A friendship, measured
Old friends often say they have known each other “forever”, and after a while it starts to feel true. There is a precise moment when it becomes true in a mathematical sense: the day you have known each other for longer than you had lived before you met. For school friends that point usually arrives in your twenties; for friends made later in life it may be decades away. The two timelines above show your life and your friend’s, with the years you have shared shaded in between.
The circles underneath show the same thing as shares of each life. Because you and your friend were different ages when you met, the friendship fills a different slice of each of your lives, and the half-life date is different for each of you too.
How we calculate it
We count the calendar days from the day you met to today. For each person we then divide that by the days they have been alive, which gives the share of their life the friendship covers. The half-life date is when the friendship has lasted as long as the person had lived before it began: we take the age on the day you met and add the same amount of time again.
| Age when you met | Friendship passes half your life at |
|---|---|
| 5 | 10 |
| 12 | 24 |
| 18 | 36 |
| 25 | 50 |
| 40 | 80 |
Worked example. Two friends met on 4 September 2002, the first day at a new school. One was born on 12 April 1990 and was 12 years and 145 days old; the other, born on 3 September 1989, had turned 13 the day before. By 30 October 2026 they had been friends for 8,822 days. The younger one passed the half-life point on 27 January 2015, the older one on 5 September 2015.
Why it matters
Long friendships are one of the most reliable sources of happiness people report, and keeping them going takes small, regular effort. A friendship milestone, like 10,000 days or the half-life date, is a lovely excuse to get in touch, especially with a friend who lives far away.
Questions people ask
What does “known them for half my life” mean here?
It is the day when the time since you met equals the time before you met. After that date, you have known your friend for more of your life than not. The page works it out for both of you.
What if I do not know the exact day we met?
Pick a date that is close, such as the first day of school or the start of a job. The percentages barely change if you are a few weeks out.
How many close friends do people usually have?
Research by the psychologist Robin Dunbar suggests most people have about five very close friends and around 15 good friends, within a wider circle of about 150 meaningful relationships.
Is anything saved?
Your own birthday is remembered on this device so other pages can use it. Your friend’s details are kept only while the page is open.
Sources. Robin Dunbar, How Many Friends Does One Person Need? (2010). Calculations run in your browser; nothing you enter is sent anywhere.