Age Calculator

What Is Age?

Age is elapsed time since birth expressed in completed calendar units. Its defining behavior is truncation rather than rounding: a person who is 35 years, 11 months, 29 days old is 35, not 36. Every legal and administrative use of age depends on that truncation, because it turns the change from one age to the next into a single identifiable day instead of a gradual approach — which is exactly what a voting roll, a driving licence, a school register, or a pension scheme needs.

Chronological age, the kind this page returns, is only one of the ages a person has. Gestational age is counted from the first day of the last menstrual period rather than from birth. Corrected age subtracts the weeks a premature baby arrived early. Biological age is an estimate from health markers and has no fixed relationship to the calendar at all. When a form, a growth chart, or a statute says “age” with no qualifier, it means the chronological figure.

The precision matters more often than people expect. School enrollment cutoffs, visa and immigration paperwork, custody schedules, insurance underwriting, and pediatric dosing all read age at one specific date in years, months, and days. Working it out by hand is where the errors creep in, because the units are not equal: months run 28, 29, 30, or 31 days, and which one you borrow from changes the answer.

Measuring between two arbitrary dates rather than from a birthday? Use the Date Calculator.

How Do You Calculate Age From a Date of Birth?

Age is worked out from the largest unit down, with borrowing — the same procedure as column subtraction, except the columns are different sizes and one of them changes length depending on which month you borrow from.

The Age Formula

years  = year of asOfDate  − year of birthDate
months = month of asOfDate − month of birthDate
days   = day of asOfDate   − day of birthDate

if days < 0:    days   += length of the month before asOfDate
                months -= 1
if months < 0:  months += 12
                years  -= 1

totalDays   = whole days between the two dates
totalMonths = years × 12 + months
totalWeeks  = totalDays ÷ 7, rounded down
totalHours  = totalDays × 24

The two halves are computed from different sources on purpose. Years, months, and days come from the calendar comparison above. The day, week, and hour totals come from the raw difference between the two dates, so every February 29 in the span is already inside them without any correction. Total months is the one figure not derived from days: it is years × 12 plus the months figure, which means it counts completed months and never approximates a month as 30 or 30.44 days.

Step by Step

• Write both dates as year, month, day. June 15, 1990 becomes 1990 / 6 / 15, and August 15, 2026 becomes 2026 / 8 / 15.
• Subtract each column on its own: 2026 − 1990 = 36 years, 8 − 6 = 2 months, 15 − 15 = 0 days.
• If the day column came out negative, add the real number of days in the month immediately before the target date, then take one off the month count.
• If the month column is now negative, add 12 to it and take one off the year count.
• Read the result as completed units — 36 years, 2 months, 0 days. Nothing is rounded up.

A borrowing example: born March 31, 2000, measured on May 30, 2026. The day column is 30 − 31 = −1, so it borrows April's 30 days to reach 29 and drops the months from 2 to 1, giving 26 years, 1 month, 29 days. Borrowing April specifically, rather than a generic 30-day month, is what keeps the answer aligned with the calendar the reader is holding.

Worked Example

Entering June 15, 1990 as the date of birth and August 15, 2026 as the measurement date returns all seven readings at once:

The internal arithmetic checks out: 36 × 12 + 2 = 434 months, and 13,210 × 24 = 317,040 hours. The 36 completed years alone account for 13,149 days, which is nine more than 36 × 365 — one extra day for each February 29 from 1992 through 2024 — and the remaining 61 days carry the count from June 15 to August 15. Move the measurement date back to September 11, 2001 and the same birth date returns 11 years, 2 months, 27 days, or 4,106 days.

Age Chart: Milestones in Months, Weeks, Days and Hours

Every row below is the calculator's own output for a birth date of August 15, 2000, read on that exact anniversary:

The months column is identical for everyone, because a completed month is a completed month regardless of length. The day, week, and hour columns move by one unit depending on how many February 29ths your own span happens to catch. Testing every birth date across a full four-year cycle, 18 years lands on 6,574 or 6,575 days, 21 years on 7,670 or 7,671, 25 years on 9,131 or 9,132, 30 years on 10,957 or 10,958, 50 years on 18,262 or 18,263, and 65 years on 23,741 or 23,742. Forty years is the one row with a single answer: every 40-year span in the current era contains exactly ten leap days, so it is 14,610 days for everybody. It slips to 14,609 only across a skipped century leap year such as 1900 or 2100.

The 10,000-Day Mark and Other Odd Milestones

Round day counts land in unremarkable places, which is half their appeal. Someone born on August 15, 2000 reaches 10,000 days old on January 1, 2028, at 27 years, 4 months, 17 days. One billion seconds — a shade over 11,574 days — arrives on April 23, 2032, at 31 years, 8 months, 8 days. Twenty thousand days comes on May 19, 2055, at 54 years, 9 months, 4 days. To find your own, put your birth date in and walk the second date forward until the total-days line hits the number you want.

The same trick works backward for anniversaries you have already passed. A birth date of June 15, 1990 crossed 10,000 days on October 31, 2017, at 27 years, 4 months, 16 days, and passed one billion seconds on February 21, 2022.

How to Read Your Result

What Each Line Means

• Years, months, days — the headline figure, in completed units only. This is what a form is asking for when it asks your age.
• Total completed months — years × 12 plus the months figure, so 36 years and 2 months reads 434. This is the line to use for infants and toddlers, whose age is normally stated in months up to about two years.
• Whole weeks — the total day count divided by seven and rounded down, so it only advances every seventh day. At 13,207 and 13,208 days it still reads 1,886 weeks; at 13,209 it ticks over to 1,887.
• Total days — the true elapsed calendar days between the two dates, leap days included. It is the only line that is fully comparable between two different people.
• Total hours — the day count multiplied by 24, so it is always a whole multiple of 24. The calculator takes no time of birth, and midnight-to-midnight is the assumption throughout.
• Born on a — the weekday of the birth date itself. June 15, 1990 was a Friday; February 29, 2004 was a Sunday.
• Next birthday in — whole days from the measurement date forward to the next anniversary. On the birthday itself it reads 365, or 366 when the year ahead contains a February 29, because that day's anniversary has already arrived.

Reading It Against an Age Cutoff

For any question of the form “am I old enough yet”, set the measurement date to the day the rule is actually applied and then read only the years figure. Anything short of the threshold is short of it, however close. A birth date of August 16, 2005 checked on August 15, 2026 returns 20 years, 11 months, 30 days with 1 day to the next birthday — one day below 21. Shift the birth date to August 15, 2005 and the same check returns exactly 21 years, 0 months, 0 days.

Run it the other way to find the date a threshold is met. Born November 2, 2008, the reading on August 15, 2026 is 17 years, 9 months, 13 days with 79 days remaining, which places the eighteenth birthday on November 2, 2026. School cutoffs work identically, with one twist worth remembering: enter the district's cutoff date as the measurement date rather than today's, because the cutoff date is the one the rule reads.

Leap-Day Birthdays and Age Systems Around the World

If You Were Born on February 29

A February 29 birthday exists only in leap years. The Gregorian rule, as the Royal Observatory Greenwich states it, is that the year number must be divisible by four, except end-of-century years, which must be divisible by 400 — so 2000 and 2024 qualify while 1900 and 2100 do not. In common years this calculator rolls the anniversary forward to March 1, the treatment most age-based eligibility rules use.

Born February 29, 2004 and measured on August 15, 2026, the result is 22 years, 5 months, 17 days — 269 months, 1,171 weeks, 8,203 days, born on a Sunday, with 198 days to the next birthday, which falls on March 1, 2027.

The rollover is visible in the year counter as it turns. On February 28, 2027 that same person reads 22 years, 11 months, 30 days with 1 day to go, and on March 1, 2027 they turn 23. In 2028, a leap year, the change comes a day earlier: February 28, 2028 reads 23 years, 11 months, 30 days, and February 29, 2028 reads exactly 24 years, with 366 days to the next birthday.

International Age vs Traditional East Asian Age

The system used here is international age: zero at birth, plus one on each birthday. Traditional East Asian reckoning counts a newborn as one and adds a year at Lunar New Year, so it can read up to two years higher — a baby born in late December is “two” within weeks of arriving.

South Korea ended a three-way split on June 28, 2023, when amendments to the General Act on Public Administration and the Civil Act took effect and standardized international age across judicial and administrative use. The third system in that mix was calendar age, which subtracts the birth year from the current year regardless of whether the birthday has happened yet — a rule that makes everyone born in the same year the same age all year long.

If a document does not say which system it means, it means international age, which is what this calculator returns.

Online Age Calculator vs Spreadsheet Formulas

The usual alternative to a web calculator is a spreadsheet, and the spreadsheet route has a documented trap sitting exactly where age calculations need it not to be. Excel's DATEDIF function has an “MD” unit that returns the leftover days after the whole months — the same quantity as the day figure here. Microsoft's own documentation warns that the MD argument “may result in a negative number, a zero, or an inaccurate result”, and states outright: “We don't recommend using the 'MD' argument, as there are known limitations with it.”

There is also a difference in how much you get back for the effort. A spreadsheet returns one number per formula, so a years-months-days answer needs three cells plus a text join, and the four totals need four more; the weekday needs a fifth function on top. This page returns all seven readings from two date entries. The trade-off is that a spreadsheet keeps a record you can audit later, while this calculator recomputes from the two dates every time and keeps nothing between visits.

Phone calendars and “time since” widgets are a third option, and they usually count days only. That is why they can disagree with a years-months-days figure without either being wrong — they never perform the borrowing step at all. If two tools differ by a single day for you, check which month each of them borrowed from before deciding one is broken.

Limits: When This Calculator Does Not Apply

Legal Age Is Reached a Day Earlier

U.S. federal rules do not treat the birthday itself as the moment an age is attained. Social Security regulation 20 CFR 404.2(c)(4) states that an individual “attains a given age on the first moment of the day preceding the anniversary of his birth corresponding to such age”, and IRS Publication 501 puts it plainly: “A person is considered to reach age 65 on the day before the person's 65th birthday.” For one day each year, then, an official record reads one year higher than this page does. Where a benefit, a deduction, or a filing status turns on the exact day, apply the day-before rule rather than the calendar figure here.

The Day Figure Can Go Negative on March 1 and 2

The borrowing step takes its number from the length of the month immediately before the measurement date. When that month is February and the birth day is the 30th or the 31st, 28 days is not enough to cover the borrow and the day figure comes out below zero: born January 31, 2005 and measured on March 1, 2026, the result reads 21 years, 1 month, −2 days. Checking every birth date across a four-year cycle against every day of 2026 — 533,265 pairs — this happens 100 times, always with a 30th or 31st birth day read on March 1 or March 2. It is the same limitation Microsoft documents for DATEDIF's MD unit, and it does not touch the other lines: the years and months figures and the whole totals row stay correct, so read those instead.

Ages This Tool Does Not Compute

• Gestational age — counted from the first day of the last menstrual period, not from birth, and normally reported in weeks plus days.
• Corrected age — a premature baby's chronological age minus the weeks born early, used on growth charts until roughly two years.
• Biological or metabolic age — an estimate drawn from health markers, with no fixed relationship to the calendar.
• Age at an instant — the calculator takes two dates and no time of birth, so the hours line is always a whole multiple of 24 and can never resolve a same-day comparison.
• Animal age in human-equivalent years, which follows species-specific curves rather than a fixed multiplier.

For the species curves rather than a straight multiplier, use the Dog Age Calculator.

Time Zones and Date Lines

Both dates are read as plain calendar dates with no time zone attached, and the gap between them is measured the same way. If you were born in Tokyo and are reading this in Los Angeles, the calculator uses the date printed on the certificate rather than the birth instant converted across zones. That is the behavior every official form wants, but it is worth knowing if you are reconciling a birth time recorded on the other side of a date line, where the certificate date and the local date of the same moment can differ by one day.