Age Calculator

Why age is not just a subtraction

Subtracting one year from another gives a number that is wrong for roughly half the population at any moment. Someone born on 3 December 1990 is 34 in January 2026 and 35 in December, and the difference matters for eligibility thresholds, insurance bands and legal age checks. Exact age requires comparing month and day, not just the year, which is where most mental arithmetic goes astray.

Months are not a fixed length

Expressing an age as years, months and days is ambiguous because months vary between 28 and 31 days. From 31 January to 28 February, is that one month or 28 days? Conventions differ, and the usual rule, followed here, is to count whole calendar months first and then the remaining days, which matches how people describe a child's age and how most legal definitions work.

Leap years and 29 February

Roughly one person in 1,461 is born on 29 February and has no birthday in a common year. Jurisdictions resolve this differently for legal purposes: some treat 1 March as the anniversary, others 28 February. For age calculation the underlying day count is unaffected, but a birthday countdown has to pick a convention and state it.

Where exact age is used

School year placement, which usually keys off a cutoff date rather than a birthday. Pension and retirement eligibility. Insurance premium bands, where crossing a birthday can change a quote materially. Medical dosing for children, which is often specified in months. In each case an off-by-one-year error has real consequences, which is why the day-level result matters more than a rounded figure.

Total days lived

The cumulative day count is a different question from the calendar age and is often the more interesting one. It accounts for every leap day and is what you want for milestone dates such as a 10,000th day. It also makes the leap-year effect visible: two people born a day apart can differ by more than one day in total days lived across a long enough span.