# How to calculate probability of seeing someone you know

1. Jun 15, 2010

### moonman239

I just thought I'd post how to calculate the probability that you'll meet someone you once knew. Now, bear in mind that there are many factors that come into play (such as a reunion-ish event taking place), thus making it very hard to accurately calculate said probability. On we go:

First, we find the estimated number of people in the US who went to high school about the same time you did. For our example, I will say 17 billion. Call this pUS.

Next, we find the estimated number of people in the area in question who went to high school about the same time you did. Call this parea.

Next, we calculate how many people will have visited the area by the time you leave, not counting repeat visitors (or if you really want, how many people from the given age group will have visited the area by the time you leave). Call this v.

Add parea to v. Divide the sum by pUS. The resulting number is the probability that we wish to estimate.

Last edited: Jun 15, 2010
2. Jul 24, 2017 at 3:45 AM

### zul8tr

What about this case that happened to me? I lived in West Palm Beach at the time. Was traveling in Salzberg Austria and on a lift to a castle. There were about 75 people on the lift from all over speaking their language not English. I need help and shouted does anybody speak English? A person across the crowded lift said I do. Turned out he was my neighbor across the yard. We hardly ever spoke when I lived there and never discussed travel plans. Probability of this chance meet?

3. Jul 25, 2017 at 7:22 PM

### stoomart

My guess for meeting any one specific person in a random part of the world would be no greater than 1 in 7.5 billion (world population).