What this calculator does
It finds the exact gap between two birth dates: siblings, a couple, a parent and child, or two historical figures. Unlike a plain date difference, it works with the dates in either order, says who's older, and shows both people's ages on any date you choose. For two adults, it also shows the "half your age plus seven" rule of thumb, with the numbers filled in.
How the gap is calculated
The gap is counted the same way as an age: whole years, then whole months, then leftover days, from the older person's birth date to the younger person's.
→ whole months ÷ 12 = years and months, then the remaining days
Decimal years = total days ÷ 365.2425
If a month doesn't have the right day (like the 31st), the month step lands on the last day of the month. 365.2425 days is the average length of a Gregorian year.
Worked example
One person was born May 15, 1990 and the other August 20, 1993. From May 15, 1990 to May 15, 1993 is 3 years. To August 15, 1993 is 3 more months, and to August 20 is 5 more days. The gap is 3 years, 3 months, 5 days, or 1,193 days (3.27 years). On October 27, 2026 they are 36 and 33.
The "half your age plus seven" rule
This is an old rule of thumb, not a scientific, medical or legal standard. Its earliest known printed version is from Max O'Rell's 1901 book Her Royal Highness Woman, which advised that "a man should marry a woman half his age, plus seven." Today it's usually applied to anyone, in both directions:
Oldest partner by the rule = (younger person's age − 7) × 2
For the example above, 36 ÷ 2 + 7 = 25, and 33 is at least 25, so the pair is within the rule. Because the gap never changes, any pair eventually meets the rule: it holds once the older person's age is at least twice the gap plus 14. The calculator only applies it when both people are adults, and it's meant as a curiosity. Plenty of happy couples fall outside it.
| Older person's age | Youngest by the rule | Younger person's age | Oldest by the rule |
|---|---|---|---|
| 20 | 17 | 18 | 22 |
| 30 | 22 | 25 | 36 |
| 40 | 27 | 30 | 46 |
| 50 | 32 | 40 | 66 |
| 60 | 37 | 50 | 86 |
Limitations
- Ages are whole years on the comparison date, the way people state their age.
- The rule of thumb says nothing about whether a relationship is healthy, legal or a good idea.
- Time of birth isn't included, so twins born minutes apart show the same birthday.