Age Difference Calculator
Enter two birth dates to see the exact age gap in years, months and days, who is older and how old each is now. Advanced adds up to six people, birth times and fun extras.
Today, unless you change it. The gap itself never changes.
Only matters when the older person was born on 29 February.
Birth times count only when both people in a pair have one. Use the same time zone for both.
How your age difference is worked out
Start from the older person's birthday and count forward to the younger one's: whole years, then months, then days.
- 1
Who is older
Alex: 12 Mar 1988
Sam: 25 Jul 1993
Alex is older - 2
Whole years
12 Mar 1988 →
12 Mar 1993
= 5 years - 3
Months, then days
+4 mo → 12 Jul
+13 d → 25 Jul
= 5y 4m 13d - 4
Total days apart
25 Jul 1993 −
12 Mar 1988
= 1,961 days
Read more: the formula, a worked example and 29 February birthdays
The age difference is the older person's age on the day the younger one was born. It is counted the way people say ages: whole years first, then whole months, then the days left over. The gap never changes, because both people age at the same speed.
The formula
years = whole years from the older birth date to the younger one
months = whole months after that
days = days left over
total days = younger birth date − older birth date
twice as old on = younger birth date + total days
% = younger's days lived ÷ older's days lived × 100
Worked example
Alex was born on 12 March 1988 and Sam on 25 July 1993. From 12 March 1988 to 12 March 1993 is 5 years. From 12 March to 12 July 1993 is 4 months, and from 12 July to 25 July is 13 days. So Alex is 5 years, 4 months and 13 days older (64 months in all).
In days: the 5 years hold one 29 February (1992), so they are 5 × 365 + 1 = 1,826 days. Then 19 days are left in March, plus 30 (April) + 31 (May) + 30 (June) + 25 (July) = 135 days. 1,826 + 135 = 1,961 days, which is 280 weeks and 1 day.
Alex was exactly twice Sam's age 1,961 days after Sam was born: on 7 December 1998, when Sam was 1,961 days old and Alex 3,922.
Born on 29 February
When the older person was born on 29 February, the count of whole years needs an anniversary in years that have no 29 February. The default uses 28 February, which is what most people do; some laws use 1 March, and you can choose that in Advanced. For example, 29 February 2000 to 28 February 2001 is exactly 1 year with the 28 February rule, but 11 months and 30 days with the 1 March rule. The total of 365 days is the same either way. If the younger person was born on 29 February, nothing special is needed: it is just the end date.
Months of different lengths
When the starting day doesn't exist in a month (say the 31st in a 30-day month), that month counts as complete on its last day. So 31 October to 31 January is exactly 3 months, or 92 days.
Birth times and twins
When both people have a birth time, the gap is worked out to the minute, and the last day only counts once it is complete. Twins born on the same day are ordered by time, so 09:02 and 09:20 on the same date gives a gap of 18 minutes.
The extras
| Extra | How it's found |
|---|---|
| Exactly twice as old | Younger birth date + gap in days |
| Younger reaches older's age | The younger person's birthday for the older person's current age in whole years |
| % of life | Younger's days lived ÷ older's days lived, on the "Ages on" date |
| Half your age + 7 | Older's whole years ÷ 2, rounded down, + 7. A popular rule of thumb, not a rule |
| Star sign | The usual Western date ranges (for example Leo 23 July to 22 August) |
| Chinese zodiac | The animal and element of the Chinese year, which starts at Chinese New Year (21 January to 20 February), not 1 January |
| Generation | Commonly used ranges (Pew Research Center): Baby Boomers 1946 to 1964, Gen X 1965 to 1980, Millennials 1981 to 1996, Gen Z 1997 to 2012; Gen Alpha from 2013 is informal. Definitions vary. Checked 14 October 2026. |
Common questions
How do you work out the age difference between two people?
Start from the older person's birth date and count forward to the younger person's: first whole years, then whole months, then the days left over. For 12 March 1988 and 25 July 1993 that is 5 years (to 12 March 1993), 4 months (to 12 July) and 13 days, or 1,961 days in all.
Does the age difference change over time?
No. The gap in days between two people stays the same for life, because both get older at the same speed. What changes is how big it feels: the younger person's age as a share of the older's gets closer to 100% every year.
When was one person exactly twice as old as the other?
On the day the younger person was as old as the gap. Add the gap in days to the younger person's birth date. With a gap of 1,961 days and a birth on 25 July 1993, that is 7 December 1998, when one was 1,961 days old and the other 3,922.
How are 29 February birthdays counted?
If the older person was born on 29 February, their 'anniversary' in other years is taken as 28 February by default, which is the usual choice. Some laws use 1 March instead, and you can switch to that in Advanced. The total in days is the same either way.
Can I compare more than two people?
Yes. In Advanced you can add up to 6 people, for example a whole family. They are sorted from oldest to youngest and you get the gap between every pair, plus the gap between the oldest and the youngest.
What is the half your age plus seven rule?
It is a popular rule of thumb (not a rule or a law) that says the youngest partner a person 'should' date is half their age plus seven years. It is shown in Advanced only for interest, when the older person is at least 14.
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