Guesstimates often form a key part of case interviews, especially in consulting and business analytics roles. Among the most common types are population-based guesstimates—questions that require you to estimate quantities or behaviors of people within a geographic or demographic group.
Think questions like:
“How many people in Delhi buy a burger every day?”
“Estimate the number of newborn babies in India per year.”
“How many people in London wear spectacles?”
These questions may appear daunting at first, but once you understand the structured approach to answering them, they become both manageable and enjoyable.
In this blog, we’ll walk through how to tackle population-based guesstimates with confidence.
Step 1: Clarify the Question
Before jumping into calculations, make sure you fully understand what is being asked.
For example:
“Estimate how many gym memberships are sold annually in Mumbai.”
Ask yourself:
Are we talking about unique memberships or total transactions?
Are online fitness platforms included?
Should we account for seasonality?
Clarifying the scope upfront not only improves accuracy but also showcases structured thinking.
Step 2: Break It Down into Layers
Population-based guesstimates are best tackled using a top-down approach. Start with the total population and apply logical filters or assumptions.
Example Question:
How many people in Bangalore go to the cinema every month?
Let’s break it down:
Population of Bangalore: ~13 million
Age filter: Let’s assume 60% are in the cinema-going age group (15–60) → 13M × 0.6 = 7.8M
Urban lifestyle assumption: Maybe only 50% of them enjoy entertainment outside the home → 7.8M × 0.5 = 3.9M
Cinema preference vs. other entertainment: Assume 40% prefer cinema over dining, shopping, etc. → 3.9M × 0.4 = 1.56M
Frequency per month: Let’s say the average moviegoer visits once a month
So, Estimated Monthly Viewers: ~1.56 million
Step 3: Make Reasonable Assumptions
You don’t need exact statistics. Interviewers are more interested in your assumptions and logic than the final number.
Here’s how to craft good assumptions:
Use round numbers for ease: If India’s population is ~1.4 billion, round it to 1.4B or even 140 crores.
Use ratios and percentages: 10% of India’s population = 14 crores.
Benchmark from known values: If you know your city has 5 million people and similar cities are being discussed, use that as a reference.
Pro Tip: Always justify your assumption. For instance, “I’m assuming 60% of the population is of working age because that’s consistent with India’s demographic profile.”
Step 4: Validate with Sanity Checks
Always ask: Does this number make sense?
If you estimated that 5 million burgers are sold daily in a city of 2 million, something’s off.
Validate your result by:
Comparing with smaller known entities
Asking if the result fits in with existing behavior (Would that many theaters exist to support this volume?)
Here’s a solved example:
Question: “Estimate how many people in Delhi eat pizza at least once a week.”
This sounds complex, but let’s solve it using 5 structured steps.
Step 1: Clarify the Scope
Ask clarifying questions—even in your own mind:
Does this include only those who order from restaurants or also frozen pizza at home?
Are we looking at city limits or NCR (National Capital Region)?
Assumption: We’ll focus on Delhi city (urban area) and include all kinds of pizza (dine-in, delivery, frozen, etc.).
Step 2: Start with the Base Population
Let’s begin with the total number of people in Delhi.
Approximate population of Delhi (urban area): 20 million
Step 3: Narrow It Down Using Logical Filters
We now apply step-by-step filters to zoom in on our target group.
Age group that’s likely to eat pizza: Let’s say people aged 10–50 years are the most active consumers.
Assume 60% of Delhi’s population falls in this group → 20 million × 0.6 = 12 million
Income and affordability: Eating out or ordering pizza is more common in middle to upper-income households.
Assume 50% of the above group can comfortably afford weekly pizza →
12 million × 0.5 = 6 million
Preference for pizza: Not everyone likes pizza or eats it regularly.
Let’s say 25% of the filtered population eats pizza at least once a week →
6 million × 0.25 = 1.5 million
Step 4: Validate the Result
We estimate that 1.5 million people in Delhi eat pizza at least once a week.
Is that reasonable?
Let’s cross-check:
Delhi has hundreds of pizza outlets (Domino’s, Pizza Hut, local joints).
If each outlet serves 300–500 customers a day, and there are ~1000 such outlets, that’s roughly 300,000–500,000 customers per day.
That supports our weekly estimate of 1.5 million unique consumers. Our estimate seems plausible.
Step 5: Present Clearly and Confidently
When presenting your answer in an interview or on paper, structure it as follows:
“Delhi’s population is roughly 20 million. If 60% are aged between 10–50, that’s 12 million. Of these, around half can afford pizza regularly, leaving 6 million. Assuming one in four eats pizza at least once a week, the final estimate is 1.5 million people per week.”
Final Thoughts:
Guesstimates are not about getting the exact number—they’re about how you approach the problem.
Next time someone throws a wild question at you, smile—you’ve got the structure to solve it.
