What a market sizing case framework is really testing
Market sizing cases test structured estimation under incomplete information. The interviewer is rarely looking for a hidden correct number. They want to see whether you can translate an ambiguous market into a logical equation, select material assumptions, calculate accurately, and interpret the result.
Top-down methods begin with a broad population or spending pool and narrow it with filters. Bottom-up methods begin with a unit such as stores, households, transactions, or productive capacity and build upward. Either can work; the best choice is the one supported by the prompt and easiest to validate independently.
A step-by-step method for a market sizing case framework
Lock the scope first, build one clean equation, calculate in manageable pieces, and then challenge the answer before using it.
Define the market
Confirm product, buyer, user, geography, time period, revenue or volume, and whether substitutes, business buyers, or ancillary purchases count.
Choose the sizing route
Select a demand-based, spending-based, supply-based, or capacity-based equation and explain why it fits the available information.
Build and populate the tree
Break the equation into non-overlapping drivers, state assumptions with brief rationale, and use segments only where behavior differs materially.
Calculate with units
Round sensibly, write units at every stage, separate annual and monthly figures, and narrate checkpoints so errors can be corrected quickly.
Validate and interpret
Cross-check with a second route, benchmark, penetration ceiling, or capacity limit; then state the result, range, sensitivity, and implication.
Worked example: a market sizing case framework
Follow the reasoning, then rebuild it for a different industry instead of memorizing the wording.
Annual home-charger installations
1Eligible households
Population ÷ household size × suitable parking
- 2 million people
- 2.5 people per household
- 45% with suitable parking
2EV demand
Eligible households × EV adoption or annual purchases
- Current EV penetration
- Annual adoption growth
- New EV buyers
3Charger need
EV households × share needing a new installation
- No charger installed
- Replacement demand
- Installer-capacity cross-check
What good looks like
- A precise market definition before arithmetic
- A compact driver tree with explicit units
- Assumptions justified by anchors rather than false precision
- A cross-check, range, sensitivity, and decision implication
Sizing mistakes
- Sizing an undefined market
- Mixing households, people, transactions, and revenue
- Creating too many segments that do not change the answer
- Stopping after the number without checking or interpreting it
Practice market sizing
Build one demand-side and one supply-side tree for the same market
Practice percentage and large-number arithmetic aloud
Complete estimates with a five-minute time limit
End every estimate with a range, sensitivity, and business implication
If voice AI isn’t part of your case prep yet, it should be.
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Frequently asked questions
Should I use a top-down or bottom-up market sizing framework?
Use the route that matches the information and produces the fewest fragile assumptions. When time permits, use the other route as a cross-check rather than averaging two weak estimates.
How many segments should a market sizing case include?
Only segment when groups have meaningfully different penetration, frequency, price, or eligibility. Extra detail that does not change the answer creates arithmetic risk without adding insight.
What should I say after calculating the market size?
State a rounded result and reasonable range, name the assumption that drives the range, compare the result with a sanity check, and explain what it means for the client decision.
Sources and further reading
- Interviewing at McKinsey: McKinsey & CompanyPrimary source for McKinsey recruiting and interview guidance.
- Consulting interview process: Boston Consulting GroupPrimary source for BCG interview stages and evaluated capabilities.
- Interviewing at Bain: Bain & CompanyPrimary source for Bain case-interview expectations and sample cases.