Unit Economics for Interviews: Explain CAC, LTV, Payback and Burn Like an Investor
A ₹299 delivery order can look like revenue, traction and growth - until you subtract the discount, rider cost, payment fee, refund risk and customer support. The real question for a new-age business is not “Did we get an order?” but “Will this customer ever pay back what we spent to acquire them?”
- Unit economics asks whether one customer, order, user or store is profitable before judging the whole company.
- CAC is the cost to acquire one new customer: acquisition sales and marketing cost divided by new customers acquired.
- LTV should be based on gross profit or contribution, not revenue; revenue LTV can make a weak business look healthy.
- LTV:CAC above 3x is a common strong signal in mature subscription businesses; below 1x means every acquired customer destroys value.
- CAC payback tells how many months it takes to recover CAC from customer gross profit; faster payback reduces funding pressure.
- Burn is acceptable when it buys repeatable, improving unit economics; dangerous when it only buys subsidised demand.
- The best interview answer links CAC, contribution margin, retention, LTV, payback, burn and runway as one operating system.
Big Picture: Growth Is Healthy Only When the Unit Works
New-age businesses often scale before profits appear at the company level. Unit economics separates good temporary losses from bad permanent losses by tracking whether each acquired customer can generate enough future contribution to repay acquisition cost and support fixed costs.
The Core Logic: One Customer Must Eventually Fund the Machine
Think of a new-age business as two engines running together:
- The growth engine spends money to acquire users, customers, merchants, drivers, sellers or stores.
- The economic engine earns contribution from usage, subscription, commission, spread, advertising or repeat purchases.
If the economic engine becomes stronger as the business scales, burn can be strategic. If the company keeps spending more to get the same weak customers, burn is not investment - it is leakage.
The Six Metrics You Must Know
These are the interview-safe measures. Use them together, not in isolation.
The Formula Chain: From CAC to Burn
The cleanest way to answer unit economics is to move from customer acquisition to cash survival.
Worked Example: Calculate CAC, LTV, Payback and Burn
Use a simple hypothetical subscription app to see the math end to end.
Step 1 - CAC: ₹10,00,000 / 2,000 = ₹500 per user.
Step 2 - Expected customer life: 1 / monthly churn = 1 / 5% = 20 months.
Step 3 - LTV: ₹100 monthly contribution × 20 months = ₹2,000.
Step 4 - LTV:CAC: ₹2,000 / ₹500 = 4x, a strong signal if assumptions are realistic.
Step 5 - CAC payback: ₹500 / ₹100 = 5 months.
Step 6 - Runway: ₹1,20,00,000 / ₹10,00,000 = 12 months. The unit economics look promising, but management still needs enough runway to scale before cash runs tight.
The Strategy Matrix: When Burn Is Good vs Bad
Burn is not automatically bad. The question is what the burn is buying. Is it buying learning, retention and future contribution - or just temporary demand?
Definitions You Can Say in One Breath
- Unit economics: Revenue and costs measured at the level of one customer, order, user, transaction, store or loan.
- CAC: Total acquisition-linked sales and marketing cost divided by the number of new customers acquired.
- LTV: Expected gross profit or contribution a customer generates over the full relationship.
- LTV:CAC: The value a customer creates divided by the cost of acquiring that customer.
- CAC payback: Months needed for customer contribution to recover the acquisition cost.
- Burn: Net cash outflow per month while the company funds operations and growth.
Many Indian D2C brands grew quickly through performance marketing on Meta, Google and marketplaces. The strategic issue was that CAC often rose as more brands bid for the same digital audience, so the winners had to improve repeat purchase, gross margin, brand recall and offline or marketplace distribution. The so what: paid growth is valuable only when retention and contribution rise faster than CAC.
Lenskart: Unit Economics in an Omnichannel Business
Lenskart shows how a new-age Indian business can improve unit economics by combining online discovery, offline trust, private-label supply and repeat eyewear purchases.
Situation: Eyewear is a trust-heavy category. Customers want fit, eye testing, style advice and easy replacement. Pure online acquisition can create reach, but it may struggle with conversion if customers are unsure about frame quality, prescription accuracy or fit.
The move: Lenskart built an omnichannel model: online discovery and app-based browsing supported by physical stores, eye-testing infrastructure, home trials in some markets and a controlled supply chain. The primary driver is control over the customer journey - from discovery to prescription to purchase to repeat. Supporting drivers include private-label margins, store-led trust, technology-enabled recommendations, manufacturing scale and repeat purchase occasions such as prescription changes, sunglasses and family buying.
Outcome or lesson: The business logic is not simply “open more stores” or “spend more on ads.” The strategic lesson is that CAC becomes more productive when the brand improves conversion, retention, average order value and gross margin together.
Takeaway: A shallow answer says Lenskart wins because it is omnichannel. A complete answer says omnichannel improves unit economics only when it lowers trust friction, raises conversion, supports repeat purchase and protects contribution margin.
How AI Changes Unit Economics in 2026
AI does not change the core math. It changes the speed and precision with which companies improve the math.
- Lower CAC through smarter targeting: AI models can score leads, personalize creatives, predict conversion probability and reduce wasted ad impressions. The risk is over-optimization to short-term clicks instead of high-LTV customers.
- Better LTV prediction: ML models can forecast churn, repeat purchase probability, basket expansion and cross-sell potential by cohort. This helps firms spend more on customers likely to pay back and less on low-quality acquisition.
- Burn control through automation: AI customer support, demand forecasting, fraud detection and workflow automation can reduce variable servicing costs and improve contribution margin. The caveat: AI tooling itself must be measured as cost per resolved ticket, saved hour or incremental conversion.
Use Perplexity to collect public interviews, DRHPs, annual reports or investor updates for a company, then use ChatGPT to create a unit economics map: unit, revenue driver, variable costs, CAC levers, retention levers, burn risk and likely interviewer questions.
Interview Relevance
“A quick-commerce or D2C company is growing revenue rapidly but is still loss-making. How would you evaluate whether the business is fundamentally healthy?”
Always say “cohort” once in your answer. Investors do not only ask whether the average customer is profitable; they ask whether newer customer cohorts are becoming cheaper to acquire, more retained and faster to pay back.
Common Mistake
The biggest mistake is calculating LTV using revenue instead of contribution. It flatters the business because it ignores discounts, delivery, refunds, servicing and payment costs. The one-line fix: use gross profit or contribution LTV, then compare it with CAC and payback.
What to Revise Next
Next, revise Indian Sector Benchmarks: What Healthy Looks Like by Industry. Unit economics tells you the formula; sector benchmarks tell you what “good” looks like for SaaS, D2C, fintech, marketplaces, quick commerce and lending in the Indian context.