Age gaps come up in relationships, families, sport and school year groups. Subtracting years is easy, but it can be a year out once you count months and days.
Because it uses full dates and not just years, the gap is exact.
How to use the Age Difference Calculator#
- Enter the first birthday. Type the day, month and year.
- Enter the second birthday. Do the same for the other person.
- Read the gap. The result shows the gap in years, months and days, and who’s older.
How the gap is worked out#
The calculator counts from the older birthday to the younger one. It subtracts the years, months and days in turn, borrowing from the next unit up when a part would be negative, just as in long subtraction.
Someone born on 15 March 1990 and someone born on 2 September 1994 are 4 years, 5 months and 18 days apart. Looking at the years alone, you would say 4, and be wrong by nearly half a year.
Why months and days matter#
For legal ages, school entry and sports age groups, exact dates decide who’s in and who’s out. If you only need the age of one person on a given day, use the age calculator.
A note on the gap changing#
The gap between two people never changes, but the ratio of their ages does. Someone who’s four years older than another was twice their age when the younger was four, and is a much smaller multiple now.
Why the ratio matters more than the gap#
A gap of five years stays five years for ever, but its meaning changes. When one person is 20 and the other 25, the older is 25 percent older. At 40 and 45, the gap is only about 12 percent. This is why age differences that feel large early in life seem less so later.
| Younger | Older | Gap | Older is this much older |
|---|---|---|---|
| 15 | 20 | 5 years | 33% |
| 20 | 25 | 5 years | 25% |
| 30 | 35 | 5 years | 17% |
| 40 | 45 | 5 years | 12.5% |
| 60 | 65 | 5 years | 8% |
Rules of thumb and what they miss#
You may have heard the “half your age plus seven” rule, which gives a minimum age for a partner. It is a rough guide that people use, not a law, and it ignores everything that matters, including maturity, circumstances and legal age limits. The law sets the age of consent, and it differs by country and by state, so check the rules where you live.
Uses beyond relationships#
Siblings, cousins and school friends often want to know exactly who is older, and by how much. School systems use cut-off dates, so two children born a day apart can be in different year groups. Sports leagues set age bands using a birth year or a cut-off date. If you need the age of one person on a specific date, the age calculator gives it in years, months and days.
Where exact gaps matter#
- School entry, where a cut-off date decides whether a child starts this year or next.
- Sports age groups, where a birth date a day either side can change a team.
- Family trees and records, where you need to know which sibling is older by months.
- Legal and administrative rules, such as the age at which someone can drive, marry, vote or take a job, which differ by country.
For any of these, enter the full birth dates and not just the years, because the months and days decide the answer.
Frequently asked questions#
Subtract the later date of birth from the earlier one, counting years, months and days.
No. It stays the same for life.
Because birthdays fall at different times in the year, so the gap can be nearly a year more or less than the difference in years.
Yes. It works for any two dates.
Compare the full birth dates. The person with the earlier date is older, even if the difference is only a day. If they were born in the same year, compare months and then days. The calculator does this for you and shows the gap in years, months and days, so you can see whether it is a matter of weeks or a decade.
In some places, yes. The age of consent, the age to marry and the rules on guardianship differ between countries and states, and some set limits on the gap between partners. Check the law where you live. The calculator only shows the gap, and does not give legal advice.