Customer Acquisition Cost (CAC) is the fully-loaded cost to acquire one new paying customer — ad spend, sales salaries, tools, and creative, divided by new customers in the period. A fully-loaded CAC includes the share of marketing, sales, and overhead salaries attributable to acquiring one customer, not just ad spend; ignoring salaries understates true acquisition cost and leads to unprofitable scale even when blended paid-CAC looks healthy. Two related distinctions sharpen the math: paid CAC isolates the cost of a customer from a specific paid channel, while blended CAC divides all acquisition spend (including organic-attributed overhead) by all new customers. Similarly, top-of-funnel CAC divides spend by signups or leads, while paid CAC divides spend by paying customers — these can differ by 10–100×, with the gap being the conversion rate.
Customer Lifetime Value (LTV or CLV) is the total gross profit a customer is expected to generate over the entire future relationship. In its simplest form for a subscription business with constant ARPU, \( LTV = \frac{ARPU \times \text{Gross Margin \%}}{\text{Monthly Churn Rate}} \). Two consequences follow directly: because LTV uses gross profit per period rather than revenue, a 50% gross margin halves the customer's lifetime value versus revenue-based math; and because LTV is inversely proportional to churn, doubling churn halves LTV, all else equal — making churn reduction the highest-leverage unit-economic move a founder can make.
The canonical measure of return on acquisition spend is the LTV:CAC ratio. The universally cited "magic number" is 3:1 — the standard benchmark for healthy SaaS unit economics. Below 1:1, each customer loses money; 1–2:1 is fragile and below investor expectations for venture-scale software. CAC Payback Period complements the ratio: it is the number of months of gross profit from a new customer required to recover the CAC spent to acquire them, and investors consider under 12 months healthy in SaaS (best-in-class B2B SaaS often achieves under 12, with under 18 months as the acceptable ceiling). A bad LTV:CAC can hide behind low churn with high CAC: customers may technically pay back, but slowly, tying up capital and breaking under interest-rate pressure — both payback (a cash-flow question) and LTV:CAC (a long-run profitability question) must be healthy.
Several related metrics complete the picture. ARPU (Average Revenue Per User) sets the upper bound on acquisition and retention spend when combined with gross margin and churn, and is distinct from ASP (Average Selling Price), which is the mean price of a single transaction or contract. Annual Contract Value (ACV) normalizes multi-year deals to an annual figure. Customer Retention Cost (CRC) is the cost to retain or serve a customer, distinct from CAC, but both must be fully loaded and both matter in LTV math. For sales-led businesses, ramp time — the months until a new sales rep hits full quota productivity — must be amortized into a ramped CAC, because non-ramped CAC ignores ramp time and understates the true cost of acquisition. Finally, implied valuation can be back-solved from unit economics: applying a multiple of ARR (e.g., 10× for SaaS) reveals what LTV:CAC and growth must be to justify a target valuation, making unit economics the bridge between operational reality and capital markets.