Interview Questions › Guesstimate

Estimate the number of tube lights in a city like Bangalore

Estimate the total number of tube lights (fluorescent/LED tube fixtures) installed across the city of Bangalore, India.

Context

Consider residential homes, commercial office spaces, educational institutes, street lighting, and retail establishments.

Clarifying questions to ask first

• Are we counting all tube fixtures or only active/working ones?
• Does this include residential, commercial, and industrial?

How to approach it

1. State baseline population assumption for Bangalore (~13-14 million people)
2. Segment into Residential, Commercial, Industrial, and Public Infrastructure fixtures
3. Calculate average household size and tube lights per household
4. Estimate commercial space square footage and light fixture density per sq ft
5. Perform quick sanity check and state top 3 sensitivity assumptions

Framework: Total Tube Lights = Residential + Commercial (Offices/Retail) + Institutional (Schools/Hospitals) + Public Streetlights

Sample answer

• Residential: ~3.5M households × 6-8 lights = ~25M
• Commercial Offices & IT Parks: 1M desk workers × ~2 fixtures = ~10M
• Retail & Schools & Hospitals: ~8M
• Streetlights: ~0.8M

1. Bangalore Pop: 14M -> 3.5M homes -> Avg 3-room home has 6-8 tube lights = ~25M residential
2. Commercial: 200M sq ft office/retail space @ 1 tube light per 25 sq ft = ~8-10M
3. Total estimate: ~40-45 Million tube lights

Executive estimate: ~42 Million tube lights in Bangalore; ~60% residential, ~30% commercial/retail, ~10% public/industrial.

What interviewers look for

• Population Base Anchor
• Household & Room Density
• Commercial Floor Space Sizing
• Sanity Check & Sensitivity Analysis