Age Calculator
Find an exact age in years, months and days from any birth date, with the leap-year and month-length cases that make the arithmetic less obvious than it looks.
A leap-day birthday is one day in 1,506
The Gregorian calendar repeats every 400 years, and that cycle is exactly 97 leap years in 146,097 days — which makes the mean year 365.2425 days, and 29 February one day in 1506.2, or 0.066% of birthdays.
Someone born on 29 February 2024 sees 20 actual birthdays in eighty years, and the law has to decide what happens in the other sixty. Most jurisdictions that bother to specify put the anniversary on 28 February; plenty of people assume 1 March. Those are different days, so a leap-day child's legal age differs for twenty-four hours every non-leap year — which matters for exactly the thresholds where it is least convenient.
The gap is not always four years, either. Someone born in 2096 waits eight, because 2100 is divisible by 100 but not 400 and so is skipped. The same rule means 1900 was not a leap year and 2000 was — which is why the 400-year cycle comes to a whole number of weeks (20,871 of them) and the calendar repeats at all.
How to use
- Enter the birth date.
- Set the date to calculate to, or leave it as today.
- Read the age in years, months and days.
- Check the total days figure for precise comparisons.
Frequently asked questions
Why is calculating age in months awkward?
Because months have different lengths, so there is no single correct answer for what a month after 31 January means. Different systems land on 28 February, 2 March, or 3 March depending on the convention chosen, and none is wrong — they simply answer slightly different questions.
How is age calculated for someone born on 29 February?
Legally it varies by jurisdiction. Some treat the birthday as 28 February in non-leap years and some as 1 March, which occasionally matters for reaching a legal age. Socially, most leap-day birthdays are celebrated on whichever suits.
Does everyone count age the same way?
No. East Asian age reckoning traditionally counted a newborn as one year old and added a year at new year rather than on the birthday, which could make someone two years old a day after birth. South Korea used a version of this until legally standardising on international reckoning in 2023.
Why does the total days figure differ from years times 365?
Leap years. Across any long span roughly one year in four has an extra day, so multiplying years by 365 undercounts by about a day for every four years. The total days count handles this properly by working through the actual calendar.
How do I calculate the number of days between two dates?
By converting both to a day number and subtracting, which sidesteps month lengths and leap years entirely. Doing it by counting months and days directly is where most manual errors come from.
What are the leap year rules exactly?
Every year divisible by four is a leap year, except those divisible by 100, unless also divisible by 400. So 1900 was not a leap year and 2000 was — a case that broke a good deal of software written by people who only learned the first half of the rule.
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