Age Calculator

How This Calculator Works

Exact age is the precise duration between a birth date and today (or any other date), broken into years, months, and days. Enter your birth date and this calculator shows that exact age — along with your total number of weeks and days lived and a live countdown to your next birthday, precise down to the minute. Total weeks lived is the same figure a pediatrician uses to describe an infant’s age (e.g. “6 weeks,” “12 weeks”) before switching to months, so it doubles as a quick way to track a baby’s age too.

Age isn’t just “today’s year minus your birth year” — that only works after your birthday has already happened this year. This calculator does the full calendar-aware subtraction, carrying over from days to months and months to years exactly the way you’d count by hand on a calendar.

The Formula

Starting from the year, month, and day difference between the two dates:

$$\text{Years} = \vA{Y_2} - \vB{Y_1} \qquad \text{Months} = \vA{M_2} - \vB{M_1} \qquad \text{Days} = \vA{D_2} - \vB{D_1}$$

where $\vA{Y_2}, \vA{M_2}, \vA{D_2}$ are the year, month, and day of the “as of” date, and $\vB{Y_1}, \vB{M_1}, \vB{D_1}$ are the year, month, and day of the birth date:

1. Subtract years, months, and days independently.
2. If the day difference is negative (the “as of” date falls earlier in the month than the birth date), borrow a month: subtract one from the month count and add that borrowed month’s length to the day count.
3. If the month difference is then negative, borrow a year the same way.

Total days lived is a separate, simpler count: the number of full days between the two dates. The next-birthday countdown finds the next date matching your birth month and day — this year if it hasn’t happened yet, next year if it has — and counts the days until it.

Worked Example

Someone born on January 1, 2000, checking their age as of June 15, 2024:

1. Years: $\vA{2024} - \vB{2000} = 24$.
2. Months: June is month 6 and January is month 1, so $\vA{6} - \vB{1} = 5$.
3. Days: $\vA{15} - \vB{1} = 14$.

That’s 24 years, 5 months, and 14 days old — and since their birthday (January 1) already passed earlier in 2024, the countdown points to January 1, 2025.