Most of the "estimate foo" problems are Fermi problems. E.g. The classic one I've heard is "estimate the number of piano tuners in the world". The route I took

Most of the "estimate foo" problems are Fermi problems. E.g. The classic one I've heard is "estimate the number of piano tuners in the world". The route I took to do so is: number of people in the world / average number people per piano * average number of times a piano is tuned per year * average amount of hours to tune a piano / average number of hours a piano tuner works per day / average number of days a piano tuner works per year So in terms of units, the flow is: people -> pianos -> piano tunings per year -> hours spent tuning pianos per year -> workdays spent tuning pianos per year -> piano tuners This is all very similar to dimensional analysis most people will learn in a highschool science class. Just to plug in some random numbers, let's pick 7 billion people, 100 people per piano, 0.3 tunings per year, 1 hour to tune a piano, 3 hours of tuning per day and 100 days per year(I imagine it's part-time for most piano tuners...). We arrive at: 7,000,000,000 / 100 * 0.3 * 1 / 3 / 100 == 70,000 And, for what it's worth, at least in my field, we do these sorts of ballpark estimations all the time to determine feasibility of systems. I've never had one of these in an interview setting, but I imagine the important thing is to demonstrate the line of reasoning rather than actually arriving at a concrete number.