How to Calculate Your Exact Age in Years, Months and Days (and Why Calculators Disagree)

“How old are you?” sounds like the easiest question in arithmetic until you need it exactly: for a visa form that wants years, months and days; for a pension calculation that counts to the day; for a child’s medical dose that depends on age in months; or for the more pleasant question of when someone hits 10,000 days alive. This guide shows how age is actually calculated, the leap-year and month-length problems that trip up spreadsheets, and the conventions different countries use.

The simple version

Subtract the birth year from the current year, then subtract one more if the birthday hasn’t happened yet this year. Born 14 March 1990, today 15 September 2026: 2026 − 1990 = 36, birthday has passed, so 36. Born 20 November 1990: 36 − 1 = 35. That covers most conversations.

Years, months and days: where it gets fiddly

Exact age is a date difference, and months are not the same length. The standard approach, which the Age Calculator uses, works from the largest unit down:

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1. Count full years from the birthday to the most recent anniversary.
2. Count full months from that anniversary to the most recent “month-day”.
3. Count the remaining days.

Born 31 January 2000, today 1 March 2026. Years: 26 (31 January 2026 has passed). Months: the next month-day would be 31 February, which does not exist, so the convention is to use the last day of February, 28 February 2026. That is one full month. Days: from 28 February to 1 March is 1 day. Result: 26 years, 1 month, 1 day. Different tools handle the “31st of a short month” case differently, which is why two calculators can disagree by a day. Neither is wrong; they are using different conventions.

Leap years

Someone born on 29 February has a birthday every four years, roughly. The rule for leap years: divisible by 4, except century years, unless the century is divisible by 400. So 2000 was a leap year, 1900 was not, 2100 will not be. For legal purposes most countries treat a 29 February birthday as 1 March in non-leap years (the UK and most of the US) or 28 February (New Zealand, Taiwan, Hong Kong). The calculator counts actual elapsed days either way, so total days are always exact; only the “years and months” reading depends on the convention.

Total days, weeks and hours

Total days is the unambiguous number: count the days between the two dates, including leap days. Someone born 14 March 1990 has been alive 13,334 days on 15 September 2026, which is 1,904 weeks and 6 days, or about 320,000 hours. The 10,000-day milestone lands around age 27 years 4 months, and 20,000 days around 54 years 9 months, which are the two people tend to want to celebrate.

Age conventions around the world

• International (most countries): you are 0 at birth and gain a year on each birthday.
• Korean age (traditional): you were 1 at birth and everyone gained a year on 1 January. South Korea officially switched to international age in June 2023, but the traditional count still appears socially.
• East Asian reckoning in parts of China and elsewhere follows the same 1-at-birth pattern for some ceremonies.
• Racehorses in the northern hemisphere all turn a year older on 1 January, which is the same idea applied to animals.

If a form or a person from another country gives an age that seems a year off, this is usually why.

Practical uses beyond curiosity

• Eligibility dates. “When does my child turn 18?” is birth date plus 18 years, but “when is she 18 for a flight booked in the US?” depends on the airline’s cut-off, so calculate the date and check the rule.
• Retirement and pension ages are often set as “66 years and 7 months”, which needs the years-and-months calculation, not just years.
• Infant medicine and milestones are tracked in weeks to 12 weeks, then months to 2 years, then years. Corrected age for premature babies subtracts the weeks early from the actual age.
• Age at a past date (how old was my grandmother when she emigrated?) is the same calculation with the end date changed. The calculator lets you set both ends.

Frequently asked questions

How do I calculate age in Excel or Google Sheets?

=DATEDIF(A1, TODAY(), "Y") gives full years; "YM" gives the extra months and "MD" the extra days. DATEDIF is undocumented in Excel but works. Be aware it uses one specific convention for month-ends.

How old am I if I was born on a leap day?

Count your birthdays as 1 March (or 28 February, depending on your country’s rule) in non-leap years. In total days you are exactly as old as anyone else born that week.

Why do two age calculators give different results?

Different handling of month-ends (e.g. 31 January + 1 month). The difference is at most a day or two and only affects the months/days part, never the years.

How many days old is a 30-year-old?

Between 10,957 and 10,958 days on their 30th birthday, depending on how many leap days fell in those 30 years.