Age Calculator

Age is just the distance between two dates, read on a calendar. We count whole years up to the last birthday, then measure the leftover months and days from there. The catch is that months aren't the same length and some years carry a leap day — so age has to be counted on real dates, not a flat 365-day year. That's why the same gap can be written two ways: as a tidy years-months-days breakdown, or as one big running total.

Three steps with one borrowing rule — the same arithmetic the tool runs for you.

1. 1

   Subtract the years

   Take the birth year from the year you're measuring to. This is a first estimate of the whole years before any adjustment.

   years = asOf.year − dob.year
2. 2

   Check the day, borrow if short

   If today's day-of-month is earlier than the birth day, borrow one month and add the number of days in the previous calendar month. This is where uneven months and leap days enter.

   if day < 0 → month− , day += daysInPrevMonth
3. 3

   Check the month, settle the years

   If the month count went negative, the birthday hasn't arrived this year — drop one year and add twelve months. What remains is the exact age.

   if month < 0 → year− , month += 12

The same procedure, walked through the four situations that trip people up — a clean subtraction, a borrowed day, a borrowed year, and a leap-day birthday.

The fastest answer when you only know a birth year. Pick a decade and read off the age for every year — the larger figure once that year's birthday has passed, the smaller until it does.

Why the totals look so big: a single year converts to a lot of small units. These are the exact factors the calculator uses to turn one age into months, weeks, days, and seconds.

Most date math is simple — until a leap day, a month boundary, or a different counting convention shows up.

A precise age — not a rounded year — decides real outcomes in a surprising number of places.

The age you reach during 2026 for birth years back to 1950 — the fast answer to “if I was born in 1980, how old am I”. The smaller figure applies until this year's birthday arrives.

Age you turn during 2026 = 2026 − birth year. Use the chart above to switch decades, or the tool for an exact date of birth.

A whole-year age converted to smaller units — the quick answer to “how many days is 18 years”. Approximate, using 365.25 days a year.

Common questions