Age Calculator
Exact age in years, months, days, hours, and minutes — plus countdown to next birthday.
Result
How to use this calculator
- Pick the year, month, and day you were born.
- Result updates to the current moment — refresh the page for an updated minute count.
- Day-of-week-born is calculated by the calendar, not approximated.
- To answer "how old was someone on a past or future date" (e.g. "how old was I on 9/11", "how old will my child be in 2030"), switch "Compute age as of" to "Pick a specific date" and set the as-of date.
- Future dates are not allowed — the calculator returns "—" if you enter one.
About this tool
Most "age calculators" stop at "you're 34 years old." The actual decomposition into years + months + days matters for forms (visa applications, insurance), milestones (a baby's first 18 months are usually tracked in days then weeks), and for the small joy of knowing exactly how many minutes you've been alive. The math has one trick: months don't have a fixed number of days. To handle this correctly we walk the calendar month by month rather than dividing by 30.44. The day-of-week-born is calculated using Zeller's congruence, the same formula used since the 1880s.
What this calculator does
This calculator returns your exact age in years, months, and days — plus the total counts in years, months, weeks, days, hours, and minutes lived since birth. It also names the day of the week you were born (using Zeller's congruence) and counts down to your next birthday, with the legally-conventional Feb-28 substitution applied for Feb-29 babies in non-leap years. The calendar math is calendar-aware: it borrows days from the previous month and months from the previous year, rather than dividing by an averaged 30.44 days.
How it works — the formula
Calendar age = years between birth date and today,
adjusting for whether this year's birthday has passed.
Total days = (today − birth_date) / 86,400,000 ms (leap years included automatically).Calendar-age arithmetic borrows days from the previous month and months from the previous year exactly as a human would on paper, so the result reads "years/months/days" rather than a decimal. The proleptic Gregorian calendar — applying Gregorian rules backward indefinitely — is the modern ISO 8601 default and is what most programming languages' Date class implements. Leap years follow the rule "divisible by 4, except century years not divisible by 400" (1900 was not a leap year; 2000 was).
Worked examples
- Inputs:
- birth = 1990-06-15, today = 2026-05-08
- Output:
- 35 years, 10 months, 23 days; 13,111 days; 314,664 hours
Birthday has not yet occurred this year — the borrow-from-previous-month step subtracts one month and pulls in April's 30 days to produce the 23-day remainder.
- Inputs:
- birth = 2000-02-29, today = 2026-03-01
- Output:
- 26 years, 0 months, 1 day (Feb 29 → Feb 28 in non-leap years per legal convention)
Most US states and the UK (Births and Deaths Registration Act 1953) treat the Feb-29 birthday as occurring on Feb 28 in non-leap years. The calculator mirrors that for the next-birthday countdown.
- Inputs:
- birth = 1985-07-20
- Output:
- Saturday (Zeller-formula or built-in Date.getDay() agree across implementations)
Zeller's congruence (1882) is a closed-form day-of-week formula. It matches JavaScript Date.getUTCDay() for all dates after the proleptic Gregorian start.
- Inputs:
- birth = 1955-02-24 (Steve Jobs), as-of = 2011-10-05 (the day he died)
- Output:
- 56 years, 7 months, 11 days
Set "Compute age as of" to "Pick a specific date" and enter 2011/10/5 to get the age on any past or future date.
When to use this vs other tools
Age Calculator is the everyday tool. For pregnancy-specific or sleep-window calculations, a more targeted tool is faster.
- Due Date Calculator
Use for pregnancy — Due Date Calculator applies Naegele's rule (LMP + 280 days) and returns the trimester schedule, which is unrelated to post-natal age math.
- Sleep Calculator
Use for sleep planning — Sleep Calculator schedules 90-minute REM cycles around a target bedtime or wake-time, a separate calendar-math problem from age accumulation.
- BMI Calculator
Use to interpret what your age implies for healthy weight — adult BMI bands (18.5–24.9) hold across ages, but the National Institute on Aging notes higher BMI may be appropriate over age 65.
- Calorie Calculator (TDEE)
Use because BMR and TDEE drop ~1–2% per decade after 30. Plug your exact age into TDEE rather than rounding.
Authority note
ISO 8601 defines the proleptic Gregorian calendar (Gregorian rules applied backward indefinitely) that every modern programming language's date library implements. NIST is the US authoritative timekeeping reference. This calculator uses both — so its output matches what a standard-compliant library on any platform would return.
Limitations
- Pre-Gregorian dates use the proleptic Gregorian convention; historical sources may use Julian and so disagree by up to 10–13 days.
- Time-zone handling: a birth recorded at 11 pm in one zone is "yesterday's" date in another. This calculator uses the browser's local time zone.
- Future birth dates (typo or pregnancy planning) return "—" rather than a negative age.
- Daylight-saving transitions and leap seconds are absorbed into the millisecond-based total-days count and have no observable effect on years/months/days output.
Age calculations are exact within the millisecond precision of the JavaScript Date object; legal age determinations should reference your jurisdiction's specific birthday-handling rules (especially for Feb 29 leap-day birthdays).