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Google·Product Manager·Onsite - Product Sense / Strategy·Intermediate

Intermediate
Apr 2026

Summary

Google PM interview with a classic estimation question about elevators in a skyscraper. Pretty short session from what I can tell, just the one question but it required more structured thinking than I expected going in.

Questions Asked (1)

Q1

How many elevators would a 100-floor building need?

Product Sense & IdeationAdaptability & Ambiguity
Author's notes

I jumped straight into a number and then had to backtrack, which felt messy.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the context—building type (office, residential, mixed-use), peak traffic patterns, and elevator technology—since these drastically change the answer. Then walk through a structured estimation using assumptions about population, arrival rates, and elevator capacity, and conclude with a recommended number plus key trade-offs.

Pro tip: Explicitly state that the answer depends on assumptions and that you'd validate them with real data (e.g., building blueprints, traffic studies) before finalizing; this shows you know when to stop estimating and start measuring.

1. Clarify the scenario

Ask questions to define building type, occupancy, peak usage times, and any constraints (e.g., budget, space, energy). This ensures your estimate is relevant.

2. Estimate demand

Calculate the number of people needing elevators during peak periods (e.g., morning rush) based on floor count, area per floor, and occupancy density.

3. Determine elevator capacity

Estimate how many people one elevator can transport per minute, considering speed, capacity, and round-trip time. Use industry benchmarks if needed.

4. Calculate required elevators

Divide peak demand by per-elevator capacity to get the minimum number, then round up and add a buffer for redundancy and maintenance.

5. Sanity-check and refine

Compare your result to real-world examples (e.g., similar buildings) and discuss trade-offs like cost, space, and wait times to validate the answer.

Key Points to Mention

  • Building type and usage patterns (e.g., office vs. residential) drastically affect elevator needs.
  • Peak traffic analysis (e.g., morning up-peak) is the critical design factor.
  • Elevator capacity depends on speed, size, and round-trip time; use standard industry metrics like handling capacity.
  • Assumptions must be clearly stated and justified; sensitivity analysis shows robustness.
  • Real-world constraints: space, cost, energy efficiency, and redundancy for maintenance.
  • Validation with data or benchmarks from similar buildings ensures credibility.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.