Oct 08, 2026 by Offer

OC&C Market Sizing 2027: Worked Example & Method

For the OC&C market sizing question "How many house moves happen in London per year?", use a top-down method: start with London's population, divide by average household size to get households, then estimate the share that moves each year. The method that works is population → households → moving rate, stating every assumption out loud and sanity-checking the final number.

What This Question Assesses in an OC&C Market Sizing Question

Market sizing is commonly reported by candidates as a frequent OC&C interview element, and it tests structured estimation under uncertainty. The interviewer does not expect the right number — they expect a logical breakdown, sensible assumptions, and clean arithmetic. Candidates commonly report that interviewers care more about how you handle not knowing something than about the final figure: do you make a reasonable assumption and move on, or freeze?

How to Answer This OC&C Market Sizing Question

Walk through the calculation step by step, narrating as you go.

Step 1 — Population: Start with London's population, roughly 9 million. State it as an assumption and note it may vary — precision here matters less than structure.
Step 2 — Households: Divide by average household size, roughly 2.4 people, giving about 3.75 million households.
Step 3 — Moving rate: Estimate the share of households that move in a given year. In a city like London, around 8–12% is a reasonable assumption — renters move more often than owners. Take 10% for round numbers.
Step 4 — Calculate and sanity-check: 3.75 million × 10% ≈ 375,000 moves per year. Then sanity-check: does that feel plausible for a city of 9 million? Roughly one in ten households moving yearly passes the smell test.

Example line: "I would start with London's population of about 9 million, convert to roughly 3.7 million households, and apply an annual moving rate of around 10%, giving approximately 370,000 moves — then I would sanity-check that against rental market intuition."

Common Mistakes With the OC&C Market Sizing Question

Chasing precision on unknowable inputs. Candidates commonly report wasting time debating whether the moving rate is 9% or 11%. Pick a defensible number, state it, and move on.
Skipping the sanity check. Always end by asking whether your answer is plausible. Catching your own absurd result impresses more than getting lucky.
Doing silent maths. Narrate every step. The interviewer is scoring your thinking, and silent calculation hides it — plus errors go uncaught.

Market sizing rewards comfort with ambiguity, which is why it appears so often. Practice five or six sizings aloud until the population-to-answer flow feels automatic, and the OC&C version will feel familiar.

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FAQ

What if I do not know London's population? Estimate it or ask. Interviewers commonly report that asking for one key fact is fine — what matters is what you do with it. Do not ask for every input, though.

Should I use top-down or bottom-up? Top-down (population → households → rate) is cleanest here. Bottom-up (estate agents × moves per agent) can cross-check your answer if time permits.

How do I handle the arithmetic? Round aggressively and keep numbers simple. 9 million ÷ 2.4 ≈ 3.75 million is fine — nobody expects long division in an interview.

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