Elevator Count Estimate for a 10,000-Occupant, 100-Story Tower
Company: Flipkart
Role: Product Manager
Category: Product / Decision Making
Difficulty: medium
Interview Round: Other
Estimate how many elevators a 100-story building needs if every floor holds 100 occupants.
The interviewer gives you nothing else. Choose the missing details yourself (what the building is used for, how large and fast the elevators are, when people travel), state each assumption as you make it, and walk through the calculation to a number you can defend.
```hint Size for the rush
The total number of rides in a day does not decide the answer. Find the busiest stretch of time, estimate how many people need a ride during it, and work out how many people a single car can move in that time.
```
### Clarifying Questions
- Is this an office tower, an apartment building, a hotel or a hospital? Each has a different rush pattern.
- Does "100 per floor" count individual people, or households or units that each hold several people?
- What service level should the design meet: a maximum wait in the lobby, or clearing the morning rush within a set time?
- Should freight and service elevators be counted, or only passenger cars?
### What a Strong Answer Covers
- Clarifying the building type and what the occupancy figure means, then stating every remaining assumption with a round number
- Identifying the busiest period as the design constraint and estimating demand within it
- Building one car's round-trip time from travel, stops, door and loading time, and turning it into people moved per hour
- Dividing demand by per-car capacity, then adjusting for how a 100-story building is laid out (zones, transfer lobbies), service cars and spare capacity
- A sanity check, a range rather than false precision, and the assumption the answer is most sensitive to
### Follow-up Questions
- How does the count change if the tower is split into zones served from transfer lobbies higher up?
- If the building is residential instead of offices, which assumptions change and in which direction?
- The developer wants fewer shafts to free up rentable floor space. How could you raise capacity without adding shafts?
Overview: A Flipkart product manager estimation question: how many elevators does a 100-story building with 100 occupants per floor need? It tests stating assumptions, sizing for peak demand rather than daily totals, working out one car's round-trip capacity, and sanity-checking the result.