How to use the age gap calculator
Type in both birth dates. Names are optional and only change the labels in the result, so you can see "Sam" and "Priya" instead of Person 1 and Person 2. It doesn't matter which date you enter first. The calculator works out who was born earlier on its own.
The second step sets the date for comparing both ages. It starts on today's date, but you can change it to a wedding day, a photo date or any day after both births. The gap between the two birthdays never changes. The ages on that date do.
- Enter a name (optional) and birth date for the first person.
- Enter a name (optional) and birth date for the second person.
- Keep today's date or pick another date to compare both ages.
- If either birthday is February 29, choose whether it moves to February 28 or March 1 in other years.
- Press Compare ages to see the gap, each person's age and the half-plus-seven check.
Age difference example: 1990 and 1998 birthdays
Take April 10, 1990 and September 25, 1998. Counting forward from the earlier date, eight birthdays take you to April 10, 1998. Five more months reach September 10, 1998, and 15 days are left. The age gap is 8 years, 5 months and 15 days, or 3,090 days.
On August 10, 2026, the older person is 36 and the younger one is 27. Their whole-year ages differ by 9 even though the real gap is only 8 years and 5 months. That happens for part of every year, between one person's birthday and the other's.
Age gap = full years + full months + remaining days, counted from the earlier birth dateDays apart = later birth date - earlier birth date (in calendar days)Weeks apart = days apart / 7, with the remainder as extra days
| Measure | Older person | Younger person | What it means |
|---|---|---|---|
| Birth date | April 10, 1990 | September 25, 1998 | The 1990 birth is older |
| Age on August 10, 2026 | 36y 4m 0d | 27y 10m 16d | Both measured on the same day |
| Exact age gap | 8y 5m 15d | 3,090 days | Fixed for life |
| Whole-year ages | 36 | 27 | Differ by 8 or 9 depending on the month |
The half your age plus seven rule
Half your age plus seven is an old rule of thumb for the youngest partner people consider socially acceptable. Divide the older person's age by two and add seven. A 40-year-old's minimum would be 27. Flip it around and the oldest partner for someone is their age minus seven, doubled.
The idea is far older than the internet. Max O'Rell wrote "a man should marry a woman half his age, plus seven" in Her Royal Highness Woman (1901), and an earlier version appears in Frederick Locker-Lampson's Patchwork (1879). A 2001 study by Buunk and colleagues found it fairly close to the youngest age men said they would date, but not a good match for women's answers.
The calculator applies the rule to both people's whole-year ages on the comparison date and tells you whether the younger person is at or above the minimum. It's a conversation piece, not a measure of whether a relationship is healthy, and it says nothing about legal age.
| Older person's age | Youngest partner by the rule | Oldest partner by the rule |
|---|---|---|
| 20 | 17 | 26 |
| 25 | 19.5 | 36 |
| 30 | 22 | 46 |
| 40 | 27 | 66 |
| 50 | 32 | 86 |
Why the age gap in years changes during the year
Two people born 8 years and 5 months apart will usually be described as 8 years apart. For a few months each year, though, their ages differ by 9. That window opens on the older person's birthday and closes on the younger person's birthday.
Nothing about the real gap changed. Whole-year ages just drop the extra months and days, and they drop them at different points in the year for each person. That's why this page shows the exact gap first and the whole-year difference on the chosen date separately.
February 29 birthdays
Someone born on February 29 has no birthday in three years out of four. Pick February 28 or March 1 as their birthday in common years and the calculator uses that choice for the gap, both ages and the next birthday.
Leap years follow the Gregorian rule documented by the U.S. Naval Observatory: years divisible by 4 are leap years, except century years that aren't divisible by 400. Laws and programs don't all agree on when a leap-day baby turns a year older, so check the rule that applies to you when it matters.
When an exact age difference matters
Most people use an age difference calculator out of curiosity: siblings, partners, friends, a parent and child, or a family tree. Setting the comparison date to a past event shows how old both people were at a wedding, a graduation or in an old photo.
School entry, sports age groups, benefits and legal age all have their own rules about cutoff dates and documents. Use this result to check the calendar math, then follow the official rule for the decision itself.
- Sibling, partner and family age gaps in years, months and days
- Both ages on a wedding, graduation or photo date
- Total days and weeks between two birthdays
- Who is older when two people were born close together
- The half-your-age-plus-seven check for a couple
Sources
These references back up the age definitions, leap-year rule and the history of the half-plus-seven rule used on this page.