsed days by 30 because calendar months have different lengths.
- Complete months: completed years × 12 + additional complete months.
- Complete weeks: total elapsed days divided by 7, keeping the whole-number part.
- Total hours: total elapsed days × 24.
- Total minutes: total elapsed days × 1,440.
- Total seconds: total elapsed days × 86,400.
The hour, minute and second totals here are date-based conversions measured at midnight on the selected reference date. An age measured from an exact birth time would also require the time of birth and the relevant time zone.
How to calculate the days until your next birthday
Find the birthday anniversary in the reference year. If it is on or after the reference date, use that anniversary. If it has already passed, use the anniversary in the following year. Count the calendar days between the reference date and that birthday.
For example, a December 10 birthday is nine days away on December 1. On December 10, the countdown is zero. On December 11, the next birthday falls in the following year. The age reached on that birthday is its year minus the birth year.
How to find the day you were born
The birth weekday is the day of the week attached to the full birth date. It depends on the year as well as the month and day, so the same month and day can fall on different weekdays in different years. For example, January 1, 2001 was a Monday.
The weekday of your next birthday is calculated from the upcoming anniversary date. Leap years shift the calendar, which is why birthdays do not always advance by exactly one weekday from one year to the next.
Frequently asked questions about age calculation
How do I calculate my age from my date of birth?
Compare your birth date with the reference date. Count completed birthdays first, then complete months after your most recent birthday, followed by the remaining days.
Why is subtracting the birth year sometimes incorrect?
Subtracting years alone ignores whether the birthday has occurred. If it has not yet happened in the reference year, subtract one from the year difference.
Does a leap year change my age in years?
Completed age in years follows birthdays. Leap days change the total elapsed days and may affect the month-and-day breakdown. February 29 birthdays also need an anniversary convention for common years.
Is dividing total days by 365 an exact age calculation?
No. It gives an approximation because some years have 366 days. Calendar years, months and days should be counted using the actual dates.
Can age in months differ from total days divided by 30?
Yes. Complete calendar months follow month boundaries, while dividing by 30 assumes every month has the same length. February and months with 31 days make those answers differ.
Does age include the day of birth?
Elapsed age is zero on the birth date. One full calendar day has elapsed on the following date. Counting both endpoints would add one day and describes an inclusive date count rather than elapsed age.