Age Difference Calculator
Enter the birth dates of two people, and see the age difference between them in years, months, and days.
How to use the calculator
- Choose the birth date of each of the two people.
- The calculator works out the age difference in years, months, and days, regardless of who's older.
How is it calculated?
The calculator finds which of the two was born first, and calculates the exact time between the two birth dates — split into whole years, whole months, and remaining days, the same method used in our age calculator.
Why the difference in years changes through the year
The gap between two birth dates is a fixed number of days that never changes. But the age difference expressed in whole years jumps between two values through the year, depending on who had a birthday most recently. Two people born eighteen months apart are «one year» apart for part of the year and «two years» apart for the rest — without anything having actually changed.
That is why this calculator gives the difference both in years, months and days, and as a total number of days. The day count is the unambiguous answer; the year figure is what people use in everyday speech, but it is a rounding that flips twice a year.
One consequence worth noting is that the difference in days is not affected by leap years, while the difference expressed in years and months is. Two people born on 1 March and 1 September are always exactly the same number of days apart, but the months-and-days breakdown shifts slightly depending on the lengths of the months in between. If you need a figure that can be compared reliably over time, the day count is the one to use.
Frequently asked questions
Does it matter which date I enter first?
No, the calculator automatically finds who's older and works out the correct difference regardless of the order you fill in the dates.
Can I use this for siblings or pets?
Yes, the calculator works for the age difference between any two dates — siblings, partners, parents and children, or pets.
Why doesn't the number of months match what I expected?
Months have varying numbers of days, so the calculation adjusts for this just like a normal age calculator — the result always gives a consistent, correct split into years/months/days.