How to Calculate the Age Difference Between Two People
Finding the age difference between two people sounds like subtracting birth years, but that shortcut can be off by almost a year. Our age calculator works from both exact dates, and the manual method below shows what it is doing.
Subtract the birth dates, not the birth years
The exact age gap between two people is the time between their two birth dates, and it never changes. Someone born on March 10, 1990 and someone born on November 22, 1995 are 5 years, 8 months and 12 days apart today, and they will still be exactly that far apart in ten years. Subtracting the birth years alone, 1995 minus 1990, gives 5 and drops the extra 8 months entirely.
Why the gap between their ages seems to change
What does change through the year is the difference between the two people's current ages in whole years. In the example above, between March 10 and November 22 the older person has had a birthday and the younger one has not, so their ages differ by 6, for example 36 and 30. For the rest of the year their ages differ by 5. Neither figure is wrong, they just answer a different question from the exact gap, which stays at 5 years 8 months.
A worked example
To count the gap by hand, go forward from the earlier birth date. From March 10, 1990, adding 5 years reaches March 10, 1995. Adding 8 more months reaches November 10, 1995, and 12 more days reaches November 22, 1995, the later birth date. So the gap is 5 years, 8 months and 12 days. When the day of the month in the later date is smaller than in the earlier one, borrow a month and count the days across the month end, the same way you borrow when subtracting.
Calculating the gap in months
For young children, an age gap is often more useful in months. Multiply the whole years by 12 and add the leftover months: 5 years 8 months is 68 months. Two siblings born 18 months apart are usually described that way rather than as a year and a half, since the month figure is more precise for spacing that close.
Common uses for an age gap
People calculate age differences for siblings, couples, coworkers, and historical figures being compared across eras. In dating and relationships, a commonly cited (though informal) guideline for an appropriate minimum age is half the older person's age plus seven years, though it is a rule of thumb rather than a fixed standard, and personal judgment matters more than any formula. Parents also use age gaps to plan school enrollment cutoffs or to compare a child's development against typical ranges for their exact age rather than their grade alone.
Checking your answer
To confirm a manual calculation, add the gap you found to the earlier birth date and check that it lands exactly on the later birth date. If it overshoots or falls short, a month or a day was miscounted. For an exact figure in years, months and days, enter the earlier birth date as the date of birth in the age calculator and the later birth date as the "age as of" date.
Frequently asked questions
Can I just subtract birth years to find an age gap?
Only as a rough estimate. Subtracting birth years ignores the months and days between the two birthdays, so it can be off by almost a year. Subtracting the full birth dates gives the exact gap.
Why does the difference between our ages change during the year?
The exact gap between two birth dates never changes, but the difference between your current ages in whole years does: it is one year larger between the older person's birthday and the younger person's birthday each year.
How do I calculate an age gap in months instead of years?
Find the gap in years and months first, then multiply the years by 12 and add the leftover months. A gap of 5 years 8 months is 68 months.
What is the age gap rule for dating?
A commonly cited guideline is half the older person's age plus seven years as a rough minimum, though it is an informal rule of thumb rather than a fixed standard.