CAC
CAC (Customer Acquisition Cost) is the average cost of landing one paying customer: everything spent on acquisition over a period, divided by the number of paying customers that spending produced. Since the denominator counts only people who actually started paying, the spend that went toward everyone else (people who never signed up, people who signed up but never converted) still gets folded in and divided across the customers who did.
Definition
CAC (customer acquisition cost) is the total acquisition spend divided by customer count — the customer-anchored cost instrument, paired with LTV on the revenue side and with SAC on the signup-anchored side.
Position in the cost-revenue grid
CAC sits in one cell of a 2×2 grid formed by two axes: cost versus revenue, and customer-anchored versus signup-anchored. The four instruments:
- CAC — customer-anchored cost: spend ÷ customer count
- LTV — customer-anchored revenue: lifetime revenue per customer
- SAC — signup-anchored cost: spend ÷ signup count
- VPS — signup-anchored revenue: value (lifetime revenue) per signup
The two anchorings answer different questions. Customer-anchored numbers measure unit economics filtered by the conversion event, which matches how finance typically tracks acquisition. Signup-anchored numbers measure unit economics across the full entry pool, which matches the signup cohort population that multiplier analysis and related apparatus operate on.
Why CAC moves with conversion rate
CAC's denominator depends on both signup volume and the signup-to-customer conversion rate. SAC's denominator depends only on signup volume. The algebraic relationship is CAC = SAC ÷ conversion rate, so a drop in conversion rate raises CAC even when spend per signup hasn't moved at all.
A movement in CAC could reflect acquisition spend efficiency, conversion rate, or both, and the number alone doesn't say which. SAC isolates spend efficiency; conversion rate is its own measurement. Keeping them as separate instruments preserves diagnostic legibility when something shifts.
CAC underlies CAC payback
CAC is a dollar figure; CAC payback is a milestone defined against that figure — the point at which a cohort's accumulated revenue crosses its accumulated acquisition cost. The names overlap because the milestone is computed from the number, but they are different kinds of object. CAC says what was spent per customer; CAC payback says how long until that spend was recouped by a specific cohort's revenue.
Example
In Q3, an offering spent $500K on acquisition and ended with 2,500 net new paying customers from 25,000 signups. CAC was $200, SAC was $20, signup-to-customer conversion rate 10%. In Q4 the spend per signup held flat but conversion rate dropped to 8%. SAC stayed at $20; CAC moved to $250. The change in CAC said nothing about how efficiently acquisition was buying signups — only about what share of those signups went on to convert.