How to use the age difference calculator
- Person 1 and Person 2 date of birth — enter them in either order. The calculator works out who was born first.
- Show both ages on — leave it on today to see current ages, or pick another date, such as a wedding day.
The headline result is the gap the way people say it: years, months and days. Below it you get the same gap as total days, weeks, months and decimal years, plus each person's age on the chosen date.
How the age gap is calculated
The difference is counted forward from the older person's birth date to the younger person's birth date, exactly like counting someone's age:
- Whole years — how many full years fit between the two birthdays.
- Whole months — the extra monthly anniversaries of the first birth date.
- Days — whatever is left after the last monthly anniversary.
When the first birth date is the 29th, 30th or 31st and a month is too short, that month is completed on its last day — so December 31 to March 31 is exactly 3 months. The decimal figure divides total days by 365.2425, the average length of a year in the Gregorian calendar.
Worked example
Person 1 was born on May 15, 1990 and Person 2 on August 20, 1995.
- Years: May 15, 1990 to May 15, 1995 is 5 years.
- Months: June 15, July 15 and August 15 follow — 3 months.
- Days: August 15 to August 20 is 5 days.
The age difference is 5 years, 3 months, 5 days. In days that is 1,923 (it includes February 29, 1992), which is 274 weeks and 5 days, or about 5.27 years. Person 1 is older.
Why "born in different years" can mislead
Subtracting birth years gives a rough answer that can be almost a full year off. Someone born December 31, 1999 and someone born January 1, 2000 are one day apart, yet their birth years differ by one. Two people born January 1 and December 31 of the same year share a birth year but are 364 days apart. When the exact gap matters — school cutoffs, sibling spacing, team age groups — use the full dates.
Age gap context: the gap is fixed, the ratio is not
The difference between two people's ages never changes, but how big it feels does. With a 5-year gap, a 10-year-old is twice as old as a 5-year-old; at 40 and 35, the same 5 years is about 14% of the older person's age. That's why gaps that look large in childhood barely register between adults.
For couples, people often quote the "half your age plus seven" rule: the younger partner should be at least half the older partner's age plus seven years. It's a social rule of thumb, often traced to Max O'Rell's 1901 book Her Royal Highness Woman, not a finding from research. For a 36-year-old the rule's minimum is 36 ÷ 2 + 7 = 25. The calculator shows it only when both people are adults on the chosen date.
For one person's exact age, use the age calculator; for any two dates that aren't birthdays, the days between dates calculator.
Frequently asked questions
How do I calculate the age difference between two people?
Count from the earlier birth date to the later one: first whole years, then whole months, then the leftover days. From May 15, 1990 to August 20, 1995 that is 5 years, 3 months and 5 days.
Does it matter which birth date I enter first?
No. The calculator always measures from the older person to the younger one and tells you who is older.
Is the age difference the same every year?
Yes. Both people get older at the same rate, so the gap in years, months and days never changes. Only the ages, and the ratio between them, change.
How is the decimal age difference worked out?
Total days apart divided by 365.2425, the average Gregorian year. 1,923 days ÷ 365.2425 ≈ 5.27 years.
How are twins born on different days handled?
Twins born either side of midnight have different birth dates, so the calculator shows a 1-day difference and names the twin born first as older.
Last updated: October 8, 2026