In this article
Before you start
This page is one step of the case interview guide to consulting cases. If you need a number you do not have, such as a market size, the market sizing guide shows how to estimate it.
Why interviewers ask this
A candidate who cannot structure a profit question is in trouble, because profit questions are among the most common case prompts. McKinsey says that in its Problem Solving Interview, sometimes called a case interview, it presents a business case to evaluate your approach to solving complex problems.
The exact answer is not the target. A general reference notes that candidates are expected to demonstrate reasoning rather than to produce the exact answer, and that the questions are ambiguous enough that you must ask questions or state assumptions. A university career center likewise describes a case as a hypothetical business situation used to assess your problem-solving skills, analytical abilities, and how well you think on your feet.
A weak answer recites a memorized list of headings. A strong answer asks for the numbers, finds what changed, and tests one cause with arithmetic. Format differs by firm: a secondary guide reports that McKinsey typically uses an interviewer-led format, while Boston Consulting Group (BCG) and Bain more often use candidate-led cases. Treat that as a tendency that varies by office.
Profitability case walkthrough from first principles
- Profit tree
- A profit tree is a diagram that splits profit into revenue and cost, then splits each of those into smaller drivers such as price, volume, fixed cost and variable cost, so that every possible cause of a profit change has one place on the page and none is counted twice.
- Contribution per unit
- Contribution per unit is the selling price of one unit minus its variable cost, the amount each sale adds toward covering fixed cost and then toward profit. It is the number that turns break-even volume into a one-line division.
In plain words
Think of a market stall. You take money in, you pay for stock and for the stall, and what is left is your profit. If your profit shrinks, there are only two places to look: less money came in, or more went out. The analogy breaks in one place. A company has costs that grow with each sale and costs that stay put, and that difference changes the answer.
In one precise sentence: profit equals revenue minus cost, revenue equals price times volume, and cost equals fixed cost plus variable cost per unit times volume.
Step 1: Clarify the goal and the numbers
The first step is to define the problem, not the symptom. A university career guide advises you to define the problem and not the symptom and notes that asking good questions is key to getting the information the interviewer holds.
Ask four things. What does profit mean here, operating or net? Over what period is it falling? How big is the fall, and what is the goal? Which products, customers or regions does the company serve? A typical prompt (illustrative) says that revenue is flat while net margin is falling, so confirm whether the client cares about profit in money or as a percentage.
Input: the prompt. Output: one agreed sentence stating the problem, the time frame and the goal. You now know what you are solving, which the next step needs to compare anything.
Step 2: Compare revenue and cost over time to see what changed
Profit falls when revenue falls, when costs rise, or both. A coaching guide puts it plainly: profits could be going down because of revenues, costs, or both. The comparison that picks your first branch is revenue and cost this period against the same figures a year earlier.
Ask whether competitors saw the same change; if the whole market moved, the cause is external (demand, a rival's price, input costs) and the recommendation changes.
Ask for those four numbers. If revenue fell, open the revenue branch. If revenue is flat or up while cost rose, open the cost branch. If both moved, size each change and start with the larger one.
Flat revenue is not a clean bill of health. The prompt above shows revenue flat while margin falls, so the cost side or the mix of what is sold is where the cause sits. Input: two periods of revenue and cost. Output: the branch to open first. This one check keeps you from spending ten minutes on the wrong half of the tree.
Step 3: Split the branch that moved into drivers
Profit equals revenue minus cost. Revenue splits into price and volume, and cost splits into fixed and variable. Pick the branch from Step 2 and split it until each leaf is something you can measure. The same guide lists the possible root causes: lower price, lower volume, higher variable costs.
For revenue, ask whether the average price fell, whether fewer units sold, or whether the customer or product mix shifted. For cost, ask whether cost per unit rose or whether fixed overhead grew. Pick the leaf your data points at, and say why before you test it.
Text description of the diagram
- Lower price, lower volume and higher variable costs are all root causes of falling profit.
- Revenue splits into price and volume, and cost splits into fixed and variable.
Input: the branch that moved. Output: a short list of testable leaves, ranked by how likely each is.
Step 4: Test the likely driver with a number
A hypothesis becomes a finding only when a number supports it. Ask for the data on your top leaf, compute the change, and compare its size with the total fall in profit. If the leaf explains most of the gap, you have the cause; if not, move to the next leaf.
Compute contribution per unit and the break-even volume, which is fixed cost divided by contribution per unit. Break-even tells you how many units must sell before profit is above zero, so it shows how much a cost rise hurts.
Check your work as you go. One guide advises you to check calculations, units, assumptions, and the recommendation's connection to the evidence. Say each assumption aloud. Input: one hypothesis and its data. Output: a sized cause.
Step 5: Recommend an action, a risk and a next step
End with a recommendation, not a summary. Say what the company should do, the evidence behind it, the main risk, and what you would check next. Keep it to a few sentences so the interviewer can follow it.
Tailor the answer to the case. McKinsey's interviewing page says, in paraphrase, that a case has no single logical solution and that tailored solutions are preferred to standard frameworks. A fixed list of headings can help you start, but your recommendation must come from this company's numbers. The frameworks that still work page covers where a framework helps and where it hurts.
Input: the sized cause. Output: one action, one risk, one next step. You now have the whole walkthrough.
Step-by-step description
- Agree the problem, period and goal.
- Compare revenue and cost over time.
- Break the branch into price, volume or cost.
- Size the likely cause.
- One action, one risk, one next step.
- Profit equals revenue minus cost.
- Revenue splits into price and volume, and cost splits into fixed and variable.
Knowing it is half the job. Saying it is the other half.
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Start Free Trial →Worked example
The numbers below are illustrative, not real data. A company sells one product. In the baseline year the price is 50 per unit, the volume is 10,000 units, the variable cost is 30 per unit and the fixed cost is 100,000. Revenue is 500,000, variable cost is 300,000, contribution per unit is 20, profit is 100,000, profit margin is 20.0% and break-even volume is 5,000 units.1
Scenario A: the price and volume do not change, but variable cost rises to 35 per unit. Profit falls from 100,000 to 50,000 while revenue stays at 500,000 in scenario A, a fall of 50%. Margin halves to 10.0% and break-even volume rises to 6,667 units.1 Step 2 finds this: revenue is flat, so cost is the branch.
Scenario B: the company cuts price to 45 and volume rises to 11,000. Revenue is 495,000, only 1.0% lower, but in scenario B the illustrative profit is 65,000, a fall of 35%, and margin is 13.1%.1 Flat revenue hid a price cut that the extra volume did not pay for.
| Quantity | Baseline | A: cost per unit up | B: price cut, volume up |
|---|---|---|---|
| Price per unit | 50 | 50 | 45 |
| Volume (units) | 10,000 | 10,000 | 11,000 |
| Variable cost per unit | 30 | 35 | 30 |
| Revenue | 500,000 | 500,000 | 495,000 |
| Profit | 100,000 | 50,000 | 65,000 |
| Profit margin | 20.0% | 10.0% | 13.1% |
| Break-even volume (units) | 5,000 | 6,667 | 6,667 |
Text description of the diagram
- Profit falls from 100,000 to 50,000 while revenue stays at 500,000 in scenario A.
- In scenario B the illustrative profit is 65,000.
Trade-offs and edge cases
The textbook tree assumes one product and clean cost categories. Real cases break that in several ways.
- Mix shift. Revenue can be flat because a high-margin product lost sales to a low-margin one. Split revenue by product before you blame price or volume.
- One-off items. A single large cost, such as a legal settlement, can explain a drop that will not repeat. Ask whether the cost is recurring.
- Shared costs. Fixed costs spread across products make per-product profit depend on an allocation rule. State the rule you assume.
- Definitions. Operating profit and net profit differ, and so do gross and net margin. Confirm which one the interviewer means in Step 1.
- Market-wide versus company-only. If competitors saw the same fall, the cause is external, such as weaker demand, a rival's price cut or higher input costs, and internal levers will not fix it. Compare against the industry before you split internal drivers.
- Format. An interviewer-led case hands you the next question; a candidate-led case expects you to choose the next analysis, as one guide reports (CaseTutor). Reported practice differs by firm and office, so ask how the interviewer wants to work.
How to answer it in an interview
Our client's profit has fallen over the last year. How would you find out why?
First I'd confirm what profit means here, the time frame and the goal. Then I'd ask for revenue and cost for this year and last, and whether competitors saw the same change, because profit can fall from revenue, cost or both. If revenue is flat, I'd go to cost per unit and product mix; if revenue fell, I'd split it into price and volume. I'd test my top hypothesis with a number, such as contribution per unit, and check units before concluding. Then I'd recommend one action, name the main risk, and say what data I'd collect next.
The spoken structure: clarify, compare the two periods, split the branch that moved, test with a number, recommend. For the McKinsey Personal Experience Interview (PEI), which comes alongside the case, see the PEI questions guide.
Common mistakes
- "Revenue is down, so that's the problem." (wrong) Even if revenue fell, cost may also have risen. (right) "Let me compare revenue and cost across both periods before I pick a branch."
- "I'll walk through my framework headings in order." (wrong) A fixed list ignores what the data shows. (right) "I'll start with the branch the numbers point to and say why."
- "Profit and margin are the same thing." (wrong) Profit is a money amount; margin is a ratio. (right) "I'll check whether the client cares about profit in money or as a percentage."
- "I'll skip the math and give the story." (wrong) A cause without a size is a guess. (right) "I'll size this driver against the total fall in profit."
- "I need to find the one exact answer." (wrong) Reasoning is what is assessed. (right) "Here is my best-supported cause, my assumption and what would change my mind."
Follow-up questions
How do you structure a profitability case?
Clarify the problem, compare revenue and cost across two periods, split the branch that moved into price, volume, fixed cost and variable cost, test the likely driver with a number, then recommend. Adapt the branches to the company in front of you.
What is the difference between profit and profit margin?
Profit is revenue minus cost, in money. Profit margin is profit divided by revenue, as a percentage. Profit can rise while margin falls if revenue grows faster than profit.
What if revenue is flat but margin is falling?
Open the cost branch first. Check variable cost per unit, then fixed overhead, then whether the product mix moved toward lower-margin items, as in scenario A and scenario B above.
Do I need to memorize a profitability framework?
No. McKinsey's interviewing page says, in paraphrase, that there is no single logical solution and that tailored answers are favored over standard frameworks. Know the profit tree well enough to adapt it, and practice on the 20 original practice cases.
How do you calculate break-even in a case?
Divide fixed cost by contribution per unit, which is price minus variable cost per unit. In the baseline example that is 100,000 divided by 20, or 5,000 units.1 The case math drills cover more.
How long is a profitability case?
For BCG, one coaching guide reports that an interviewer typically asks one or two fit or Personal Experience Interview questions for about ten minutes, then moves to a case of about 30 to 40 minutes; a profitability case fills that case slot, and timing varies by firm.
- Profit falls because of revenue, cost or both, so compare two periods first.
- Split the branch that moved into price, volume, fixed cost and variable cost per unit.
- Flat revenue can hide a falling margin; test cost per unit and mix.
- Size every cause with a number and state your assumptions.
- End with one action, one risk and one next step.
Sources
- Interviewing at McKinsey: the Problem Solving Interview — McKinsey & Company (accessed 2026-10-26), 26 Oct 2026
- Case interview — Wikipedia (general reference, accessed 2026-10-26), 26 Oct 2026
- Preparing For Case Interviews — Colorado State University Career Center (accessed 2026-10-26), 26 Oct 2026
- BCG Case Interview - How to Prepare — IGotAnOffer, a coaching firm (secondary, accessed 2026-10-26), 26 Oct 2026
- 10 Most Common Types of Cases Applied in BCG Interviews — Medium, individual author (secondary, used for prompt wording only, accessed 2026-10-26), 26 Oct 2026
- McKinsey Case Study Interview: The Complete Prep Guide — CaseTutor (secondary, accessed 2026-10-26), 26 Oct 2026
