Calculate the exact age difference between two people. Enter two birthdates to find the gap in years, months, and days — along with each person's current age and which person is older.
| Relationship Type | Typical Age Gap |
|---|---|
| US married couples (average) | 2–3 years |
| Siblings (consecutive) | 1–5 years |
| Parent to first child | ~27 years |
| Grandparent to grandchild | ~50 years |
The difference between two birth dates is constant in absolute terms — two people born four years apart stay four years apart forever. What changes is how significant that gap is relative to their ages.
A four-year gap between a 6-year-old and a 10-year-old is enormous developmentally. The same gap between 40 and 44 is barely noticeable.
Whole-number ages change on birthdays, so for part of each year the apparent difference shifts by one. Two people born 18 months apart will be described as 2 years apart for several months and 1 year apart for the rest, depending on whose birthday has passed.
Calculating from full dates rather than ages in years removes this and gives the true interval in years, months and days.
Children born days apart can land in different school years if a cut-off date falls between them, which is why two children with a tiny age gap may be a full academic year apart — and why those born just after a cut-off are often the oldest in their cohort.
Research on relative age effects in school and youth sport has found this can matter more than the small difference in months would suggest.