How age is calculated
Your age in years is the number of birthdays you have had. After that come the full months since your last birthday, and then the remaining days. It sounds simple, but months have different lengths and leap years add an extra day, so mental arithmetic is often a day or even a month off.
The calculator works in three steps:
- Years: the number of complete years between the date of birth and the chosen date.
- Months: the number of complete months since the last birthday.
- Days: the days left over after that.
age in days = chosen date − date of birth (calendar days)
Example: someone born on 15 May 1990 is 36 years, 4 months and 21 days old on 6 October 2026. That is 436 months, 1,899 weeks or 13,293 days. They were born on a Tuesday, and their next birthday is 221 days away, on 15 May 2027.
Born on 29 February
If you were born on a leap day, your birthday only exists every four years. In other years the calculator treats 28 February as your birthday, which is how most calendars and software handle it. Someone born on 29 February 2000 turns exactly 27 on 28 February 2027. Some countries legally use 1 March instead, so check local rules for official purposes.
Age on a specific date
You often need someone’s age on a particular date:
- School entry and sports: age is often measured on a cut-off date, such as 1 September for school years in England or 31 December for youth sports.
- Legal ages: driving, voting and drinking ages differ by country and state; you reach a new age on your birthday.
- Retirement: the US full retirement age for Social Security is 67 for people born in 1960 or later; in the UK the State Pension age is rising from 66 to 67 between 2026 and 2028.
- Insurance and pensions: premiums and benefits often depend on age at a specific date.
Fill in that date as the second date and the calculator shows the age on that day.
Fun facts about your age
- You are 10,000 days old at around 27 years and 4 months. Use the days between dates calculator to find the exact date.
- One billion seconds is roughly 31 years and 8 months.
- Want to know which week of the year your birthday falls in? Check the week number calculator.
Age in Excel and Google Sheets
Use =DATEDIF(A1,TODAY(),"Y") for full years, "YM" for the remaining months and "MD" for the remaining days. Be aware that "MD" can return wrong results around month ends in some Excel versions; this calculator handles those cases correctly.
Exact age versus age in years
In daily life we round age down to whole years: you are 35 until the day you turn 36. For some purposes you need more precision. Paediatricians use months and weeks for babies, schools use cut-off dates to decide in which year a child starts, and pension and benefit rules often depend on your age on a specific date. Insurance companies sometimes use your "nearest" age, rounding to the closest birthday instead of down. This calculator always shows the exact age in years, months and days, plus totals in months, weeks and days, so you can use whichever figure the rule requires. Read more on our blog: how many days old am I? and age by birth year.
Frequently asked questions
How old am I in days?
Enter your date of birth and leave the second date empty. The calculator shows your age in days up to today. Someone born on 15 May 1990 is 13,293 days old on 6 October 2026.
Is the day of birth counted?
The calculator counts the difference between two dates, which is how age normally works. On the day you are born you are 0 days old, and on your first birthday exactly 1 year.
How do I calculate someone’s age on a past date?
Enter the date of birth and set the second date to the past date. It also works for historical figures.
How many days until my birthday?
See “Next birthday” in the result. If it is your birthday today, the calculator says so.
What day of the week was I born?
The result shows it under “Born on a”.
Is my date of birth stored?
No. Everything is calculated in your own browser and nothing is sent or saved.
Last reviewed: 2026-10-06. Results are estimates for information only.