Windows in New York City

Guesstimate questions are a staple of product management interviews. The actual number you arrive at is rarely the point. Instead, the interviewer wants to see how you think under uncertainty, how you structure your analysis, and how clearly you communicate your reasoning.

If you jump straight into math without clarifying the problem or making justified assumptions, you lose the interviewer’s confidence. The trap is to treat these questions like math quizzes. The actual job is to demonstrate your analytic thinking and communication skills.

Clarify the problem before you start calculating

Almost every guesstimate question is ambiguous by design. Talvinder often asks candidates, "How many flights fly out of Bangalore airport in a day?" or "How many washing machines sell in India per year?" The correct answer is unknown — no one has that number memorized.

The first step is to ask clarifying questions to reduce the scope and define the problem space. For example, if asked "How many windows are there in New York City?" you might clarify:

• Are we counting only building windows, or also windows in cars, subways, and other places?
• Are we including residential, commercial, and industrial buildings?
• Do we consider only occupied buildings or all structures?

These clarifications show that you are comfortable with ambiguity and want to align expectations. They also help you avoid wild guesses.

Make justified assumptions and explain them

You will never have perfect data. The skill lies in making reasonable assumptions with clear justifications. Talvinder stresses that vague or unsupported assumptions undermine your answer.

For example, in estimating windows in New York, you might assume:

• NYC population is 10 million (rounded for simplicity).
• Average household size is 2.5 people.
• Each residential unit has 6 windows on average.
• Commercial buildings have roughly 10 windows per employee.
• Office buildings have 15 windows per employee.

Explain why you chose these numbers. For instance, "I assumed 6 windows per residential unit because typical apartments have 2-3 rooms with 2 windows each." This shows you are not guessing blindly.

Your assumptions form the backbone of your solution. If you cannot justify them, the interviewer will doubt your analytic rigor.

Break the problem into smaller parts

Talvinder recommends the top-down or bottom-up approach to structure your answer.

• Top-down: Start from a large number (e.g., total population) and break it into segments (residential, retail, office), estimating windows per segment.
• Bottom-up: Start from small units (e.g., windows per apartment) and multiply by number of apartments.

Breaking down complex problems into smaller, manageable pieces makes your reasoning easier to follow and less error-prone.

For example:

• Residential windows = (population / household size) × windows per household
• Retail windows = number of retail employees × windows per employee
• Office windows = number of office employees × windows per employee
• Total windows = sum of all categories

Communicate your roadmap and calculations clearly

Your answer is a narrative — take the interviewer through your thought process step by step.

Talvinder advises you to:

1. State your clarifying questions and assumptions upfront.
2. Outline your roadmap: "I will estimate residential windows, retail windows, and office windows separately, then sum them."
3. Show intermediate calculations clearly.
4. Round off numbers to keep math manageable.
5. Offer caveats and acknowledge uncertainties.

For example:

"First, assuming 8 million people live in residential areas with 3 windows each, that gives 24 million residential windows. Retail has 1 million people with 4 windows each, totaling 4 million. Offices have 1 million people with 5 windows each, totaling 5 million. So, about 33 million windows in New York City."

After your calculation, offer thoughtful caveats:

"But this estimate might be low because many new buildings have glass facades with many more windows than older buildings."

This shows curiosity and critical thinking.

The goal is the process, not the number

Talvinder emphasizes: no one expects a precise answer. The interviewer is grading your structured thinking, assumptions, and communication.

If you get a number wildly off, but your logic is sound and assumptions justified, you score well.

If you rush to a number without structure or clarity, you fail.

Field exercise: Practice a guesstimate question

Common mistakes to avoid

• Skipping clarifications: Leads you to solve the wrong problem.
• Making vague assumptions: Weakens your credibility.
• Doing complex math mentally: Increases risk of errors; use pen and paper.
• Not communicating your process: Interviewers can't follow your logic.
• Ignoring caveats: Shows lack of curiosity and critical thinking.

Judgment exercise: Windows in New York City

From the field: Why clarity and structure matter

Where to go next

• If you want to sharpen your interview problem-solving skills: PM Interview Question Types and Approaches
• If you want to master user problem discovery: User Research Methods
• If you want to learn how to communicate clearly as a PM: Effective Communication for PMs
• If you want to practice more estimation questions: Practice PM Guesstimate Questions

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