Fermi estimation case: sizing the airport count of the United States and the globe

Estimate how many airports there are, first in the United States and then across the entire world. Walk through how you structure each estimate, label your assumptions, and finish each part with a figure and a range. ### Clarifying Questions - What counts as an airport: only airports with scheduled airline service, every public-use airfield, or every landing site including private strips, heliports and seaplane bases? - Should military air bases be included? - Is an order-of-magnitude answer enough, or should the figure be within a factor of two? ### Part 1 — The United States Estimate the number of airports in the United States. ```hint Count something you can picture Settle the definition of an airport first, then choose a unit you can count with some confidence and estimate how many airports each unit supports. ``` #### What This Part Should Cover - The definition of "airport" in use, stated before any arithmetic - A top-line equation built on a countable unit - Labeled assumptions, arithmetic that checks out, and a range ### Part 2 — The world Extend your estimate to the whole world. ```hint Do not copy the US Air travel is spread very unevenly across countries; decide which differences between countries change airport density the most before you scale up. ``` #### What This Part Should Cover - A way to scale beyond the US that reflects how much countries differ in air travel - Assumptions for every piece of the world estimate, with totals that add up - A cross-check of the world figure against the US result ### What a Strong Answer Covers - One definition of "airport" applied consistently across both parts, or a deliberate, stated switch - A structure for each part that can be stated in a single sentence - Round, labeled assumptions, each with a short reason grounded in how air travel works - Correct, unit-consistent arithmetic with a sanity check from a second angle - Final figures with ranges, and how they shift under the other definitions of an airport ### Follow-up Questions - Which single assumption moves your world figure the most, and how would you tighten it? - Where might your method undercount or overcount, and how would you adjust for it? - What public data would you use to check your answers, and what would you watch for in how that data defines an airport?