Unit Economics: Does This Business Actually Work?
Master unit economics, contribution margin and LTV:CAC, and prove to any interviewer you can separate real growth from subsidized growth.
Unit economics is the single calculation that separates a business that works from one that just looks like it works. Every McKinsey, BCG, and Bain interviewer who hands you a growth or profitability case is waiting to see if you instinctively reach for it.
This guide teaches you what unit economics actually measures, how to calculate it under interview pressure, and how to use it to reach a recommendation that sounds like a partner wrote it. Read it once, practice the exercises at the end, and you will walk into your case interview knowing exactly what to look for when a client says "we're growing fast but losing money."
What Unit Economics Actually Measures
Unit economics answers one question: does the business make money on a single transaction before you ever think about overhead?
Imagine a lemonade stand. You collect $3 per cup. Sugar, lemons, and the cup cost $1.80. You make $1.20 on every cup you sell. That $1.20 is your contribution margin per unit. It is the engine. Every other number, rent on the table, the sign you painted, your little brother's salary, runs on top of that engine. If the engine produces less than zero, no amount of marketing spend or Series B funding fixes the underlying machine.
Contribution margin per unit = Revenue per unit minus Variable cost per unit. That formula is the foundation of every unit economics analysis you will run in a case.
Why LTV and CAC Change Everything for Subscription and SaaS Businesses
For any business where a customer buys more than once, the single-transaction view is incomplete. You need to know the lifetime value of a customer (LTV) versus what it cost to acquire them (CAC).
Think of it like a fishing boat. Every time you cast a net, you pay a fuel cost. That fuel cost is your CAC. The fish you pull in over the entire voyage is your LTV. A smart captain does not ask "did I catch more fish than fuel this cast?" She asks "over the full voyage, does the haul justify every drop of fuel I burned?" If your LTV is $300 and your CAC is $100, your LTV:CAC ratio is 3:1. That is the consulting benchmark for a healthy customer acquisition model. Below 1:1, you are burning dollars to make dimes.
McKinsey and BCG use the LTV:CAC ratio constantly in tech, retail, and financial services cases. Know the 3:1 benchmark. Know that payback period (how many months until CAC is recovered) matters just as much to operators who have tight cash cycles.
How to Spot a Business Subsidizing Growth
The most dangerous slide in any board deck is top-line revenue growing 40% year over year with no mention of margin per customer.
Dropbox in 2008 is the classic example. Freemium users cost real money to serve: storage, bandwidth, support. The question was never "how many users signed up?" The question was "what does it cost to convert a free user to paid, and does the lifetime value of that paid user cover both the conversion cost and the original acquisition cost?" Growth that does not answer that question is not growth. It is subsidized volume.
In a case interview, the moment a client brags about user growth, your job is to ask: "What is the blended contribution margin per user, and how does CAC compare to LTV?" That question signals you think like a business owner, not an analyst.
Practice this framework on a real case: the [dropbox-growth-2008] case on BoardroomIQ puts you in the room.
Practice this framework
Work through the Dropbox 2008: Growth When Ads Don't Work case with AI coaching.
The Two Numbers You Must Calculate First in Any Growth Case
When you open a growth or profitability case, calculate contribution margin per unit and LTV:CAC before you touch market size or competitive dynamics.
Contribution margin tells you if the core transaction works. LTV:CAC tells you if the growth engine is sustainable. Everything else, market share, product roadmap, geographic expansion, is noise until you confirm those two numbers are healthy.
A quick framework for structuring your math: start with revenue per unit, subtract all variable costs (COGS, payment processing, delivery, support), land on contribution margin. Then estimate average customer lifetime in months, multiply by monthly contribution margin to get LTV, divide by CAC. State your ratio. State whether it clears the 3:1 bar. Then, and only then, recommend a growth path.
How to Practice Unit Economics Before Your Interviews
The mechanics are simple. The pressure is not. Here is how to build the muscle before interview day.
Narrate your math out loud. Pick any consumer subscription you use (streaming, gym, meal kit) and calculate its unit economics from public data. Say every step aloud as if a partner is in the room. Interviewers grade your reasoning, not just your answer.
Stress-test the inputs. Once you have a baseline unit economics model, cut the price by 10% and recalculate. Then raise CAC by 20% and recalculate. Consulting interviewers push on assumptions. Knowing how sensitive your answer is to each variable is the difference between a good answer and a great one.
Time yourself on cold cases. Set a five-minute timer and calculate contribution margin and LTV:CAC from a case prompt you have never seen. Speed and structure under pressure is the actual skill being tested.
The best way to practice unit economics is under realistic pressure, with a case that fights back. Open the Dropbox Growth case on BoardroomIQ and see how fast you can isolate the unit economics before the interviewer redirects you.