Unit economics for PM interviews: the four numbers that decide if a business works
Hook: A product can grow fast and still run out of cash. The four numbers below tell you whether each new customer makes the business stronger or weaker. Interviewers ask about them because a PM who cannot reason about unit economics will ship features that burn the runway.
What unit economics actually measures
Unit economics asks one question. Stripped of overhead and office costs, does a single customer bring in more money than the full cost to win and serve that customer? Marty Cagan argues that product managers own the business viability of their product, and viability starts with this question. When the average customer loses you money, scale makes the bleeding worse.
Four numbers carry the weight here: lifetime value, customer acquisition cost, payback period, and contribution margin. Each one answers a different question about the engine. Together they tell you whether the engine runs on fuel or on fumes.
Lifetime value (LTV)
LTV is the total gross profit you earn from a customer across the full relationship with your company.
The simple formula for a subscription product:
LTV = ARPU × Gross Margin % × Average Customer Lifetime
Here is a worked case. A fitness app charges $10 a month. Gross margin sits at 80% after payment fees and hosting costs. The average user stays for a lifetime of 20 months. LTV lands at $10 × 0.80 × 20, which is $160 per user.
Two traps catch candidates at this step. The first trap is confusing revenue with profit. LTV uses gross profit, so a product with razor-thin margins has a much smaller LTV than the top-line number suggests. The second trap is ignoring churn. When monthly churn rises from 5% to 8%, average lifetime drops from 20 months to 12.5 months, and LTV falls by 37%. Lenny Rachitsky has written that retention is the single biggest lever for consumer subscription businesses, and this formula shows the reason.
Customer acquisition cost (CAC)
CAC is the fully loaded cost to win one new customer.
CAC = Total Sales and Marketing Spend ÷ New Customers Acquired
Include paid ads, content, sales team salaries, tooling, and agency fees in the numerator. A common candidate mistake is counting only ad spend. That number is called paid CAC, which is useful for channel math but misleading as a whole-business number.
Blended CAC includes organic customers in the denominator. Paid CAC does not include them. In an interview, say which one you mean so the interviewer can follow your math. Picture a company that spent $50,000 last quarter and brought in 500 customers. Blended CAC works out to $100 per user. If 200 of those customers came from word of mouth, paid CAC on the remaining 300 is $167 per user.
CAC on its own tells you nothing. A $500 CAC is wonderful for enterprise software and ruinous for a $3 mobile game.
The LTV to CAC ratio
This ratio is the fastest way to judge a business model.
LTV ÷ CAC = ratio
The rough industry rule of thumb is that a ratio below 1 means you lose money on every customer. A ratio around 3 is considered healthy for SaaS. When the ratio sits above 5, it often signals that the company is underinvesting in growth and leaving customers on the table.
Using the fitness app above, LTV was $160. At a CAC of $40, the ratio is 4, which is solid territory. If CAC climbs to $80 because paid channels got more expensive, the ratio drops to 2, and the business needs attention before anyone adds more ad spend.
Payback period
Payback period is the number of months it takes to earn back the CAC from a single customer.
Payback = CAC ÷ (ARPU × Gross Margin %)
For the fitness app with $40 CAC, $10 ARPU, and 80% gross margin, payback is $40 ÷ ($10 × 0.80), which equals 5 months.
Payback matters because it controls how fast a company can grow without running out of cash. Two businesses with the same LTV to CAC ratio can have very different survival odds. At a 12-month payback, you front the cost of every customer for a full year. With a 3-month payback, you recycle cash four times as fast and can pour it back into more growth.
As a rule of thumb, consumer subscription businesses aim for payback under 12 months. Enterprise SaaS often accepts 18 to 24 months because contract values and retention run much higher in dollars.
Contribution margin
Contribution margin is the profit left over from one unit of sale after you subtract the variable costs tied to delivering that unit.
Contribution Margin = Revenue per Unit − Variable Cost per Unit
For a food delivery order with $30 of revenue, $22 in food cost, driver pay, and packaging, the contribution margin is $8 per order, or 27%.
Contribution margin tells you whether scaling helps the business. At a positive contribution margin, every new order pays down fixed costs like engineering salaries and office rent. With a negative contribution margin, growth makes losses worse, and no amount of volume fixes the problem. Early WeWork and MoviePass both had negative contribution margins, which explains the ending of those stories.
How interviewers use these numbers
Expect prompts like "walk me through the unit economics of this feature." Another common one: "this business is growing 40% a quarter but losing money. What is your first check on the numbers?"
A strong answer follows a pattern:
- State the formula for the metric in question.
- List the inputs that you need as rough estimates.
- Walk through a rough calculation with stated assumptions.
- Name the lever you would pull to improve the weakest number.
At a poor LTV to CAC ratio, the levers are raising prices, reducing churn, cutting acquisition cost, or improving gross margin. For a payback that runs too long, the levers are charging annually upfront, upselling fast, or shifting the acquisition mix toward cheaper channels. Jeff Gothelf points out that PMs should pair metric changes with hypotheses about customer behavior, so always tie the lever back to a specific user action.
A worked example
A B2B SaaS tool charges $200 per seat per month. Gross margin is 85%. The average account stays 30 months. Paid CAC per account is $2,400.
LTV is $200 × 0.85 × 30, which works out to $5,100 per account. The LTV to CAC ratio is $5,100 ÷ $2,400, which equals 2.1. Payback is $2,400 ÷ ($200 × 0.85), which is about 14 months.
Here is the read. The ratio sits below the SaaS target of 3, and payback runs longer than most early-stage boards tolerate. Retention is the biggest lever, because every extra month of lifetime multiplies through the LTV calculation. A second lever is annual billing with a discount, which would cut effective payback in half by collecting cash upfront.
What to practice
Memorize the four formulas until you can write them without any hesitation. Then practice estimating each input from a short business description. Given any company, you should be able to guess ARPU, gross margin, churn, and CAC within the right order of magnitude. Most interview failures on this topic come from candidates who know the definitions but freeze when asked to plug in real numbers.
When you can walk an interviewer through LTV, CAC, payback, and contribution margin for any business they name, you have cleared one of the highest-signal bars in the PM interview loop.
Works Cited
Cagan, Marty. "The Four Big Risks." Silicon Valley Product Group, https://www.svpg.com/four-big-risks/.
Gothelf, Jeff. "The Lean UX Canvas V2." Jeff Gothelf, https://jeffgothelf.com/blog/lean-ux-canvas-v2/.
Rachitsky, Lenny. "What is Good Retention." Lenny's Newsletter, https://www.lennysnewsletter.com/p/what-is-good-retention-issue-29.
Kevin Armstrong is a product leader with an MBA from the University of Chicago Booth School of Business. He writes about PM interviews, payments, and iOS development at kevinarmstrong.io.