Fermi estimate of total car parking capacity in San Francisco
Company: Google
Role: Product Manager
Category: Product / Decision Making
Difficulty: medium
Interview Round: Other
Estimate how many places to park a car there are in San Francisco, and walk through the assumptions behind your number.
```hint Split by where cars sit
Cars park at the curb, at home off the street, and in garages or lots at their destinations. Each kind grows with a different driver, so one formula for all of them hides most of the reasoning.
```
### Clarifying Questions
- Does the count cover only the city of San Francisco, or the wider Bay Area?
- Should private spaces such as home garages, driveways and employee garages count, or only spaces open to the public?
- Do unmarked stretches of curb where parking is legal count, or only marked and metered spaces?
- Cars only, or motorcycle spaces as well?
### What a Strong Answer Covers
- A defined scope (city limits, public versus private, marked versus informal curb) settled before any numbers
- A breakdown of the total into components, each tied to its own driver
- Round, labeled assumptions and arithmetic that can be followed step by step
- At least one sanity check against an independent anchor
- Which assumptions move the total most, and a plausible range rather than one precise number
### Follow-up Questions
- Which single assumption swings your total the most, and how could you pin it down cheaply?
- How would your approach change if the question were how many spaces sit empty at 6 pm on a weekday?
- If a maps product needed curb-space counts block by block for a parking-availability feature, how would you estimate them?
Overview: A product manager estimation exercise asking how many car parking spaces exist in San Francisco. It tests scoping what counts as a space, structuring the estimate into components with labeled assumptions, clean arithmetic, and sanity checks that turn a single guess into a defensible range.