This free online age calculator tells you exactly how old you are today. Enter your date of birth and it returns your age in years, months, days, weeks, hours and seconds — calculated to the day, with leap years handled correctly. You can also set any past or future reference date to find what your age was, or will be, on that day.
What is chronological age?
Chronological age is the exact elapsed interval between an individual's date of birth and a reference date, expressed in years, months, and days using the Gregorian calendar. It serves as the baseline measurement in medical dosing, legal eligibility, retirement filings, school admissions, and demographic research. This age calculator applies ISO 8601:2019 interval arithmetic — the same standard used by clinical software, government agencies, and actuarial systems worldwide.
How does the age calculator work?
The age calculator subtracts your date of birth from today's date using exact Gregorian calendar arithmetic. It outputs the result in six time units simultaneously: years, months, days, weeks, hours, and seconds. Leap year correction prevents one-day errors on February 29 birthdays. Enter your date of birth in the form above to see all units computed instantly without page reload.
How do you calculate age from a date of birth?
The formula for chronological age in complete years equals the current year minus the birth year, reduced by 1 if the birthday has not yet occurred in the current year. For complete precision, the calculator then computes the remaining months and days after that whole-year count, borrowing from the previous month using its real length (28, 29, 30 or 31 days) rather than an average.
How do I calculate my age manually step-by-step?
- Write today's date and your date of birth as Year-Month-Day.
- Subtract birth year from current year.
- Check if your birthday has passed this year. If not, subtract 1.
- For months: subtract birth month from current month. If the result is negative, add 12 and subtract another year.
- For days: subtract birth day from current day. If negative, borrow days from the previous month using its exact length (28, 29, 30 or 31).
How does the age calculator handle leap years?
The age calculator handles February 29 birth dates using the international civil registration standard: in non-leap years, February 28 serves as the legal age-increment anniversary. This prevents "chronological drift" — the cumulative error that would otherwise occur every four years. The Gregorian leap year rule: a year is a leap year if divisible by 4, except century years (divisible by 100) which are not leap years unless also divisible by 400.
How is age counted differently around the world?
Three distinct age systems exist globally, and the age calculator on this page supports both the Western and traditional Korean methods. The Western system — used in most legal and medical contexts worldwide — starts age at 0 at birth and increments by 1 on each birthday anniversary. The Korean traditional system (Sae-neun-nai) starts age at 1 at birth, acknowledging the gestation period, and increments by 1 on every January 1 regardless of the individual birthday. South Korea officially retired this system for legal purposes in June 2023, but it remains culturally common.
When do you need an exact chronological age calculation?
- Medical dosing: Paediatric weight-based dosing and clinical drug trials require age in days for the first month and age in months thereafter.
- Retirement filings: Full retirement age, penalty-free withdrawal thresholds, and enrollment windows all require exact-to-the-day age.
- Legal documents: Wills, trusts, and beneficiary clauses tied to specific age thresholds require precise calculation rather than approximate years.
- School enrollment: Cutoff-date compliance — many regions use a fixed date that determines grade placement to the day.
- Insurance underwriting: Life insurance premium brackets shift on exact age anniversaries.
- Genealogical research: Lifespan calculation from historical records, often requiring calendar conversion for pre-1923 dates.
What is the difference between chronological age and biological age?
Chronological age measures elapsed time since birth using the calendar. Biological age measures cellular and physiological aging using markers like telomere length, DNA methylation, VO₂ max, cardiovascular fitness, and inflammation biomarkers. Chronological age is the only legally and clinically recognised measure for documentation purposes; biological age is increasingly used in longevity research but carries no legal standing.
For a separate estimate based on lifestyle markers, try the Biological Age Calculator.
How is age in days, hours, and seconds calculated?
Total days alive equals the number of calendar days from your date of birth to today, counted continuously through every leap year encountered. Multiply total days by 24 for total hours, by 1,440 for total minutes, by 86,400 for total seconds. The live counter above updates every second using the browser's animation-frame API.
Frequently Asked Questions
This age calculator uses ISO 8601:2019 interval arithmetic — the same standard applied in clinical software, government systems, and actuarial calculations worldwide. Results are exact to the day, with full leap year correction, including the century-year exception rule.
Different tools use different month-counting methods. This age calculator uses the Gregorian calendar method: complete months between dates of the same day number. Tools using a fixed 30-day month produce different results, particularly across February.
Use the "At Date" tab above. Enter your date of birth and the target date — past or future — to find your exact age as of that specific day. Useful for insurance application age verification, legal document age thresholds, and historical research.
The live counter shows the exact number of seconds elapsed since your date of birth, updated every second in real time. It accounts for leap days and is computed from a Unix timestamp baseline for second-level precision.
Yes. The age calculator outputs age in the format required by most government, medical, and legal documents: years, months, and days, using ISO 8601:2019 interval arithmetic with full leap year handling. For sensitive filings, verify the result against your own records or with a professional advisor before signing documents.
For people born on February 29 in a leap year, the age calculator uses February 28 as the legal age-increment date in non-leap years and February 29 in leap years. This follows the international civil registration standard and prevents the one-day annual drift that would otherwise occur.
Traditional Korean age (Sae-neun-nai) counts the gestation period as the first year, so newborns are age 1 at birth. Every person also gains a year on January 1, not on their birthday. South Korea officially retired this system for legal purposes in June 2023, but it remains culturally common.
Yes. Enter your date of birth and the calculator uses today's date automatically, so a date of birth is the only thing you have to supply. Change the reference-date field if you need your age as of some other day.
Yes. Every calculator on this site is free, runs entirely in your browser, and needs no sign-up. Your date of birth is never transmitted to a server or stored — the arithmetic happens on your own device.
Not directly on this page, but the arithmetic is straightforward: subtract the age from the reference date. Use the "DOB Finder" tab above for the fastest way to do this.
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