Fermi estimate: sizing the global number of mobile handsets

Estimate the total number of mobile phones that exist worldwide. Lay out how you structure the estimate, label each assumption, and finish with a figure and how confident you are in it. ```hint Choose what you count Decide whether you will count people and the phones each one carries, or phones sold and how long each stays in use. Either way, keep phones in active use and phones that are idle or unsold in separate terms. ``` ### Clarifying Questions - Should the count include only phones in active use, or also old phones kept at home and new phones sitting in stores and warehouses? - Do basic feature phones count, or only smartphones? - Are cellular devices that are not phones, such as tablets, smartwatches and connected cars, outside the count? - How precise does the answer need to be: an order of magnitude, or within about 20%? ### What a Strong Answer Covers - A scoped definition of which devices count and whether idle phones count, settled before any arithmetic - A top-line equation that links population or sales to the number of phones - Labeled, round assumptions, each with a short rationale about who has a phone and how many - Arithmetic that is correct, unit-consistent and easy to follow - A sanity check from an independent angle, and a final figure with a range ### Follow-up Questions - Which single assumption moves your answer the most, and how would you narrow it? - How would your estimate change if you had to count only smartphones? - Using your answer, roughly how many phones would you expect to be sold worldwide each year?