Calculate exact age from birth date.
Guide to Age Calculation
How does the age calculator work?
The age calculator calculates the exact time elapsed from the date of birth to the chosen target date. It calculates the difference in years, months, and days, accounting for the varying number of days in each month. Optionally, you can calculate age for any date in the past or future.
Age and health
Biological age may differ from chronological age. Healthy lifestyle, physical activity, proper diet, and avoiding habits can slow aging. Regular preventive check-ups allow early detection of age-related diseases such as hypertension, diabetes, and cancer.
Age-related milestones
- 18 years - legal adulthood, full citizen rights
- 21 years - legal drinking age in the USA
- 65 years - retirement age in many countries
- 70 years - higher retirement benefits in some systems
- 100 years - centenarian status, special honors
Fun facts about birthdays
The rarest birthday is February 29 - people born on this day celebrate birthdays only every 4 years. On average, each person celebrates 79 birthdays (with average life expectancy of 79 years). In Poland, the most popular birthdays are in fall and winter, while April is the least popular.
A 29 February birthday gets 49 real anniversaries in 200 years, not 50
Leap years are not simply every fourth year. Century years are skipped unless divisible by 400, so 1900 was not a leap year and 2100 will not be one, while 2000 was. Across 1900 to 2099 that leaves 49 leap days rather than 50 — and someone born on 29 February gets a genuine anniversary once every 4.08 years on average, not every 4.
How it works
- Calculates exact age in years, months and days between a birth date and any reference date.
- Applies the full leap-year rule, including the century exceptions that catch most naive implementations.
- Handles 29 February birth dates, where the convention has to be stated rather than assumed.
a year is a leap year if: divisible by 4 AND not divisible by 100 UNLESS also divisible by 400 1900 → not a leap year 2000 → leap year 2100 → not a leap year 400-year cycle = 146,097 days → average year = 365.2425 days
Worked example
Counting leap days across two centuries and what it means for one birth date.
- 1900 to 2099 contains 200 years
- naive every-fourth-year counting gives 50 leap days
- 1900 fails the century rule and 2100 fails it too
- 2000 passes, being divisible by 400
- the true count is 49
One leap day every 4.08 years rather than every 4. Anyone born on 29 February 1896 waited eight years for their first real birthday, because 1900 was skipped — the longest such gap the calendar produces.
Reading the result
- The 400-year rule exists because the solar year is 365.2425 days, not 365.25. Adding a day every four years overshoots slightly, so three leap days are removed every four centuries to bring the calendar back into line.
- Age in years is not the elapsed days divided by 365.25. Legal and everyday age counts completed anniversaries, so someone is 40 until the day before their forty-first birthday regardless of how the fractional years accumulate.
- For 29 February birthdays the legal convention varies by jurisdiction. Some treat 1 March as the anniversary in common years and some treat 28 February; it matters for coming of age, licences and contracts, and it is worth checking rather than assuming.
- Month arithmetic is genuinely ambiguous at the ends. One month after 31 January is 28 or 29 February in most implementations, which means adding a month and then subtracting one does not always return the original date.
Common questions
- Was 1900 a leap year?
- No. It is divisible by 4 and by 100 but not by 400, so the century exception applies. 2000 was a leap year because 400 divides it, which is why a rule that most people never see in action happened to be invisible to anyone born after it.
- When does someone born on 29 February have a birthday?
- Genuinely, once every four years — 49 times across two centuries. In common years the convention depends on jurisdiction and institution, with 28 February and 1 March both in use, so the practical answer is whichever your local rules specify.