Growth strategy cases: size the gap before you brainstorm

What is a growth strategy case interview?

A profitability case hands you a problem: profit fell, find out why. A growth case hands you an ambition. The client wants to be bigger, faster or in a new place, and you have to work out how much bigger, by which route, and whether the route is worth taking. That difference changes what the interviewer is listening for. In a diagnosis they want to see you eliminate branches. In a growth case they want to see you size a gap, generate options without drowning in them, and choose.

None of the three firms publishes a list of case types on its careers pages, so "growth case" is a label from candidates and prep sites rather than a firm taxonomy. What the firms do publish is worth reading closely. Bain's practice case FashionCo starts from a women's fashion company whose revenue has fallen for five years and asks why, and what would drive revenue back up. As published it runs through a structure, an analysis of market and consumer trends, a brainstorm of solutions with pros and cons, a numerical comparison of two options, and a recommendation with risks and next steps. BCG's guidance describes realistic business challenges where you structure, ask questions, analyse data and do quick calculations, and says plainly that there is no single right answer. Bain's main careers page says its cases reward precision and also creativity.

Put those together and you have the shape of every growth case we run: establish the size of the ambition, find where growth can come from, compare the options with numbers, and recommend one. The firms change their pages and their practice cases, so check them before you rely on our summary, but the shape has been stable for as long as we have interviewed.

The prompt arrives in a handful of disguises, and it pays to recognise which one you have, because the first move differs.

Organic or inorganic: which do you start with?

Organic growth is what the client builds itself: more customers, more visits, bigger baskets, new products, new places. Inorganic growth is what it buys or borrows: acquisitions, joint ventures, licensing, partnerships. Every prep site draws this line, and it is a fair place to start, but the useful question is which side to size first.

Start organic. You can size an organic lever from the client's own numbers (its customers, its frequency, its basket), and the interviewer will hand you those numbers when you ask. An acquisition needs a target, a price, a synergy estimate and an integration plan, none of which exist yet in the room. That makes it a poor place to spend your first ten minutes.

Raise inorganic growth as one option in the ranking, and say what would have to be true for it to win: the gap is too large for organic levers to close in the time available, or the client lacks a capability it could not build fast enough. In the cases we mark, candidates who lead with "they should acquire someone" almost never have a target, a price or a reason, and they read as though they are avoiding the analysis. The candidate who ranks it fourth, with a stated trigger for promoting it, reads as someone who has seen a client buy the wrong company.

If the interviewer steers you towards a deal, follow. At that point the case has changed type, and the market entry and private equity guides describe the structures you will need.

How do you break down a revenue growth target?

Ask twenty candidates how a retailer could grow revenue and you will get twenty lists: loyalty programme, online, new stores, price rises, promotions, private label, partnerships. Some of the ideas are good. The trouble is that a list has no way to tell you whether it is complete, no way to say which idea is bigger than another, and no way to show the interviewer that you were in control of the problem rather than reaching for whatever came to mind.

Decomposing first fixes all three. Revenue is customers, times how often each buys, times the size of each purchase. That is an identity, so the branches are collectively exhaustive by arithmetic, the same argument we make for the profit tree in the profitability guide. Every idea on your list now belongs to one branch only, and you can ask of each branch how much growth it could plausibly carry.

Two adjustments make the tree fit a client. First, change the words to match the business: a B2B supplier has accounts, orders per account and order value; a subscription service has subscribers and revenue per subscriber, with churn under the first term; a network has outlets and sales per outlet. The multiplication survives, and the interviewer sees that you built the tree for this client rather than recalling one. Second, keep the fourth branch. A pure multiplication describes the business as it is, and the most interesting growth ideas usually sit outside it.

There is a second split worth holding in your head, and it is older than the tree: revenue growth equals market growth plus share change plus anything new. The tree tells you which lever to pull. The market-and-share split tells you how hard you must pull, which is where the next section starts. For the mechanics of keeping branches distinct and exhaustive, the MECE and issue trees guide goes further than we can here, and for the four-box structures people memorise, case interview frameworks explains why we would not open with one.

How much growth do you need to find?

Every growth target hides a rate. "€800m to €1bn in three years" sounds like a big number and turns out to be a modest one once you convert it, and the conversion takes ninety seconds at the whiteboard. Total growth is 1,000 divided by 800, which is 1.25, or 25%. You need a steady yearly rate r such that (1 + r) multiplied by itself three times gives 1.25.

You will not solve that to the decimal in your head, and you do not need to. Try 7%: 1.07 cubed is about 1.225. Try 8%: 1.08 cubed is about 1.260. The target of 1.25 sits about three-quarters of the way from the first to the second, so the answer is around 7.7% a year. The exact figure is 7.72%, and the check is quick: 1.0772 cubed comes to 1.2499. In a live case, saying "a bit under 8% a year" with the working behind it is a full answer.

The tempting shortcut is to divide 25% by three and announce 8.3%. It is wrong in a direction that looks safe. At 8.3% a year, €800m becomes about €1,016m, so you have set a target €16m higher than the client asked for. Over a longer horizon or a bigger base the error grows. The case maths guide covers the growth-rate shortcuts worth memorising, and it is worth an hour before you meet one of these cases.

The rate alone is only half of the sizing. Next ask the question most candidates skip: how fast is the market growing? If the whole category grows 6% a year and the client simply keeps its share, €800m becomes about €953m, because 1.06 cubed is 1.191. The client is roughly €47m short, which is a gap of around €50m, and closing it needs a share gain of about 5% in relative terms. A company with a 20% share would need 21%.

Change one assumption and the case changes character. If the market grows 3% a year, the client reaches about €874m on share alone (1.03 cubed is 1.0927) and the gap is around €125m. The relative share gain needed is about 14%, so that same 20% share must reach nearly 23%. The ambition is identical in the prompt, and the two versions call for entirely different answers: a modest gap invites incremental levers, and a large one demands a new area or a bold move.

Say the gap out loud once you have it: "At 6% market growth we are about €50m short, so I am looking for a five per cent share gain or one new revenue line worth €50m." That sentence tells the interviewer you have turned an ambition into a search with a size. Then ask whether the market number is one you should trust, because you will be asked to defend it. If you need to build a market estimate yourself, the market sizing guide covers how.

How do you prioritise growth options?

By the time you have a driver tree and a sized gap, you will have six or seven ideas and minutes to choose between them. The failure we see most is ranking by taste. "I would start with loyalty, because customers like being rewarded" is a preference, and the interviewer cannot mark a preference. Five criteria turn it into a comparison.

Size of prize comes first because it is the only criterion you can compute, and computing is what the interviewer wants to watch. The others adjust it. A simple way to show that adjustment is to multiply the prize by your estimate of the odds, which gives an expected value you can rank. It is crude, and the interviewer knows it, but a crude number you can defend beats a qualitative score you cannot.

Notice what the answer does with the arithmetic. It uses it to decide (two options are enough at 6%, four are barely enough at 3%), where a weaker version stops at a ranking. Sizing the gap first is what makes that possible. Without the €50m and the €125m, five ranked options are just five ranked options, and you have no way to say when to stop.

Two cautions about tables like this one. Options are not additive in real life: two levers aimed at the same customer will overlap, so treat the total as a ceiling. And a two-by-two of prize against ease of execution is a perfectly good alternative when time is short. We prefer the numeric version in a case because the interviewer can watch you multiply, and a grid asks them to trust your placement of the dots. Whichever you use, finish by saying what the ranking implies for the gap, since a ranking alone answers nothing.

How is a new product launch case different?

A launch question is a growth case where one option has been picked for you and the interviewer wants to know whether it is a good one. The structure changes to fit. You no longer need the whole driver tree, and you need four questions answered well: does a customer want it, does it make money, what does it take from the client's existing products, and how would the client sell it.

Start with the customer, because it is the branch that can end the case in two minutes. If nobody has the problem, or the people who do already have a good enough answer, the economics are irrelevant. A useful habit is to ask what the customer does today, with no product at all. The answer is your real competitor, and it is frequently a spreadsheet, a habit or a neighbour rather than another company.

Cannibalisation is the branch candidates forget, and it is where launch cases are won by the candidate who remembers. A new product's headline revenue overstates its contribution to growth by whatever it takes from the range you already sell. The size of the correction depends on two numbers, the share of buyers who would have bought the old product anyway and the margin gap between the two, and you can work both out on the whiteboard.

Then go-to-market. Which channel reaches the customer at the lowest cost? Does the client have the sales team, the distribution or the brand permission to sell this, and how long would it take to build any that is missing? A launch that needs a capability the client lacks belongs in the ranking, with a longer time to impact and a lower probability, and it may lose to a plainer option for that reason. Add one competitor question: if the product works, what would the strongest rival do in the first year?

When the "product" is a new service added to an existing network or customer base, the same four branches apply with a fifth question folded in: can the client's existing operation deliver the service reliably, or does the service change what the operation has to be? Ask it early. It tends to be the branch that decides the answer.

What does the interviewer reward in a growth case?

Growth cases lean harder on creativity than a diagnostic case does, and candidates are slow to hear it. Diagnostic cases have a right answer somewhere in the data. A growth case has a set of defensible answers, and interviewers separate candidates by whether they produce an option nobody else would have. That is why the firm pages we read say there is no single right answer and that they are looking for how you think.

Creativity of that kind is a habit, and you can build it in a structured way. Use the driver tree as a generator: one idea per branch, then one more from outside the tree. That gives you a shortlist that is broad by construction rather than random. To find the outside idea, ask a few questions we have seen work. What is the customer's next problem after this purchase? What does the client own (trucks, stores, data, a trusted name) that it uses for one thing and could use for another? Could the pricing model change, from a one-off sale to a subscription, or from a price per item to a price per outcome? Who else is already in front of this customer, and would they rather partner than compete?

A creative option still has to be sized. The candidate who says "they could rent out spare warehouse capacity" and stops has offered an idea. The candidate who adds "if a fifth of the warehouse space sat idle and rents at a plausible rate, that is about €15m, so it is useful but cannot close a €50m gap on its own" has offered an option, and that is the version that scores. Pushing back on the obvious answer earns credit too. If the interviewer asks about a loyalty programme and your arithmetic says it is worth €10m against a €125m gap, say so.

Interviewers also read growth cases for commercial judgement, a different thing from ideas. When you propose a growth lever, say what it does to margin. Growth bought with discounts can raise revenue and lower profit, and a client who wanted "more sales" almost always wanted more money. Ask early whether the target is revenue, profit or both.

How do you open and close a growth case?

The first three minutes decide how much of the case you get to spend on options, because everything before the options is set-up. The opening below is the order we would run it in.

The close is where growth cases are most often lost. You have generated options, ranked them and sized them, and now you have sixty seconds to give a partner something they can act on. Synthesis is marked as its own skill, and the shape it rewards is the same across case types: recommendation first, then the reasons with numbers, then the main risk, then the next step. In a growth case the reasons must include the gap, because it tells the listener why you chose two options rather than five.

Every figure in that close was established earlier. The 7.7% came from the opening, the €50m and €125m from the gap, the €78m from the ranking (36 plus 42). Nothing new appears in the last minute, which is why the close takes a minute and stays credible. Keep your notes as you go: a written gap, a written ranking and a written list of assumptions are what a good close is assembled from.

One limit before you practise: a growth case with an interviewer-led format will not let you run the close the way we have written it, because the interviewer will ask the next question rather than wait for your recommendation. That is fine. The habit still carries, and our guide to interviewer-led and candidate-led cases explains how to adapt without losing the structure.

What mistakes do we see in growth cases?

These are the patterns that recur in the growth transcripts we mark, roughly in order of how much score they cost.

• Brainstorming a long list before sizing anything. Twelve ideas with no gap feels productive and gives you no way to choose, and it is the first thing a partner would strike.
• Never asking how fast the market is growing. Without it you cannot say how much of the target the client gets for free, so you cannot say how big the search is.
• Dividing the total growth by the number of years. As shown above, 25% over three years is 7.7% a year, not 8.3%, and the error is a small but visible arithmetic slip in the first five minutes.
• Treating revenue as the goal without asking. Buying growth with discounts can shrink profit, and interviewers score the candidate who checks the margin higher than the one who finds the extra sales.
• Ranking by liking. "I would do X because customers love it" has no number attached, and the interviewer cannot tell it apart from a guess.
• Ignoring time to impact. If the deadline is three years, an option that matures in year four does not count toward it, however large its prize.
• Adding up unrisked prizes. Five options each worth €50m do not make €250m of growth once you weight for odds and overlap.
• Leading with an acquisition. It is rarely the right first idea, and without a target, a price and a reason it reads as avoiding the analysis.
• Ending on a shopping list. Five equal recommendations tell the listener you never decided which cause or which lever was biggest. Recommend one path, sized, with the risk you volunteered.

The remedy for most of these is the same, and it is dull: size before you speak. A candidate who does the arithmetic at each step has less room to make any of the nine mistakes above. Once you can do that on paper, practise it out loud, because speaking under time pressure is a separate skill from thinking at a desk. The AI coaches on MBB Ready are personas built by ex-MBB interviewers, and they will hold you to the arithmetic, though they are no substitute for an experienced human partner giving you a view on how you come across in the room. If you have access to both, use both. For the wider set of habits that cost points across every case type, see our guide to case interview mistakes.