How to Calculate Your Age from a Date of Birth
Understand how calendar dates are compared to calculate age in completed years.
Key idea
Understand how calendar dates are compared to calculate age in completed years.
Formula or method
Worked example
Start with the values in your question, substitute them into the formula, and keep the units consistent. For example, if a rate is given as 15%, use 0.15 when a formula requires a decimal.
Common mistakes
- Mixing percentage points with percentage change.
- Using different units for the same calculation.
- Rounding too early when several steps are involved.
Why the result can differ
Real-world calculations can include assumptions, rounding, fees, tax rules, calendar conventions, or other details. Treat simple calculator results as estimates when the underlying situation is more complex.
Birthday-based age versus elapsed days
Age in years is normally based on completed birthdays, while elapsed-day calculations measure the distance between two dates. These methods answer different questions and can produce numbers that should not be substituted for one another.
Leap years and calendar boundaries
A date calculation has to account for different month lengths and leap years. Simple subtraction of calendar years does not always give a person’s completed age.
Use the right tool
Use the Age Calculator for birthday-based age and the Date Difference Calculator when your question is the elapsed interval between two dates.