How Age Calculators Work (And Why Leap Years Complicate the Math)

Calculating someone's age sounds like it should be simple subtraction — current year minus birth year — but the actual calculation is more nuanced than that, especially once you account for whether a birthday has already happened this year, and how leap years affect day-based age calculations.

Why "current year minus birth year" isn't quite right

Simple year subtraction only works if you're calculating age on someone's exact birthday. For any other day of the year, you need to check whether their birthday has already occurred:

For example, someone born on November 20, 1990 is only 34 (not 35) on a date in June of 2026, because their birthday hasn't occurred yet that year. This "has the birthday happened yet" check is the part most manual age calculations get wrong.

How leap years complicate day-based calculations

A standard (non-leap) year has 365 days; a leap year has 366, with the extra day added as February 29. Leap years occur roughly every 4 years, following this rule:

• A year is a leap year if it's divisible by 4
• Unless it's also divisible by 100 — then it's not a leap year
• Unless it's also divisible by 400 — then it is a leap year after all

This is why 2000 was a leap year but 1900 was not, even though both are divisible by 100. When calculating the exact number of days someone has been alive, every leap year in that span adds one extra day that has to be accounted for — otherwise a "days lived" calculation will drift off by roughly a day for every four years.

The special case of a February 29 birthday

People born on February 29 (sometimes called "leaplings") technically only have a true calendar birthday once every four years. Most systems that calculate age or legal birthdays for these individuals treat either February 28 or March 1 as their "observed" birthday in non-leap years, though which convention applies can depend on jurisdiction or context.

Calculating age in different units

Beyond years, age is sometimes needed in other units — total days lived, weeks, or months. Converting years to days isn't just years × 365, because it needs to account for every leap year crossed in that span:

For a precise day count between two specific dates (like a birth date and today), it's more reliable to calculate the exact number of calendar days between the two dates directly, rather than trying to approximate with a years-and-leap-years formula — which is exactly what a proper date-based age calculator does under the hood.