Transparent method
The formula
Worked example
Example inputs
How to interpret the result
Age in completed years only advances on an actual birthday, which means two people born just one day apart in the same year can be counted as different ages for almost an entire year depending on where today's date falls relative to each of their birthdays. The calculation counts full completed years, not a rounded average based on birth year alone.
Before you rely on it
What to check
Confirm the calculation is counting full completed years from the actual birth date, not simply subtracting birth year from the current year, which overstates age before a birthday has actually occurred that year.
The common error
Where people go wrong with age calculator
Calculating age by subtracting birth year from the current year alone. That method overstates age by one full year for anyone whose birthday hasn't yet occurred in the current calendar year.
Sensitivity evidence
How birth date changes the age in years
| Birth date | Age in years | Total days lived |
|---|---|---|
| May 13, 1986 | 40 | 14,717 |
| May 29, 1988 | 38 | 13,970 |
| June 15, 1990worked example | 36 | 13,223 |
| July 1, 1992 | 34 | 12,476 |
| July 18, 1994 | 32 | 11,729 |
Every input, tested
Which input moves the age in years most
| Input | Tested from | To | Age in years at each end | Swing |
|---|---|---|---|---|
| As of date | December 28, 2020 | April 27, 2032 | 30 to 41 | 11 (31%) |
| Birth date | May 29, 1988 | July 1, 1992 | 38 to 34 | 4 (11%) |
Two variables at once
Age in years by birth date and as of date
| Birth date \ As of date | April 29, 2015 | August 28, 2026 | December 27, 2037 |
|---|---|---|---|
| May 13, 1986 | 28 | 40 | 51 |
| May 29, 1988 | 26 | 38 | 49 |
| June 15, 1990 | 24 | 36 | 47 |
| July 1, 1992 | 22 | 34 | 45 |
| July 18, 1994 | 20 | 32 | 43 |
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Birth date | June 15, 1990 | |
| As of date | August 28, 2026 | Defaults to today. |
| Age in years | 36 | |
| Total days lived | 13,223 | |
Inputs, definitions and assumptions
Birth date
As of date
Defaults to today.
Where the inputs come from
Enter the birth date and the date you want to measure to, which defaults to today. The calculator counts full years between the two dates, and the total days lived. Use the birth date from a birth certificate, since some forms record dates in different formats, and be careful with day and month order. For an age on a future date, such as when a child will start school or an employee will reach retirement age, change the reference date.
When the result stops applying
Legal ages in some places are counted differently, and some cultures count a person as one year old at birth. Leap-day birthdays, February 29, are treated as February 28 or March 1 for legal purposes depending on the jurisdiction. Time zones can matter for births near midnight. The tool counts calendar days, not accounting for time of day.
What to compare it with
See the interval between other dates on the date difference calculator, count down to a date on the day counter, and find the weekday on the day of the week page. The add days to date and business days calculators plan schedules. For retirement ages, the Social Security pages use full retirement age.
People also ask
Frequently asked questions
How do I calculate age from a birth date?
Count the number of full years from the birth date to the reference date, adding a year on each birthday, and count remaining months and days if you want more precision.
When does someone with a February 29 birthday turn a year older?
In non-leap years many places treat the birthday as February 28 or March 1, and the rule differs by jurisdiction and purpose.
Can I calculate a future age?
Yes. Enter a future reference date to see how old someone will be on that date.