Age Calculator

An age calculator looks like the simplest tool on the internet: subtract one date from another. It is simple until the date of birth is the 31st of a month, or 29 February, or the question is asked in Seoul rather than London. This page gives your exact age in years, months and days on any as of date you choose, and it tells you which rule it used at the two places where honest calculators disagree.

How the exact age is counted

Your age in completed years is the number of birthdays that have happened on or before the as of date. Months come next: the calculator steps forward from your date of birth one calendar month at a time, always measuring from the original day rather than from the previous step, and stops at the last month anniversary that is not after the as of date. The days are whatever is left between that anniversary and the as of date. The day you were born is day zero, so a child born on 15 March is 3 days old on 18 March, not 4. That is the same counting convention Calculator Soup documents, and most of the leading calculators follow it.

The totals in the tiles are counted from the calendar, not from averages. Some tools convert a year to 365.25 days and a month to 30.4375 days before dividing, which is fine for a rough figure and wrong at the edges. Here the total days are the real number of midnights between the two dates, every 29 February included, and the total months are the whole calendar months from the exact age. Weeks are total days divided by seven, with the remainder shown rather than rounded away. Hours are total days multiplied by 24, which assumes a birth at midnight; if you know the time you were born, your true figure is within 24 hours of the one shown.

The month end problem, and why two calculators can disagree

Months are not all the same length, so "one month after 31 January" has no perfect answer. There are two conventions in common use, and they give different results for anyone born on the 29th, 30th or 31st.

• Anchored months, used here. A month anniversary that does not exist is moved to the last day of that month. Someone born on 31 January 2026 completes one month on 28 February 2026 and two months on 31 March 2026. Calculator.net describes the same idea when it counts 20 February to 20 March as one month.
• Borrowed days, the pencil and paper method. Subtract years, months and days column by column, and when the days go negative borrow the length of the previous month. Under that method 31 January to 28 February 2026 is 0 months and 28 days, and 31 January to 30 March 2026 is 1 month and 27 days rather than the 1 month and 30 days shown here.

Neither is a mistake; they answer slightly different questions. Years and total days never change between the two. Only the months and days split moves, and only near the end of a month. When your dates fall in that zone the calculator prints the borrowed result underneath, so you can match whatever figure a form, a school or another website expects.

Leap day birthdays: 28 February or 1 March?

The Gregorian calendar adds 29 February in every year divisible by 4, except century years that are not divisible by 400. That gives 97 leap years in every 400 and an average year of 365.2425 days, close to the 365.2422 day tropical year. A four year block therefore holds 1,461 days, and only one of them is a 29 February, which is why leaplings are rare and why the law has had to decide what their birthday is in the other three years.

It has decided both ways. In the United Kingdom the legal birthday in a common year is treated as 1 March, and Hong Kong fixed 1 March in section 5 of its Age of Majority (Related Provisions) Ordinance in 1990. Taiwan's civil code has pointed to 1 March since 1929. California and New York go the other way: their statutes count 29 February and the day before it as a single day, so rights tied to age arrive on 28 February. Some rules only cover one purpose. Maine's driver licensing law deems a leapling born on 1 March, and Michigan's vehicle code does the same for that code only, while Nevada and Oregon use 28 February for licences. Where no statute speaks, the result turns on whether a court counts the day of birth: exclude it and the age arrives on 1 March, include it and it arrives on 28 February.

The calculator defaults to 1 March and lets you switch to 28 February. The choice changes the exact age only for a few days a year, but those days matter: someone born on 29 February 2000 was legally 21 on 28 February 2021 in California and not until 1 March 2021 in the UK. 2026 is a common year, so every leapling had that decision made for them this year, and the next real 29 February is Tuesday 29 February 2028.

When exactly you turn a year older

Even the moment of turning an age varies. Under the old common law of England a person attained an age at the first moment of the day before the birthday. The Family Law Reform Act 1969 replaced that for anniversaries after 1 January 1970: you now attain an age at the start of the anniversary itself. Japan kept the older idea. Its Age Calculation Act of 1902 says age increases at the end of the day before the birthday, which is why a child born on 1 April joins the school year above a child born on 2 April. This calculator follows the modern UK convention, so the count turns over on the birthday.

East Asian age reckoning and South Korea's 2023 switch

Traditional East Asian age reckoning, once used in China, Japan, Korea and Vietnam, starts a baby at 1 and adds a year at the New Year rather than on the birthday. That puts the traditional age 1 or 2 years above the international age, with the full 2 year gap for anyone whose birthday has not yet come round since New Year. Japan legislated against the system in 1902 and again in 1950. In China the traditional count moves at the lunar New Year, which in 2026 fell on 17 February.

South Korea kept three systems side by side until recently. The National Assembly passed the change on 8 December 2022 and from 28 June 2023 international age became the default for contracts, medical care and official documents; legal and medical papers had already used it since the early 1960s. A September 2022 government survey found 86 percent of South Koreans planned to use it in daily life. The switch was not total. A "year age", the current year minus the birth year, still governs military service, school entry and the Youth Protection Act limit of 19 for alcohol and tobacco.

The year age produces a 2026 result worth knowing: anyone born in 2007 could legally buy alcohol in South Korea from 1 January 2026, even with a birthday in December. Under the calculator's tiles you will find the year age and the traditional counting age for the as of year, so you can see all three figures together.

Using the as of date

The second field starts on today, in your own time zone, but any date works. Set it to a past date to find how old someone was at an event, on a marriage certificate or at the time of a census. Set it to a future date to check eligibility ahead of a deadline: the age on the first day of a school year, on an election day, or on the date a pension or visa rule looks at. If the as of date is earlier than the date of birth the calculator says so rather than printing a negative age.

Reading the tiles and the milestones

The exact age tile is the one most forms want. Total months is the figure usually asked for when giving a baby's age. The next birthday tile counts calendar days from the as of date and names the weekday it falls on, taking your leap day rule into account. The weekday you were born on comes from the same calendar arithmetic, so a date in 1875 is as reliable as one in 2025. As a check on long spans, the calculator reproduces the longest verified human life, Jeanne Calment's, from her dates of 21 February 1875 and 4 August 1997: 122 years and 164 days. The milestones column on the right lists dated round numbers, such as your 18th and 21st birthdays and your 10,000th day, and marks each as passed or counts down to it. For dates tied to a pregnancy, the pregnancy calculator does the same kind of calendar counting forward from a due date.