The age difference between two people is the time between their two dates of birth, counted from the earlier date to the later one in years, months and days. For example, someone born on March 4, 1988 is 3 years, 2 months and 5 days older than someone born on May 9, 1991 — 1,161 days.
The gap never changes as the two of you get older, but the ratio does: the calculator also shows each person's age on any date, your combined age, and the day the older person was exactly twice as old as the younger.
Worked example
First person, date of birth: March 4, 1988Second person, date of birth: May 9, 1991Ages on: October 30, 2026
Age difference: 3 years, 2 months, 5 days
March 4, 1988 to May 9, 1991 is 3 years, 2 months and 5 days, or 1,161 days. The older one was exactly twice the younger's age on July 13, 1994, when the younger one was 1,161 days old.
Open this example in the calculator →How to use the Age Difference Calculator
- Enter the first person's date of birth.
- Enter the second person's date of birth. The order does not matter.
- Read the age gap in years, months and days, and who is older.
- Change "Ages on" to see both ages on another date.
How the age gap is calculated
Take the earlier birth date and count forward to the later one: whole years first, then whole months, then days. From March 4, 1988 to March 4, 1991 is 3 years; to May 4 is 2 more months; to May 9 is 5 days. That is the age difference — 3 years, 2 months and 5 days — and it stays the same for the rest of both lives.
The days figure is the plain count of days between the two birth dates, with leap days included.
The "twice as old" date
When the younger person is born, the older one is infinitely many times older; the ratio then falls towards 1 for ever. There is exactly one day on which the older person is twice the age of the younger: the day the younger one is as old as the gap. With a gap of 1,161 days, that is 1,161 days after the younger person's birth — on that day one has lived 2,322 days and the other 1,161.
Is an age gap "too big"?
There is no rule. The often-quoted "half your age plus seven" is a 20th-century rule of thumb, not research. Surveys of couples find the typical age gap between partners is around two to three years, with the older partner more often being the man, but couples with much bigger and smaller gaps are common. The calculator just gives you the exact number.
Siblings, parents and generations
For siblings, the gap is the spacing between births. For parents and children it is the parent's age at the child's birth. To see whether two people belong to the same generation, use the generation calculator.
Frequently asked questions
How do I calculate the age difference between two people?
Count from the earlier date of birth to the later one: whole years, then whole months, then days. Someone born March 4, 1988 is 3 years, 2 months and 5 days older than someone born May 9, 1991.
Does the age difference change over time?
No. The gap between two birth dates is fixed. Only the ratio of the ages changes, falling towards 1 as both people get older.
When was one person twice as old as the other?
On the day the younger person was as old as the gap between them — the younger person's birth date plus the gap in days. The calculator shows the date.
What is the average age gap between couples?
Studies typically find an average of about two to three years between partners in heterosexual couples, with wide variation.
Can I compare a date that has not happened yet?
Yes. You can enter a future date, such as a baby's due date, to see the gap to an older sibling.