Practice drill · Problem Solving & Consulting · Foundation

Market sizing, step by step: estimate anything in five moves

How many cups of coffee does a campus sell in a day? Estimation questions test structure, sensible assumptions and clean arithmetic, not trivia. Here is the method.

Metanoia Editorial · 11 Oct 2026 · 8 min

Market sizing, step by step: estimate anything in five moves

What is happening

A market-sizing question asks you to estimate a number with little or no data: the size of a market, the revenue of a shop, the number of something in a city. Nobody expects the true figure. The interviewer watches how you break the number into parts, which assumptions you choose, whether you state them clearly, how cleanly you calculate, and whether you check the result against common sense.

There are two broad approaches. Top-down starts from a large known population and narrows it: people, then the share who buy, then how often, then at what price. Bottom-up starts from one unit and scales it: one shop’s capacity, times the number of shops. Strong candidates often do one and use the other as a sanity check.

Why it matters

Estimation is the most portable consulting skill. Business plans, budgets and every “is this worth doing?” conversation start with a number nobody has measured yet.

The framework

Five moves for any estimate

Clarify the unit and scopePer day or per year? Units or rupiah? Which geography? Ask before you calculate.
Write the equation firstSay it aloud: cups per day = people on campus × share who buy × cups per buyer.
Choose round, defensible assumptionsExplain each in one phrase. Round numbers make the arithmetic safe.
Calculate step by step, aloudWrite intermediate results. Track zeros explicitly.
Sanity-check and state the so-whatCompare with a second method or a known anchor, then say what the number means for the decision.

Worked example

Coffee sold on one campus per weekday

Equation: cups per weekday = people on campus on a weekday × share who buy coffee on campus × cups per buyer.

Assumptions: the university has 20,000 students and 2,000 staff; about 60% are on campus on a given weekday, so roughly 13,000 people. Perhaps one in four buys coffee on campus, about 3,250 people, and buyers average 1.2 cups. Estimate: about 3,900 cups, call it 4,000 per weekday.

Sanity check, bottom-up: if the campus has 10 coffee points and each serves about one customer a minute for six busy hours, that is 360 cups each, or about 3,600 in total. The two methods agree within roughly 10%, which is good enough. So-what: a new kiosk aiming for 400 cups a day would need about a tenth of the whole campus market, which is ambitious unless it has a clearly better location.

All figures are illustrative assumptions for practice, not data about any real campus.

What this means for you

StudentPractise one estimate a day out loud on your commute. Speed comes from repetition, not from memorising facts.

Fresh graduateLearn a few anchors you can defend: Indonesia’s population is roughly 280 million; a working year has about 250 weekdays. Anchors make assumptions credible.

Early professionalUse the same method for internal proposals. A sized opportunity with stated assumptions gets approved faster than an adjective.

What to do

Memorise five anchors: national population, households, working days, hours in a year, and one price you know well.
Always write the equation before any number. Interviewers forgive a rough assumption, not a missing structure.
Run every estimate twice, top-down and bottom-up, until checking becomes a habit.
End every estimate with one sentence on what the number means for the decision.

Practice drill

Ride-hailing trips by one student body8 min, timed

Estimate how many ride-hailing motorcycle trips the students of one university with 30,000 students take in a typical teaching week. State the equation first, then your assumptions, then a sanity check.

Segment by how students live: near campus, far from campus, with their own vehicle.
Count trips per user per week, not per day.
Check against a bottom-up view of the campus gates.

Show a model answer

Equation: weekly trips = students × share who use ride-hailing × trips per user per week.

Segments (illustrative): 40% own a motorcycle and rarely use the service; 35% live in boarding houses near campus and use it about twice a week for errands or rain; 25% live farther away and use it for about four trips a week. Users: 35% × 30,000 = 10,500 at 2 trips = 21,000; 25% × 30,000 = 7,500 at 4 trips = 30,000; owners add perhaps 1,000. Total about 52,000 trips a week.

Sanity check: over five teaching days that is about 10,000 trips a day; across four main gates over about 12 active hours, roughly 200 pick-ups or drop-offs per gate per hour, three to four a minute. That feels plausible for a busy campus. A good answer says the result is sensitive mainly to the share of students living far away.

Reflect

Which assumption in your answer were you least sure of, and how could you check it cheaply?
Did you state the equation before the numbers? If not, what made you skip it?

Do it with the pillars

The Rope
The Rope’s Lesson 6.4, Estimation, Numbers and Business Cases, takes this method into a full interview with scoring.The Pack
The Pack’s Case Interview Practice module gives you live estimation cases with a mentor.

Sources & provenance

Method synthesised from standard case-interview practice, including Marc Cosentino, “Case in Point” (Burgee Press). Population anchor: Statistics Indonesia (BPS) mid-decade projections put the population at roughly 280 million; check BPS for the current figure. All campus figures are illustrative.

Written for

StudentFresh graduateEarly professional