Age calculator

How to calculate age by hand

Write the date you are measuring on above the date of birth, as year, month and day, and subtract from the right. Take someone born on 28 November 1984, measured on 9 March 2026:

1. Days: 9 − 28 is negative, so borrow one month. February 2026 has 28 days, so the day column becomes 9 + 28 = 37, and 37 − 28 = 9 days. The month column drops from 3 to 2.
2. Months: 2 − 11 is negative, so borrow one year. The month column becomes 2 + 12 = 14, and 14 − 11 = 3 months. The year drops from 2026 to 2025.
3. Years: 2025 − 1984 = 41.

The age is 41 years, 3 months and 9 days. The calculator gets there by counting forward instead: the last birthday was 28 November 2025, three whole months reach 28 February 2026, and nine more days reach 9 March. It also reports 15,076 days, or 2,153 whole weeks.

When borrowing breaks

Borrowing fails when the day of birth is larger than the month you borrow from. For someone born on 31 January 1990 and measured on 1 March 2026, the day column gives 1 + 28 − 31 = −2 days. Counting forward has no such gap: one month after 31 January 2026 is 28 February, the last day of that month, and one more day reaches 1 March. The age is 36 years, 1 month and 1 day, and on 28 February it was 36 years, 1 month and 0 days. Python's dateutil library gives the same answer. Excel's DATEDIF does not always: Microsoft's DATEDIF page says its "MD" unit "may result in a negative number, a zero, or an inaccurate result". Its "Y" unit returns complete years and has no such warning.

The same month-end rule applies to babies. A baby born on 31 August turns one month old on 30 September, because September has no 31st.

Age in months, weeks, days and hours

The totals do not multiply years by 365. The calculator converts both dates to day numbers and subtracts them, so every 29 February between the two dates adds a day. Two people who are both exactly 18 can have lived different numbers of days: someone born on 15 February 2008 reaches 18 on 15 February 2026 after 6,575 days, because 29 February 2008 falls inside the span, while someone born on 15 March 2008 reaches 18 after 6,574 days.

Weeks are whole weeks (days ÷ 7, rounded down). Months are 12 per year plus the months since the last birthday. Hours are days × 24, counted from midnight on the date of birth, not from the time of birth.

Every age that is a multiple of 4 has a single day count, because any span of 4 whole years contains exactly one 29 February. That holds for spans between 1901 and 2099; 2100 is not a leap year, so a span that includes February 2100 has one day fewer. The leap year checker shows which years qualify.

Birthdays on 29 February

In the Gregorian calendar, 97 of every 146,097 days are 29 February, so if births were spread evenly, about 1 person in 1,506 would have a leap-day birthday. Ignoring the century rule gives 1 in 1,461.

Which date counts as the birthday in a common year depends on where you live, and only some legal systems say:

The choice matters most for ages that carry legal rights. Someone born on 29 February 2008 turns 18 in 2026, a common year: on Saturday 28 February under the 28 February setting, or on Sunday 1 March under the 1 March setting, which is the Scottish rule.

Japan's rule has a second effect. A year of age there ends at the close of the day before the birthday, so a child born on 1 April reaches six on 31 March. The School Education Act, art. 17 starts compulsory schooling at the first school year after a child reaches six, and school years begin on 1 April, so children born on 1 April start school a year earlier than those born on 2 April of the same year.

East Asian age reckoning

Korea has used three counts and China a fourth, and they can differ by two years for someone born late in December. For a person born on 15 December 2005, measured on 10 January 2026:

Korea's Civil Act, art. 158, counts age in full years including the day of birth. Year age survives in two laws. The Youth Protection Act, art. 2, defines a minor as under 19 but excludes anyone who has reached 1 January of the year they turn 19, so everyone born in 2007 could legally buy alcohol and tobacco from 1 January 2026, even at 18. For the timing of service duties, the Military Service Act, art. 2, reads "from age X" as from 1 January of the year a person turns X.

Chinese nominal age moves at Lunar New Year rather than 1 January. The Hong Kong Observatory's 2026 calendar table puts the first day of the first lunar month on 17 February 2026. Japan abandoned its counted age, kazoedoshi, by the Act on Age Counting, in force from 1 January 1950, which requires public bodies to state ages in full years.

Mistakes that change the answer

• Time zones. A birth at 00:30 on 1 January in London happened at 19:30 on 31 December in New York. Use the date on the birth certificate, which is the local date where the birth took place.
• "In your 36th year". This means aged 35: the first year of life runs from birth to the first birthday.
• Hours from the time of birth. The hour total runs from midnight at the start of the date of birth to midnight at the start of the chosen date. Someone born at 18:00 has lived 18 hours fewer than the figure shown.
• Multiplying years by 365.25. A spreadsheet formula such as years × 365.25 gives 6,574.5 days for an 18-year-old, when the true count is a whole number, 6,574 or 6,575, depending on how many 29 Februaries the span holds. For a span that isn't a birthday, such as the days between two events, use the days between dates calculator, and the day of the week calculator to see which weekday a date of birth fell on.