How to use the age calculator
- Choose your date of birth. On a phone this opens the date wheel; on a computer you can type the date or use the calendar.
- Leave “Age at the date of” on today, or pick another date to find out how old someone was, or will be, on that day.
- Read your exact age in years, months and days, plus the same span in total months, weeks, days and hours, the weekday you were born and the countdown to your next birthday.
Birth dates are personal data, so it matters that this page never sends yours anywhere. The calculation runs on your device using your own clock.
How age is calculated
Age is counted in whole calendar units, the same way a birthday cake counts it. The tool subtracts the birth date from the second date year by year, month by month and day by day, and borrows when a part comes out negative:
years = end year − birth year months = end month − birth month days = end day − birth day if days < 0 → months − 1, and add the length of the month before the end date if months < 0 → years − 1, and add 12 months
The tool counts whole months forward from the start date (a start day that does not exist in a shorter month, such as the 31st, is moved to that month's last day), then counts the leftover days. The answer always lands back on the correct date when you add it up again.
Worked example
Born on 15 March 1990, measured on 10 June 2025:
- Years: 2025 − 1990 = 35
- Months: 6 − 3 = 3
- Days: 10 − 15 = −5, which is negative. Take one month away (3 becomes 2) and add the length of May, 31 days: −5 + 31 = 26
The age is 35 years, 2 months, 26 days. You can check it by adding back: 15 March 1990 + 35 years = 15 March 2025; + 2 months = 15 May 2025; + 26 days = 10 June 2025.
The same span is 12,871 days, which is 1,838 weeks and 5 days, or 308,904 hours. The next birthday falls on 15 March 2026, 278 days after the measurement date, and 15 March 1990 was a Thursday.
Leap-day birthdays and other edge cases
People born on 29 February have a real birthday only in leap years. In other years, countries choose either 28 February or 1 March. The UK and Hong Kong, for example, treat 1 March as the legal birthday in a non-leap year, while New Zealand uses 28 February. This tool counts the next birthday as 1 March in a non-leap year and says so by showing the date it used.
Total hours assume 24-hour days from midnight to midnight and ignore the 23- or 25-hour days that occur when clocks change, so it can differ from a stopwatch count by an hour or two over a lifetime. The exact time of birth is not used, only the date.
Common reasons to calculate an exact age
- School and sports cut-offs: many programmes ask for age on a fixed date, such as 1 September, rather than today.
- Forms and eligibility: passports, driving tests and pension schemes usually ask for age in completed years on a given day.
- Pets and milestones: a total of days or weeks is handy for a baby's age or for the date a puppy came home.
- Genealogy and history: set the second date to a past event to see how old someone was at the time.
Accuracy and limits
The Gregorian calendar is used for all dates, including those before it was adopted in 1582 (and before 1752 in Britain and its colonies), so very old historical ages can differ from sources that use the Julian calendar. Ages are shown in completed calendar units, which is the convention for legal and medical forms. Some East Asian systems count a newborn as one year old, which this tool does not.
Frequently asked questions
How is my age in months worked out?
It counts the number of completed months between the two dates. A month is complete only when the day of the month has been reached, so from 15 March to 10 June is 2 completed months, not 3.
Why does my age differ from another site by a day?
Some calculators count the end date as well, or use 30-day months. This tool uses real calendar months and counts from the birth date up to, but not including, the later date.
Can I find someone's age on a past or future date?
Yes. Change the “Age at the date of” field to any date after the birth date. This is useful for eligibility cut-offs and planning milestone birthdays.
Is my date of birth stored or sent anywhere?
No. The calculation runs in your browser, and the page does not save the dates. Refreshing the page clears them.
How are leap years handled?
Month lengths come from the real calendar, including 29 days in February in leap years. Leap-day birthdays count their next birthday on 1 March in non-leap years.