Age calculator — exact years, months and days
Enter a date of birth and get the exact age in years, months and days using real calendar arithmetic, not an average of 365.25 days a year. You also get the total days and weeks lived, the weekday you were born on, and how long until the next birthday. Leave the second date blank to use today, or fill it in to work out an age on any past or future date.
How to use the Age Calculator
- Type the date of birth as YYYY-MM-DD (for example 1990-05-15).
- Leave “Age on date” blank to use today, or enter another date to get the age on that day.
- Press Calculate age.
- Read the exact age, the totals, and the countdown to the next birthday.
How an exact age is worked out
Ages are counted the way a calendar counts them, not by dividing days by 365.25. The method is to add whole years to the date of birth for as long as the result stays on or before the target date, then whole months, then count the days left over. Adding a month to the 31st of a month lands on the last day of a shorter month, which is the behaviour almost everyone expects.
Worked example — born 1990-05-15, measured on 2026-03-10. Adding 35 years reaches 2025-05-15, which is still before the target; 36 years would overshoot, so the answer starts with 35 years. Adding 9 more months reaches 2026-02-15; a tenth month would overshoot. From 2026-02-15 to 2026-03-10 is 23 days. The answer is 35 years, 9 months and 23 days — 13,083 days in total.
Why the number of days in a month matters
Because months are 28, 29, 30 or 31 days long, "one month" is not a fixed quantity. Someone born on the 31st has no anniversary in a 30-day month, and someone born on 29 February has one only in leap years. This calculator borrows the length of the month immediately before the target date, which is the convention used for statutory ages in the UK and in most date libraries.
Leap-day birthdays
A person born on 29 February has a real birthday only once every four years. For the "next birthday" figure in non-leap years, this tool uses 1 March, which matches the rule in England and Wales for when a person attains a given age. Some other jurisdictions treat 28 February as the date, so if the distinction matters legally, check locally.
Totals and what they are good for
| Figure | How it is calculated |
|---|---|
| Total days | Difference between the two dates in whole days, counted in UTC so daylight saving never adds or removes an hour |
| Total weeks | Total days ÷ 7, with the remainder shown separately |
| Total months | Years × 12 + months from the exact age |
| Day of the week born | Taken from the calendar date itself |
Leap years, briefly
A year is a leap year if it divides by 4, except centuries, which must divide by 400. So 1900 was not a leap year but 2000 was, and 2100 will not be. Any calculator that assumes "every fourth year" drifts by a day across century boundaries; this one uses the proper rule.
Common uses: checking whether someone meets an age threshold on a specific date, working out a pet's age, filling in forms that ask for age in months, or settling an argument about which day of the week you were born on.
Frequently asked questions
How old am I in months?
Multiply the completed years by twelve and add the completed months. The calculator shows this total for you alongside the years-months-days breakdown.
Does it count leap years correctly?
Yes. It uses the full rule — divisible by 4, except centuries, which must be divisible by 400 — so 1900 is not treated as a leap year but 2000 is.
Can I work out my age on a date in the future?
Yes. Put the future date in the “Age on date” field and you will get the age you will be on that day, along with the days remaining until then.
What date format does it accept?
YYYY-MM-DD, for example 1990-05-15. This avoids the ambiguity between British day/month/year and American month/day/year ordering.
Privacy
This tool runs entirely inside your browser using WebAssembly and the Canvas/File APIs. Your files are never uploaded to ToolFlint or any third party; you can verify this in your browser's network tab or by switching to airplane mode after the page loads. Read how we process files.
Last updated 2026-09-23.