How it works
Your age is counted the same way you count birthdays. First the calculator finds your last birthday on or before the date you picked, which gives your age in whole years. Then it counts the whole months since that birthday, and finally the days left over.
It also counts the same span in months, whole weeks, and days, and works out when your next birthday is and how many days away it is. Only calendar dates are used, so time zones and daylight saving time never change the answer.
The formula
Starting from the date of birth:
- Years: the most whole years you can add to the date of birth without going past the date.
- Months: the most whole months you can add after that, again without going past the date.
- Days: the days from there to the date.
A few details:
- Adding a month keeps the day of the month (March 15 plus one month is April 15). When that day doesn’t exist, the last day of the month is used instead, so January 31 plus one month is February 28, or February 29 in a leap year.
- Birthdays on February 29 are counted on February 28 in years that don’t have a February 29.
- Total days counts from the date of birth to the date, so a baby is 0 days old on the day they’re born and 1 day old the next day.
- Whole weeks are the total days divided by 7, rounded down.
Worked example
Want this with your own numbers? See Show the work above.
Someone born on April 5, 1978. How old were they on September 17, 2003?
- Years: their last birthday on or before September 17, 2003 was April 5, 2003. From April 5, 1978 to April 5, 2003 is 25 years.
- Months: from April 5, 2003, whole months go to May 5, June 5, July 5, August 5, and September 5. That’s 5 months. One more would be October 5, which is past the date.
- Days: from September 5 to September 17 is 12 days.
So they were 25 years, 5 months, 12 days old. That’s 305 months, 1,328 weeks, or 9,296 days. Their next birthday, their 26th, was April 5, 2004, 201 days later.
FAQ
How do I work out my age by hand?
Subtract your birth year from this year. If your birthday hasn’t happened yet this year, take away 1. That’s your age in years. For months and days, count forward from your last birthday: whole months first, then the days that are left.
How is age counted for someone born on February 29?
In leap years the birthday is February 29. In other years this calculator counts it on February 28, so someone born on February 29, 2000 turned 1 on February 28, 2001. Laws differ from place to place, and some count the birthday on March 1 instead, so check the rules where you live if it matters for something official.
Why does a month sometimes end on the 28th or 30th?
Months have different lengths. If you were born on January 31, there’s no February 31, so your first month is counted to the last day of February. That’s why an age can occasionally read something like “1 month, 30 days” instead of “2 months” near the end of a month.
Is the day I was born counted?
The day of birth is where the count starts, the same as a stopwatch starting at 0. On the day you’re born you’re 0 days old, and on your first birthday you’re 1 year (365 or 366 days) old.
Does the time or time zone of birth matter?
Not here. The calculator uses calendar dates only, so it gives the same answer wherever you are. Someone born late in the evening turns a day older at midnight like everyone else, even though a full 24 hours hasn’t passed yet.
Why does the total in days not match years times 365?
Leap years add an extra day about every 4 years. Someone born on April 5, 1978 lived through 6 February 29ths by their 25th birthday, so turning 25 took 9,131 days, not 25 × 365 = 9,125.
Sources
- Adding months and years, month ends, and February 29, with a worked age example: dateutil documentation, relativedelta examples
- Complete years and days between two dates: Microsoft Support, DATEDIF function
- Counting days between dates and to the next birthday: Python documentation, datetime module
Cite this page
APA (7th edition)
Mathlark. (2026, September 30). Age calculator. https://mathlark.com/everyday/age-calculator/
MLA (9th edition)
"Age Calculator." Mathlark, 30 Sept. 2026, mathlark.com/everyday/age-calculator/.