How many times do you forget a stranger?

  1. How many people does Mike walk past every day?
  2. How many people does Mike pass on more than one occasion?
  3. How many people can Mike remember in total?
Photo by Ingo Joseph from Pexels

How many people does Mike walk past every day?

Let’s set a fairly realistic scene. Mike lives and works within a bustling neighborhood situated in a large city. We’ll call this neighborhood Fermitown. It’s roughly one square mile and holds about 20,000 residents.¹ Rather conveniently for this analysis, Fermitown is perfectly square, spanning twelve blocks in each dimension.² In total, Fermitown has 144 blocks and 576 sidewalks.

How many people does Mike pass more than once?

According to this model, Mike walks past 240 residents, or 1.2% of Fermitown’s population, every day. We can use that to approximate a 1.2% chance of seeing a specific resident on the sidewalk on any given day. After one year, Mike will have seen about 93% of the population more than once.⁵ After two years, assuming residents move every nine years, Mike will have seen about 21,900 faces at least twice, and a quarter of those faces at least ten times.

How many people can Mike remember?

Photo by jesse orrico on Unsplash

Putting it all together

After two years of living in Fermitown, Mike has unknowingly passed by at least 16,900 people on more than one occasion.⁶ Assuming facial recognition capacity is limited to 5,000 faces, he would continue passing the rest of his neighborhood’s citizens multiple times without being the wiser. In reality I think Mike might perform much better than we’ve estimated. But even if he recognized 50% of the people he passed, that would still leave thousands unrecognized.


  1. Using San Francisco as a guide, the average population per square mile is a little over 18,000. However, most neighborhoods have higher population densities.
  2. I’m assuming a block length of 350 feet and street widths of 60 feet, using San Francisco as a guide once again.
  3. Americans don’t seem to walk much at all.
  4. It also implies that over half of the neighborhood’s residents are walking at any given time, which sounds less reasonable.
  5. I originally tried a naive equation of 1-.988³⁶⁵ but it didn’t match my (most likely more accurate) simulation. In the meantime I’m going to look into a better way to model this distribution.
  6. Remember this number only includes residents.



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Zachary Kitt

Zachary Kitt

Writer of code. Interested in data-driven policy. Graduate of @JacksonYale and @UCSBGlobal.