Age gaps matter for siblings, couples and genealogy. The “half your age plus seven” rule is a light-hearted guideline from the early 20th century, not a scientific measure.
How to use the Age Difference Calculator
- Person 1 date of birth
- Person 2 date of birth
- Compare on
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter both dates of birth and the date on which to compare their ages.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Person 1 date of birth | A date |
| Person 2 date of birth | A date |
| Compare on | A date |
What each result means
- Age gap = floor([Months apart]÷12)
- and months = [Months apart] – 12×floor([Months apart]÷12)
- and days = [Later birth date] – addmonths([Earlier birth date], [Months apart])
- Gap in days = [Later birth date] – [Earlier birth date]
- Gap in years (decimal) = ([Later birth date] – [Earlier birth date])÷365.2425
- Older person’s age = floor(monthsbetween([Earlier birth date], [Compare on])÷12)
- Younger person’s age = floor(monthsbetween([Later birth date], [Compare on])÷12)
- Half-plus-seven minimum for the older person = [Older person’s age]÷2 + 7
Formula
Age gap = later date of birth − earlier date of birth (years, months, days)Half-plus-seven: youngest socially acceptable partner age = older age ÷ 2 + 7Worked example
Inputs — Person 1 date of birth: 1990-03-12; Person 2 date of birth: 1994-11-30; Compare on: 2026-10-25.
Results — Age gap = 4 years; and months = 8; and days = 18; Gap in days = 1,724; Gap in years (decimal) = 4.72; Older person = Person 1; Older person's age = 36; Younger person's age = 31; Half-plus-seven minimum for the older person = 25.
Frequently asked questions
How do I calculate an age gap?
Subtract the earlier date of birth from the later one, counting whole years, months and days.
What is the "half your age plus seven" rule?
A rule of thumb that the youngest partner one "should" date is half one's age plus seven.
Does the age gap change over time?
No – the gap in time stays the same, but the ratio between the ages shrinks.
What formula does the Age Difference Calculator use?
Age gap = later date of birth − earlier date of birth (years, months, days).