Calculating an age exactly is less trivial than it looks. Months differ in length, leap years insert an extra day irregularly, and different conventions disagree about what happens when a birth date has no exact match in the target month.
How the calculation works
This calculator counts complete years first, then complete months, then remaining days — the convention used almost everywhere in everyday life and in most legal contexts.
When the day of the target date falls before the day of birth, one month is borrowed and the days of the preceding month are added. Because that borrowed month varies in length from 28 to 31 days, the day remainder depends on which months the span crosses.
Leap years
A year is a leap year if it is divisible by 4, except centuries, unless divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be.
leap = (year % 4 == 0 AND year % 100 != 0) OR year % 400 == 0
People born on 29 February have no birthday in common years. Jurisdictions differ on whether the legal anniversary falls on 28 February or 1 March.
Other systems
- Traditional East Asian age counting starts at 1 at birth and increments at Lunar New Year, making someone up to two years "older" than their Western age. South Korea moved to international reckoning in June 2023.
- Insurance age nearest rounds to the closest birthday, so you become a year older six months before your actual birthday.
- Gestational age is counted in completed weeks and days from the last menstrual period, not from conception.
Worked example
Using the values pre-loaded in the calculator above:
| Input | Value |
|---|---|
| Date of birth | 1990-01-01 |
| Age at date | 2026-07-23 |
| Output | Value |
|---|---|
| Age | 36 years, 6 months, 22 days |
| Total months | 438 |
| Total weeks | 1,907 |
| Total days | 13,352 |
| Total hours | 320,448 |
| Total minutes | 19,226,880 |
| Day of the week born | Monday |
| Next birthday | Friday, January 1, 2027 |
Frequently asked questions
How do I calculate age from a date of birth?
Subtract the birth year from the current year, then subtract one if the birthday has not yet occurred this year. This calculator also reports the exact remaining months and days.
How is age handled for 29 February birthdays?
Most systems treat 1 March as the anniversary in common years, though some use 28 February. Legal definitions vary by jurisdiction.
Why do different calculators give different day counts?
Because months vary in length, the order of operations changes the remainder. Counting years then months then days is the most common convention and the one used here.
How many days old am I?
The calculator reports total days, weeks, hours and minutes alongside the conventional years-months-days breakdown.