How to Calculate Age: A Step-by-Step Guide
Calculates exact age in years, months and days.
Dates and durations matter for notice periods, leave planning, project deadlines and contracts. Counting days on a calendar is slow and easy to get wrong, especially across months of different lengths.
What you need
- Date of birth
- Age on this date
Step by step
- Enter the date of birth.
- Enter the age on this date.
- The results update as you type, or press Calculate.
- Read the exact age first, then the supporting figures below it.
The formula
Count whole years, then months, then days.
Worked example
For example, with these inputs:
- Date of birth: 9 Aug 1990
- Age on this date: 5 Oct 2026
the calculator returns:
- Exact age: 36 years, 1 months, 26 days
- Total days: 13,206 days
- Total months: 433 months
Practical tips
- Write dates in an unambiguous format such as 5 Oct 2026 when sharing deadlines.
- Leap years add a 29 February every four years, except most century years.
- Decide upfront whether the end date should be counted, because it changes the answer by one day.
- For overnight shifts, add 24 hours when the end time is earlier than the start time.
Background
Time durations are easier to handle in minutes. Convert each time to minutes after midnight, subtract, then convert back to hours and minutes, adding 24 hours for shifts that run past midnight.
When sharing dates internationally, write the month as a word to avoid confusion between day-month and month-day formats.
The Gregorian calendar has months of 28 to 31 days and adds a leap day almost every four years, which makes counting days by hand surprisingly error-prone. Converting dates to day numbers and subtracting avoids the problem.
Related calculations
- Hours Worked Calculator: Totals a shift's paid hours after breaks.
- Days Between Dates Calculator: Counts days between two dates.
- Add Days to a Date Calculator: Finds the date a number of days after another date.
- Subtract Days from a Date Calculator: Finds the date a number of days before another date.
Want to skip the arithmetic? The free Age Calculator does all of this instantly and updates as you type.
Run your own numbers: Age Calculator