Estimate the worldwide installed base of iPhones versus Android phones
Company: Google
Role: Product Manager
Category: Product / Decision Making
Difficulty: medium
Interview Round: Other
Estimate how many iPhones and how many Android phones there are in the world. For this estimate, assume that each person has only a single phone. Walk through your equation, the assumption behind each number, and a final figure for each platform.
```hint Split before you multiply
Both owning a smartphone and choosing an iPhone over an Android phone depend heavily on income and country, so one global percentage for each would hide most of the reasoning. Decide how to divide the world's population first.
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### Clarifying Questions
- Does "one phone each" mean that every person on Earth, young children included, owns a phone, or only that nobody who owns a phone carries a second one?
- Are we counting phones in active use, or every iPhone and Android phone still in existence, including old devices kept in drawers?
- How should basic feature phones and smartphones on other operating systems be treated: left out, or folded into one of the two platforms?
- Do Android-based phones sold without Google's apps and services, which are common in some markets, count as Android?
### What a Strong Answer Covers
- A clear equation that runs from world population to phone owners to the split between the two platforms, applying the one-phone assumption explicitly.
- Segments, such as income level or region, chosen because smartphone adoption and iPhone share differ sharply between them.
- Explicit treatment of people who own no smartphone and of phones that are neither an iPhone nor Android.
- Round, labeled assumptions with correct arithmetic, and a sanity check against figures the candidate already knows.
### Follow-up Questions
- Which single assumption moves your iPhone figure the most, and what public data would you use to tighten it?
- If we drop the one-phone assumption so that people can own both a work phone and a personal phone, how does each total change?
- How would you turn your installed-base estimate into an estimate of how many iPhones are sold each year?
Overview: A market-sizing estimate that asks how many iPhones and Android phones exist worldwide when each person is treated as having just one phone. It tests segmenting the population, making adoption and platform-share assumptions, doing clean arithmetic, and sanity-checking the final figures.