Subtracting birth year from current year gets you close, but exact age - the kind needed for legal cutoffs, school enrollment, visa forms, and pediatric checkups - is a calendar calculation, not simple arithmetic. Months have 28 to 31 days, February grows a day every fourth year, and different cultures have counted age differently for centuries. Our Age Calculator handles all of it; this guide explains what it is doing.
Years, Months, Days: How the Borrowing Works
Exact age is computed like column subtraction, largest unit first. Born March 15, 2000, on July 12, 2026 you are: 26 full years (birthday already passed), then March 15 → June 15 is 3 full months, then June 15 → July 12 is 27 days. Result: 26 years, 3 months, 27 days.
The interesting case is borrowing. Born August 30, 1995, on July 12, 2026 your birthday has not arrived yet, so you are 30, not 31. Count months from August 30, 2025: ten full months reach June 30, 2026, and June 30 → July 12 is 12 days: 30 years, 10 months, 12 days. Because the days borrowed come from whichever month you land in - 28, 30, or 31 days long - two people born the same number of days apart can differ slightly in their "months and days" phrasing. The day count is exact; the wording is convention.
Leap Years and the February 29 Problem
The calendar adds Feb 29 in years divisible by 4 (century years only if divisible by 400 - so 2000 was a leap year, 1900 was not). For age math, leap days simply make some years 366 days long; day counts stay exact. The only genuine puzzle is a Feb 29 birthday: the date exists once every four years, so in off-years the birthday is observed Feb 28 or Mar 1 - and where it matters legally, the rule varies by country (Mar 1 in the UK and Hong Kong; Feb 28 in New Zealand and Taiwan). A "leapling" born Feb 29, 2000 turned 26 in 2026 while having had only 6 actual Feb 29 birthdays.
Your Exact Age, Down to the Day
Years, months, days, total days alive, and a countdown to your next birthday - leap years handled correctly
Open the Age Calculator →Not Everyone Counts Age the Same Way
The international system - born at 0, +1 on each birthday - feels universal but is not. Under the traditional East Asian system, a baby was 1 at birth (counting the year of life in progress) and everyone aged together at New Year, so a child born in late December turned 2 within days. Versions of this system were historically used across China, Japan, Korea, and Vietnam.
South Korea was the last country using it in daily life, often quoting two or three different ages for one person. On June 28, 2023, Korean law standardized on international age for legal and administrative purposes - a change that made most Koreans one or two years younger on paper overnight. Traditional counting still appears informally and in some contexts (like the drinking-age year rule), which is why Korean-age converters remain a popular calculator feature.
Weeks for Babies, Days for Fun
Pediatricians track infants in weeks up to about three months and in months to age two, because vaccination schedules and milestones are defined that way - an "18-week-old" (126 days, about 4.1 months) is a precise clinical age. At the other end of the whimsy scale, day-counting produces great trivia: you cross 10,000 days old at about 27 years 4.5 months. And since a "year" is just one orbit, your age varies by planet: a 30-year-old on Earth has seen about 16 Mars years (687-day orbits) and roughly 124 Mercury years (88-day orbits). Same person, same days lived - the unit is the only thing that changed, which is really the theme of all age math.
Frequently Asked Questions
How old is someone born on February 29?▼
Their exact age in days is unambiguous - only the birthday celebration is tricky. A real Feb 29 only exists in leap years, so in other years the anniversary is observed on Feb 28 or Mar 1. Where the date matters legally (turning 18), jurisdictions pick a rule: the UK and Hong Kong treat Mar 1 as the birthday in non-leap years, while New Zealand and Taiwan use Feb 28.
Why do two age calculators give different months-and-days answers?▼
Because months are unequal, "X months and Y days" depends on the counting convention - most tools add whole years first, then whole months, then leftover days, but some resolve end-of-month edge cases differently (is Jan 31 + 1 month Feb 28 or Mar 3?). The age in total days is always identical; only the months-and-days phrasing can vary by a day or two.
What was Korean age and why did Korea change it?▼
Under the traditional system you were 1 at birth and everyone gained a year each Jan 1 - a baby born on Dec 31 turned 2 the next day. Since June 28, 2023, South Korea standardized on international age (0 at birth, +1 each birthday) for legal and administrative purposes, making everyone one or two years "younger" on paper overnight.
Why are baby ages given in weeks and months instead of years?▼
Because development moves too fast for years to be useful - the difference between an 8-week-old and a 20-week-old is enormous. Pediatric milestones and vaccination schedules are specified in weeks (up to about 3 months) and then months (up to age 2), so "18 weeks" carries information that "0 years" cannot.
How do I find the exact number of days I have been alive?▼
Count the days between your birth date and today, including leap days - roughly your age × 365.25. A 27-year-old is around 9,860 days old and crosses the 10,000-day mark at about 27 years 4.5 months. An age calculator does the exact count instantly, including the date of your next round-number "day birthday".
How Old Are You, Exactly?
Exact age in years, months, and days, total days alive, and your next birthday - free, instant, and entirely in your browser.
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