📅 Age Calculator
Calculate exact age in years, months, days, weeks, and hours from your date of birth.
Calculating exact age, and the leap-year problems nobody warns you about
Age sounds like simple subtraction and is not, because months have different lengths and some years have an extra day. Subtracting two dates in a spreadsheet and dividing by 365 gives an answer that drifts by a day every four years, which matters for anything legal, medical or actuarial.
The correct method
Work from largest unit to smallest, borrowing where needed:
- Subtract the birth year from the current year.
- If the current month is earlier than the birth month, subtract one year and add twelve months.
- If the current day is earlier than the birth day, subtract one month and add the number of days in the preceding month.
Born 18 September 1991, today 25 October 2026: years 2026 − 1991 = 35, months 10 − 9 = 1, days 25 − 18 = 7. So 35 years, 1 month and 7 days. Had today been 10 October, the day subtraction would go negative, so you borrow 30 days from September: 35 years, 0 months and 22 days.
Leap years
A year is a leap year if it is divisible by 4, except years divisible by 100, unless they are also divisible by 400. So 2024 was a leap year, 1900 was not, and 2000 was. The rule exists because the Earth's orbit takes about 365.2422 days, and the three-part correction keeps the calendar accurate to roughly one day in 3,030 years.
| Year | Divisible by 4 | Divisible by 100 | Divisible by 400 | Leap year? |
|---|---|---|---|---|
| 1900 | yes | yes | no | No |
| 2000 | yes | yes | yes | Yes |
| 2024 | yes | no | — | Yes |
| 2100 | yes | yes | no | No |
People born on 29 February have no birthday in common years. Most jurisdictions treat 1 March as the legal anniversary, though some use 28 February. Over a lifetime of 80 years a leapling has about 20 actual birthdays.
Why 365.25 is not good enough
Dividing total days by 365.25 is a common shortcut and it is wrong in a way that compounds. Someone who has lived 12,784 days is 35.0 years old by that method, but their exact age depends on how many leap days fell inside that span, which depends on when they were born. For age verification, insurance or clinical dosing, count calendar dates rather than dividing days.
Other ways to express age
The same span looks different depending on the unit. A 35-year-old has been alive for roughly 12,784 days, 1,826 weeks, 306,816 hours and 18.4 million minutes. Their heart has beaten something like 1.3 billion times. None of these are more correct than the others; they just change what the number feels like.
Age in different systems
Western reckoning starts at zero and increments on each birthday. The traditional East Asian system counted a newborn as one year old and added a year at lunar new year rather than on the birthday, which could make a baby born in December two years old within weeks. South Korea formally moved to the international system in June 2023, having previously used three parallel systems in different contexts.
A worked example
Born 29 February 2004, calculating on 25 October 2026. Years: 22, since the February anniversary has passed. Months from 28 February to 25 October: 7 months and 25 days, with the day borrow handled from September. Exact age: 22 years, 7 months and 27 days, or 8,275 days. Of those years, six contained a 29 February: 2004, 2008, 2012, 2016, 2020 and 2024.
Frequently Asked Questions (FAQs)
How do I calculate age in Excel?
Use DATEDIF(birthdate, TODAY(), "Y") for whole years, and swap the last argument for "YM" and "MD" to get the remaining months and days. Subtracting dates and dividing by 365 or 365.25 drifts by a day or more and should be avoided for anything that matters.
When do people born on 29 February celebrate?
Most jurisdictions treat 1 March as the legal anniversary in common years, though a few use 28 February. Socially, people split roughly evenly between the two. The legal date matters for things like driving licences and age-restricted purchases.
How many days old am I?
Count the days between your birth date and today, including every leap day in between. At 35 years it is about 12,784 days, at 50 about 18,262, and at 80 about 29,220. The calculator counts calendar dates, so the leap days are handled automatically.
What is the difference between age and age next birthday?
Age is completed years. Age next birthday is that plus one, and it is used in some insurance pricing because it loads the premium toward the age you are approaching. Actuarial tables sometimes also use age nearest birthday, which rounds to whichever is closer.
Why does my age in months not match years times twelve?
Because months have different lengths, so a month boundary does not always fall where a twelfth of a year would. Twelve months after 31 January is 31 January, but counting 365 days from the same date in a leap year lands on 30 January.
How is gestational age different from age?
Gestational age is measured in weeks from the first day of the last menstrual period and is used only during pregnancy. Age after birth is counted from the date of birth. For premature babies, clinicians often also track corrected age, which subtracts the weeks of prematurity.