Why year subtraction alone isn't enough
Subtracting birth year from the current year works only if your birthday has already happened this year. If it hasn't, that simple subtraction overstates your age by exactly one year. Someone born in December, checked in January of the following year, is a clear example: the year subtraction says one age, but their birthday is still 11 months away.
The actual method
Subtract your birth year from the current year. Then compare your birth month and day to today's month and day. If today's month/day comes before your birth month/day, subtract 1 from the result, your birthday hasn't happened yet this year. If today's date is on or after your birthday, the subtraction already stands.
Getting a full years, months, days breakdown
For more than just a whole-number age, count the months and days separately after you've settled the years figure above. Compare the day-of-month first: if today's day-of-month is earlier than your birth day-of-month, borrow a month, using the actual number of days in the month you're borrowing from, not a flat assumption of 30. Then compare months the same way. This is the same trap that shows up calculating days between any two dates, February being 28 or 29 days is what usually throws off a manual count.
A worked example
Someone born March 15, 1994, checked as of August 21, 2026:
Frequently asked questions
How do I calculate my age manually?
Subtract your birth year from the current year. Then check whether your birthday has already happened this year, if it hasn't, subtract 1 from that result. For a full years, months, and days breakdown, count the months and days separately, using the actual number of days in each month rather than assuming every month has 30 days.