How to calculate age in days
Enter a birth date and a reference date.
1
Enter the birth date
Use the calendar date, not a due date.
2
Choose the reference date
Today by default, or any future date.
3
Read the units
Use days for milestones, years for ordinary age.
What does this tool do?
Age is usually expressed in years, but days and weeks are useful for projects, paediatric milestones and curiosity. This calculator uses calendar dates and can measure age at any reference date.
The Formula
Days = Reference date − Birth dateFrequently Asked Questions
Why is the age in years not exactly days ÷ 365?
Leap years and different month lengths make calendar years irregular. The calculator uses calendar anniversaries for the year count.
Can I calculate age on a future date?
Yes. Set the reference date to the future date; the result must be after the birth date.