Unit Economics: CAC, LTV, Margin
Customer Acquisition Cost (CAC) = sales & marketing spend ÷ new customers.
Lifetime Value (LTV) ≈ average gross profit per customer per period × expected periods retained.
A common health check is LTV ÷ CAC of 3 or more, with CAC recovered within about 12 months.
Contribution margin tells you what each sale adds after variable costs — growth with negative margin accelerates losses.
Monthly customer churn is the percentage of customers who leave during a month, measured against the customers present at the start of that month. A steady 2% monthly churn rate means assuming that 2 out of every 100 remaining customers leave each month. Using the simplified subscription model, LTV equals monthly gross profit per customer divided by monthly churn expressed as a decimal: $70 ÷ 0.02 = $3,500.
Cash runway is how long available cash can cover the business’s net cash outflows; six months of cash on hand means roughly six months of runway at the current spending rate. An 18-month CAC payback means acquisition costs take about 18 months to recover. Scaling marketing requires cash upfront and can shorten the runway further. Even with a strong 4:1 LTV:CAC ratio, the business could become insolvent—unable to pay its obligations when due—before recovering those acquisition costs.
The main LTV calculation error to avoid is treating revenue as customer value without subtracting delivery costs. Revenue is the amount earned from sales; gross profit is revenue minus the direct costs of providing the product or service. Net profit subtracts other business expenses as well, including overhead, interest and taxes. This simplified LTV model uses gross profit, not revenue or net profit. Using revenue inflates LTV and the LTV:CAC ratio; using gross profit rather than net profit is not itself an error. Refunds should also reduce the customer value estimate where material, rather than automatically being treated as acquisition costs.
You, before you double your marketing budget, calculate your current LTV:CAC ratio and payback period to ensure you're funding profitable growth, not just expensive activity.
Learning objective
Build a consistent CAC and LTV model and stress its assumptions before scaling.
Understand the mechanism
Define the unit, cohort and period. CAC should include the costs required to acquire that cohort, not just ad spend if sales labor is essential. LTV should use gross profit or contribution consistently rather than revenue masquerading as value.
A simple subscription LTV approximation is monthly gross profit divided by monthly churn, assuming steady churn and margins. That assumption often fails for small or changing cohorts. Report observed retention alongside the model and avoid precision unsupported by data.
Put it to work
Measure payback in months and compare it with cash capacity. A 3:1 LTV:CAC ratio is a heuristic, not a universal requirement or proof of health. Long payback can strain an otherwise profitable business.
Stress higher acquisition cost, lower margins and worse retention together. Include onboarding and refunds where material. Growth should be funded against credible cohort economics, not an optimistic blended average.
Worked example
- 1
Monthly revenue per customer is $50; gross margin 60% means $30 gross profit.
- 2
At assumed steady 5% monthly churn, simplified LTV = $30 ÷ 0.05 = $600.
- 3
CAC $200 gives 3:1 and about 6.7 months payback. At 10% churn LTV falls to $300 and ratio to 1.5:1.
Retention assumptions can change a scaling decision more than a headline growth rate.
Your practice lab
Revenue $80, gross margin 50%, CAC $240, monthly churn 4%. Find modeled LTV and payback; repeat at 8% churn.
Personal scratchpad · not graded or saved
The Scale-Up Squeeze at DataStream Analytics
DataStream Analytics, a B2B SaaS firm, achieved a celebrated 5:1 LTV:CAC ratio. Their LTV was $5,000 ($100 monthly gross profit / 2% monthly churn) and CAC was $1,000. Confident in this blended metric, the board approved a massive scaling of the sales and marketing budget to accelerate growth.
However, the push for growth targeted larger, more complex enterprise clients. This unexpectedly increased CAC to $2,500 due to longer sales cycles and higher-touch onboarding. At the same time, the new cohort of demanding customers required more support, dropping the gross margin and leaving only $75 in monthly gross profit. The LTV for this new cohort fell to $3,750 ($75 / 2%), and their LTV:CAC ratio was now an unprofitable 1.5:1.
The company burned through its cash reserves much faster than anticipated, funding negative-margin growth. They had to pause expansion and lay off staff—a painful lesson in relying on a single, blended metric instead of analyzing cohort-specific economics.
A healthy blended LTV:CAC ratio can hide unprofitable unit economics in newer customer cohorts. Always measure and monitor economics by cohort before and during a scaling push.
Illustrative example — names and figures are composites for teaching.
Model your venture's key unit economics. Define your 'unit' and estimate your Customer Acquisition Cost (CAC), the components of your Lifetime Value (LTV), and your target LTV:CAC ratio and payback period.
Example: Unit: 1 subscribed consulting client. Est. CAC: $500 (LinkedIn ads, discovery call labor). Est. LTV: $4,500 ($250 monthly contribution margin x 18 months average retainer). Target LTV:CAC is >3:1 and payback <6 months. Our current model shows a 9:1 ratio and a 2-month payback, which strongly supports scaling.
Go deeper — outside resources
- Personal finance course · Khan Academy
Key ideas
- Know CAC and LTV
- Aim for LTV:CAC ≥ 3
- Never scale negative contribution margin
Try it: Estimate CAC and LTV for your business or one you know.
Quick check
1. You spend $10,000 and win 50 customers. CAC is…
2. LTV $600, CAC $200. Ratio?
3. What is a major risk of a long CAC payback period, even if the LTV:CAC ratio is healthy?
4. A company has a monthly gross profit per customer of $70 and a steady monthly churn rate of 2%. Using the simplified subscription model, what is the estimated LTV?
5. Your business has a strong 4:1 LTV:CAC ratio but an 18-month CAC payback period. With only 6 months of cash on hand, what is the most urgent threat of scaling marketing?
6. What is the most common error that creates a misleadingly high LTV:CAC ratio?