Manholes Guesstimate – How to make more educated guesses

There is a common interview question that asks the candidate to estimate the amount of manhole covers in North America.

ontario manhole

This is a particularly nice interview question, as there are a dozen different ways to approach it, no clear right answer, and no easily searchable result online. Here I present my quick take on it, and some assumptions that could impact results. I will make every effort to avoid searching anything online (during my guess phase), and present purely my guesstimates for all values. To do this, I decided to try out a very nice online tool:

Manholes Method

To get an easily tweak-able, reasonable value, I decided to start with the population of North America. Taking the percentage of the population that lives in urban (or suburban) centers, I figure out how many people have sewer access. I then work from the other direction, and count the rough amount of sewer mainlines needed per person. Lastly, I make a wild guess (and this is the part people should take a peak at) that there are 1-5 manhole covers per mainline.


This gives me a rough answer of 22 million Manholes in North America, with reasonable lower to upper bounds of 19-46 million.

Manholes Sensitivity

The result above is very sensitive to final “Manhole Covers per Mainline” step. My reasoning to take a 1 to 5 guess is:

  • Each mainline should have at least 1 manhole to check for issues/do repairs
  • Each mainline would stretch at most 5 km, more that this and likely 2 or more mainlines would connect togeter
  • In the 5km case, a maintenance worker would not be expected to walk 2.5km underground, in full gear.
  • Ideally, the worker would have to walk a maximum of 1km. Hence, 5 covers for 5km mainline.

The other values and explanations are listed in the model. Take a look at the model here:

Manholes Guess & Check

The EPA estimates that there are about twenty million manholes in the United States [*]

If we take EPA’s 2012 estimate to be accurate, we are getting roughly 16 people per Manhole. This would give us (if we take US numbers to be fairly average for North America) about 29 million manholes in North America. This means my guess above turned out to be a fairly accurate, a 30% error!

Canadian Space Agency Astronaut – My Letter

Hello world,

This year, I had submitted my candidacy to the Canadian space agency for their 2016 recruitment drive. I had worked at the St.Hubert headquarters, and quite liked the environment and experience. Though I was not accepted (and had very little chance anyway 😉 ) I wanted to put up my cover letter. Here it is in its entirety.

To the Canadian Space Agency

I strongly believe that space exploration and its peaceful use are essential to the long term success of Canada. Living and working in space is not a goal in and of itself, it is instead the path towards better work and life for all Canadians. This can be seen not only from the economic benefits that space brings, but also from social and environmental benefits. Technologies developed for space exploitation are seen everywhere, from GPS in cell phones to remote monitoring of forestry. Space exploration has also been a driving force for improving the study of software and robotics, two technical fields that are revolutionizing modern industry. However, the societal impacts of our space program are just as important; they introduce the next generation of students to STEM fields. For Canada to remain competitive internationally, future generations must develop new technologies and entirely new disciplines. Lastly, development of space industries is an environmental responsibility; it allows for clean energy, efficient mining and agriculture research. Solar technologies, battery storage, efficient communication systems: these were all developed for space applications, and now contribute to reducing our carbon footprint.

Ever since reading The Norby Chronicles as a child, I have been fascinated with two things in life: robotics and space exploration. From building Lego as a child, to launching model rockets and making small walking robots, I have always had a passion for exploration and technology. My parents immigrated to Canada from Moscow in the early 1990s, looking for more opportunities in scientific research. Since my arrival in Canada, I have studied French and English, and spoken Russian at home. Bilingualism has been invaluable to me; on my first Co-op term from Waterloo University, I was given the chance to work with the satellite communications department at the St. Hubert station of the Canadian Space Agency. I spent my first term developing satellite tracking software for the CSA that is still in use today. In my second term, I assisted with the launch of an ESA satellite. Both experiences were amazing opportunities, showing the immense talent and co-operation of the CSA. My favourite part of each day at the CSA was walking by the control centre and seeing a team of highly talented people working together to accomplish a goal. The experience I gained there has been a great help in my robotics career since.

I have always thrived under pressure, working best under tight timelines and difficult goals. My passion for my work comes across in presentations, talks to the public, as well as open source projects I contribute to. In University, I often simultaneously worked with several start-up companies, completed my full course load, and found time to help student robotics teams. I have also won awards for technical speaking and presentations, a goal I set for myself to improve my public speaking. Lastly, in starting to manage engineering teams, I have had to take on leadership roles and responsibilities. This is new territory for me, however my department has received the highest personal satisfaction ratings in the company. I mention these characteristics purely to show that I am an excellent candidate for the position, as Canadian modesty and team spirit is something I take to heart.

The future of space exploration, and of manufacturing in Canada, will undoubtedly involve robotics. Employing my expertise will prove of great benefit to the Canadian Space Agency. I thank you for the opportunity to apply to be Canada’s next astronaut.

Best wishes,
Ilia Baranov

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