Age Formula Explained, With Examples
Every age result comes from one formula. Once you understand what goes into it, you can sanity-check any figure you are given, whether by an employer, a bank, a teacher or another website.
The formula
Count whole years, then months, then days.
What each input means
- Date of birth: the value you enter in the calculator.
- Age on this date: the value you enter in the calculator.
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
Why it matters
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.
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.
When sharing dates internationally, write the month as a word to avoid confusion between day-month and month-day formats.
Age is calculated in completed years, months and days. Someone born on 29 February is usually treated as turning a year older on 28 February or 1 March in non-leap years, depending on the rule being applied.
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.
Use the Age Calculator to plug in your own numbers.
Run your own numbers: Age Calculator