How age is calculated

Updated 31 October 2026 · 7 minute read

Age looks like a single number, but it is really a comparison between two dates. Most of the difficulty comes from the calendar: months have different lengths, leap years add a day, and a month is not a fixed number of days. Once you accept that, the counting becomes simple and repeatable.

This guide explains the method our age calculator uses, why the “divide by 365” shortcut drifts, and where a legal age can differ from the arithmetic.

Three ways to state the same age

On a form, age usually means complete years — the number of birthdays already passed. A child who turns seven tomorrow is still six today, even though they are closer to seven than to six. That rule exists because it is unambiguous: everyone agrees on the date the number changes.

For some purposes the exact form is more useful:

• Years, months and days — used for child development, medical records and some visas. It says 36 years, 5 months and 16 days rather than just 36.
• Total days — used for day counts in contracts, insurance and residency rules, where every calendar day matters.
• Months — used for benefits, tenancies and instalment plans that run on calendar months.

The calculator shows all of them at once so you can use whichever the form asks for.

The counting rule

Start from the date of birth and count complete months. A month is complete when the same day number is reached in a later month. Then count the days left over.

From 15 October to 31 October there are 16 more days, so the full answer is 36 years, 5 months and 16 days. The same span is 437 complete months, or 13,318 calendar days.

Two details make the rule robust:

• A month is not 30 days. 15 March to 15 April is one month even though it contains 31 days; 15 April to 15 May is also one month with 30 days.
• Short months are clamped, not skipped. A birth on the 31st becomes the last day of a shorter month. 31 January to 28 February counts as one month in a common year, because there is no 31 February to reach.

Why “days ÷ 365” is not good enough

A common shortcut divides the total number of days by 365. It looks close, but it is wrong in a way that grows with time:

• A year is 365 or 366 days, so the divisor is never exact for a whole lifetime.
• Leap years add roughly one day every four years — about 0.27% — so a 40-year-old is off by around 10 days.
• The months drift too: dividing by 30.44 (the average month) can put a birthday on the wrong side of a month boundary.

Counting real calendar months avoids all of this. It is also the method used by official age calculators and by most legal systems.

Leap years and 29 February

A leap year adds 29 February. The rule in the Gregorian calendar is:

• Every year divisible by 4 is a leap year,
• except years divisible by 100,
• unless they are also divisible by 400.

So 2000 was a leap year, 1900 was not, and 2100 will not be. Our calculator works with real dates, so leap days are included automatically in the total-day count.

A birthday on 29 February needs a convention in common years. The calculator treats it as 28 February, which is the most common legal and informal practice; some families celebrate on 1 March instead, but that does not change the age. In a leap year the birthday is 29 February as usual.

Official age can differ from arithmetic

Arithmetic gives one answer; the law may give another:

• New-year age. Some East Asian traditions counted everyone as one year old at birth and added a year at New Year rather than on the birthday. South Korea stopped using this for official documents in June 2023, but it is still used socially.
• Time zones. A birth at 11:40 pm on 30 June in one country can be 1 July in another. Passport and school records use local time at the place of birth.
• Cut-off dates. School entry and some benefits use a fixed date in the year, not the birthday itself.
• Time of birth. For a strict 24-hour count, a person born at 6 pm is not a full day older until 6 pm the next day. Calendar-date tools ignore the clock.

For anything official — passports, visas, pensions, school admissions — the date on the birth certificate and the rules of the relevant authority decide. Use the calculator to check a number, not to replace the document.

Worked example

Born 15 May 1990, checking age on 31 October 2026:

Every figure follows from the two dates. Change either one and the calculator re-counts from scratch.

Age on a future date

The second date does not have to be today. Set it to a deadline and the tool shows the age that will be reached on that day. This is useful for:

• Visas and immigration forms with “age at arrival” or “age at application” rules.
• Sports categories that switch on 1 January.
• School cut-off dates and university entry requirements.
• Pension and retirement ages, which often change on a specific birthday.

Using the calculator

Enter the date of birth and, if needed, the date to measure against. The result card shows the exact age, the totals and a countdown to the next birthday. The copy button puts a plain-text summary on your clipboard for a form or an email.

The calculator works entirely in your browser. Dates are never sent anywhere, which matters because a date of birth is personal data.

Open the age calculator →