How it works
Your exact age is calculated by comparing your date of birth with today's date, accounting for varying month lengths and leap years.
Age Calculator — How Old Am I from Date of Birth
Type your date of birth and the page reports your age in three forms — calendar age (years + months + days, the legal-age format used on IDs and forms), total days lived (a single number useful for milestone counts like "my 10,000th day"), and a countdown to your next birthday. It also surfaces the day of the week you were born (Zeller's congruence — the same algorithm Doc Brown was muttering at the start of Back to the Future). Useful for filling in age-required forms with the exact correct number on the day, planning a milestone birthday party, settling "how old were you when X happened" debates, or just curiosity about your own life arithmetic.
About this tool
Calendar-aware age calculation is harder than it looks: simply dividing the day-difference by 365.25 produces the wrong answer near birthdays because months have 28 to 31 days and four years out of every four hundred (the centurial-non-divisible-by-400 rule) skip the leap day. This page does the legally-correct calculation: subtract birth year from current year, then if the current month is before the birth month (or same month with the current day before the birth day) subtract one. Months and days are computed by the same calendar-aware comparison. Total days lived uses straight `Math.floor((today - birth) / 86_400_000)` since milliseconds-per-day is constant for non-DST math at UTC. The Gregorian calendar's 365.2425-day average year (achieved by the leap-day rule with century exceptions) means 1,000 days = ~2.74 years, 10,000 days = ~27.4 years, 25,000 days = ~68.4 years (a common retirement-era milestone). Use cases: filling in legal/medical forms requiring exact age in years AND months, planning a milestone birthday far enough in advance, transcribing an old date for genealogical work, comparing ages across siblings precisely, sanity-checking a database age column.
- Calendar-aware age (handles leap years and varying month lengths)
- Years + months + days breakdown — same as legal/medical forms
- Total days lived single number (useful for milestone counts)
- Days until next birthday countdown
- Day-of-week you were born (Zeller's congruence)
- Reactive — recalcs as you change date of birth
- No upload — your DOB stays in your browser
- Handles future DOBs gracefully (negative-age check)
- Works for any modern Gregorian date (1582 onward)
- Useful for legal/medical forms, milestone birthdays, genealogy
Free. No signup. Your inputs stay in your browser. Ads via Google AdSense (consent required).
Frequently asked questions
How does calendar-aware age calculation differ from days/365.25?
Naive division of the day-difference by 365.25 produces wrong results near birthdays because months have 28-31 days and the Gregorian calendar drops 3 leap days every 400 years (the centurial-non-divisible-by-400 rule). The legally-correct calculation: subtract birth year from current year, then if the current month is before the birth month (or same month with current day before birth day) subtract 1. Months and days are computed by the same calendar-aware comparison. Worked example: born 1990-03-15, today 2026-03-14 → naive (today − birth) / 365.25 = 35.999 → rounds to 36; correct calendar-aware = 35 (today is one day before the 36th birthday). The difference matters most for legal forms requiring exact age (passport renewals, insurance enrollment, school cutoffs, voting registration, military draft).
What is Zeller's congruence (the day-of-week algorithm)?
Zeller's congruence is a closed-form formula by Christian Zeller (German mathematician, Acta Mathematica volume 9, 1886, pages 131-136) to compute the day of the week for any Gregorian date without lookup tables. The formula: h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7, where q = day of month, m = month (March=3 to February=14, treating Jan/Feb as months 13/14 of the previous year), K = year mod 100, J = ⌊year / 100⌋, and h = 0 (Saturday) through 6 (Friday). Used in Donald Knuth's Art of Computer Programming Volume 1 §1.3.2, Numerical Recipes, and most calendar libraries. Pop-culture cameo: the opening scene of Back to the Future (1985) shows Doc Brown muttering the algorithm aloud as he computes a date.
How does Korean / East Asian "counting age" differ from Western age?
Traditional East Asian counting age (Korean 세는 나이, Chinese 虚岁, Vietnamese tuổi mụ) treats a baby as 1 year old at birth and adds 1 every Lunar New Year (or January 1 in modern Korean usage). So a baby born 31 December 2025 could be "2 years old" by 1 January 2026 — only one day after birth. Western age (Korean 만 나이) starts at 0 and increments on each birthday, the convention used internationally. South Korea officially abolished traditional counting age in legal contexts via Civil Code Article 158 amendment passed December 2022 and effective 28 June 2023; all administrative + judicial processes (passports, voting, military service, contracts) now use Western age by default. Traditional age persists informally in China (虚岁) and Vietnam (tuổi mụ) for cultural occasions but legal documents use Western age.
What ages matter legally in major jurisdictions?
United States: drinking 21 (federal National Minimum Drinking Age Act 1984, Public Law 98-363; states with MLDA below 21 lose 10% of federal highway funds under 23 U.S.C. § 158); voting 18 (26th Amendment ratified 1 July 1971); driving permit 14-16 + licence 16-18 by state; Social Security full retirement 67 for those born 1960+ (1983 amendment); Medicare eligibility 65; military enlistment 17 with parental consent / 18 without. European Union: voting 18 (Austria 16); drinking 18 typically (16-18 wine/beer in Germany, Italy, Belgium); driving licence 17-18; retirement age varies by country 65-67. UK: drinking 18; voting 18 (Scotland + Wales 16 for devolved elections); driving 17. The age calculator's "years + months + days" breakdown matches the format US SSA, USCIS immigration, and most European civil-registry forms expect.
Why does the page show "Feb 28" for a Feb 29 birthday in non-leap years?
Because civil-law convention in most jurisdictions treats Feb 29 birthdays as observed on Feb 28 in non-leap years (some use March 1 — usage varies by jurisdiction and family preference). The US Social Security Administration, USCIS immigration, and most insurance systems clamp the observation date to Feb 28 for legal-age computation. The Gregorian leap year rule (codified by Pope Gregory XIII, 1582 Inter gravissimas: divisible by 4 EXCEPT divisible by 100 EXCEPT divisible by 400) means leap days occur 97 times per 400-year cycle. A baby born 29 February has a real birthday roughly every 4 years (with the rare 100/400 exception); civil-law clamping to Feb 28 ensures continuous age incrementing without skipping a year of legal status.
Sources (5)
- International Organization for Standardization (2019). ISO 8601-1:2019 — Date and time format YYYY-MM-DD (only locale-unambiguous + lex-sortable plain string); foundational for legally-correct birthdate handling on forms (passports, driver's licenses, USCIS immigration, healthcare records). ISO Technical Committee 154 (TC 154); supersedes ISO 8601:2004.
- Pope Gregory XIII (1582). Papal bull Inter gravissimas (24 February 1582) — Gregorian calendar reform; leap year rule (year divisible by 4, EXCEPT divisible by 100, EXCEPT divisible by 400); mean Gregorian tropical year 365.2425 days; foundational for accurate age arithmetic across leap-day birthdays (Feb 29 → Feb 28 clamping in non-leap years for civil purposes). Papal States; adopted by Catholic Europe 1582-1584, Britain 1752, Russia 1918, Greece 1923.
- Zeller, C. (1886). Kalender-Formeln (Acta Mathematica 9:131-136) — Zeller's congruence: closed-form formula h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ − 2J) mod 7 to compute day-of-week from any Gregorian date. Used in Numerical Recipes and most calendar libraries; popularised in pop culture by the opening scene of Back to the Future (1985). Christian Zeller (German mathematician), Acta Mathematica volume 9, 1886, pages 131-136; the modern formula is also derived in Donald Knuth, The Art of Computer Programming, Volume 1 §1.3.2.
- Republic of Korea — National Assembly (2022). Civil Code Article 158 amendment + Act on the Standardization of Age Calculation (대한민국 민법 제158조 개정안) — abolished traditional Korean counting age (세는 나이) in legal contexts; effective 28 June 2023, all administrative + judicial processes use Western age (만 나이) by default. Traditional Korean age = age at birth + 1; +1 every Lunar New Year (so a 31 December baby could be "2 years old" by 1 January). Republic of Korea National Assembly; Civil Code amendment passed December 2022, effective 28 June 2023; aligns Korean age with international Western age in passports, voting, military service, contracts. Similar traditional age systems persist informally in China and Vietnam.
- World Wide Web Consortium (W3C) (2018). Web Content Accessibility Guidelines (WCAG) 2.1 — Success Criterion 4.1.3 Status Messages. W3C Recommendation 5 June 2018; carried unchanged into WCAG 2.2 (Recommendation 5 October 2023).
These are the original publications the formulas in this tool are based on. Locate them by journal name and year on Google Scholar or PubMed.