Age Calculator
Calculate age from date of birth
Fast, accurate calculations with clear results. Built for speed and ease of use.
Formula
Calculate difference between datesHow to Use Age Calculator
- Provide your input values.
- Understand the calculation method shown.
- Get immediate and accurate results.
- Use the output for planning or analysis.
Examples
An age calculator answers a question that looks trivial and is not quite: how many full years, months, and days have passed since a birth date, which matters for eligibility rules, legal age checks, and anywhere 'age' has to be an exact whole number rather than an estimate.
The formula
age = (current year - birth year) - 1 if (current month, day) < (birth month, day)- current year
- the year of the date you are calculating age on
- birth year
- the year of birth
- current month, day
- the month and day of the calculation date
- birth month, day
- the month and day of birth
Worked example
Someone born on 29 February 2000, a leap day, turns 26 during 2026, a year with no 29 February to land on. Most systems treat the birthday that year as falling on 28 February, so the person is considered 26 years old from 28 February 2026, while a system that instead waits for 1 March would count them a day later, and the two conventions disagree by exactly one day every non-leap year.
What trips people up
- Leap day birthdays are the sharpest edge case: with no 29 February in three years out of four, different systems pick 28 February or 1 March, and they do not agree.
- Age in months at the end of a month is ambiguous. Someone born on 31 January is not cleanly '1 month old' on 28 February, since February never reaches the 31st.
- Time zones can shift a birth date by a day for anyone calculating age close to midnight in a different zone from where they were born.
- 'Years since birth' and 'calendar years touched' are different numbers; someone born in December is in their second calendar year after only a few weeks alive.
Frequently asked questions
- Why might two age calculators give different answers for a leap day birthday?
- Because there is no single agreed rule for a birthday that lands on 29 February in a non-leap year; some tools treat it as 28 February, others as 1 March, and the choice shifts the reported age by a day.
- How is age in months calculated?
- By counting whole months from the birth date to the target date the same way years are counted, comparing the day-of-month components, which is why a birth date late in a month can produce an ambiguous month count against a shorter month.
- Does the calculator need to know the time zone of birth?
- Only for edge cases right around midnight; for the vast majority of birth dates the calendar date alone is enough, and time zone only matters if someone is calculating age within hours of a birthday.
- Why does my age in days not match age in years times 365?
- Leap years add an extra day roughly every four years, so the exact day count depends on how many 29 Februaries fall within the span, not on a flat 365 day year.
- Can this calculate age as of a date other than today?
- Yes, the same year, month, day comparison works for any target date, which is how eligibility checks calculate age as of a specific cutoff date rather than the current date.