How would you estimate the number of petrol stations in the UK?
Structured estimation — a clear method, sensible assumptions, sanity checks. The final number matters less.
Answer it out loud firstTake 30–60 seconds, as you would in the room. Then compare with the model answer.
Show the model answer
Work from demand: about 30 million cars, each filling up roughly once a week — around 30 million fills a week, or 4.3 million a day. A station with 8 pumps, each doing about 6 fills an hour over 15 hours, at 60% average utilisation handles roughly 430 fills a day. So about 4.3 million ÷ 430 ≈ 10,000 stations. Then sanity-check: one station per 6,500 people sounds plausible. The real figure is around 8,000.
The full answer
A method that works for any sizing question
- Clarify: petrol stations only, not charging points; the whole UK.
- Choose an angle: demand (cars and fills) or supply (stations per town), and say why.
- Break it down into numbers you can estimate.
- Do the arithmetic out loud, rounding generously.
- Sanity-check with another angle.
Demand side
| Step | Assumption | Result |
|---|---|---|
| Cars | 30m | |
| Fills per car per week | 1 | 30m a week |
| Fills per day | ÷ 7 | ≈ 4.3m |
Supply side
| Step | Assumption | Result |
|---|---|---|
| Pumps per station | 8 | |
| Fills per pump per hour | 6 | |
| Hours open | 15 | 720 at full capacity |
| Average utilisation | 60% | ≈ 430 fills a day |
Stations ≈ 4.3m ÷ 430 ≈ 10,000.
Cross-check
A town of 30,000 people might have 4–5 stations → one per 6,500 people → about 10,000 for 67m people.
Mistakes to avoid
Giving a number with no structure.
Calculating silently — the interviewer wants to hear the reasoning.
Follow-ups they may ask
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