Unit economics: CAC, LTV and payback

A company can grow ARR and still be dying. Unit economics asks a narrower question: if we acquire one more customer of this type, do we make money on them — and how long until we get our cash back?

If the unit loses money, scaling marketing or hiring more salespeople makes the hole deeper. If the unit is profitable but payback is 30 months, you need a lot of capital to grow. The math is simple. The traps are in definitions.

Pick the unit that matches how money moves: a customer for B2B SaaS, an order for a marketplace, a first purchase for consumer, an API key for usage. This map uses a subscription customer, the common software case.

The formula LTV = (ARPA × gross margin) / churn is a steady-state shortcut. Once you have months of data, plot cohort contribution: revenue × margin minus variable support, minus CAC, month by month. Healthy SaaS crosses zero and keeps rising. Mixed segments lie in the blend — $8k SMB LTV and $400k enterprise LTV with a $15k blended CAC is not "2.7×."

As an engineer you rarely own CAC, but you own surfaces that move it: activation, time-to-value, self-serve checkout, seat expansion, churn from reliability. A 10% relative drop in monthly churn (2.0% → 1.8%) lifts LTV by about 11% in the simple model. Sit with finance once and ask for the contribution curve on the segment your team ships for. Then prioritize against payback and retention, not only against engagement vanity metrics.

Concepts in this section

The numbers every board has seen, in one place:

ARPA $1,000 / mo  ×  80% GM  =  $800 contribution
Churn 2% / mo                 →  LTV = $40,000
CAC $15,000                   →  payback ≈ 19 months
LTV / CAC                     ≈  2.7×   (shy of 3×)

Common mistakes

• Celebrating ARR growth without asking whether one more customer is profitable.
• Ad-only CAC and revenue LTV. Both flatter the ratio.
• One blended number across SMB and enterprise. The blend hides the motion that is underwater.
• Treating LTV as a fact from a short history. Prefer payback and cohort curves when young.

Key takeaways

• Unit economics ask whether one more customer is profitable.
• CAC is fully loaded S&M / new customers. Track blended and paid.
• LTV ≈ (ARPA × GM) / churn. The classic case is $40k.
• Payback is the cash question. $15k CAC → about 19 months in that case.
• LTV/CAC > 3 is a heuristic. Distrust long lifetimes. Segment everything.

Further learning

Startup Killer: the Cost of Customer Acquisition (David Skok)In the many thousands of articles advising entrepreneurs on what they have to focus on to build successful startups, much has been written about three key factors: team, product and market, with particular focus on th…For Entrepreneurs

The Dangerous Seduction of the Lifetime Value Formula (Bill Gurley)Many consumer Internet business executives are loyalists of the Lifetime Value model, often referred to as the LTV model or formula. Lifetime value is the net present value of the profit stream of a customer. This con…Above the Crowd