Channel-level CAC payback is the single best heuristic for whether to scale a paid channel. If LinkedIn ABM pays back in 9 months and Meta pays back in 22 months, you scale LinkedIn and rework Meta — even if Meta has lower headline CAC. The payback metric captures both acquisition cost and revenue quality in one number.
Calculating it
CAC by source = paid spend ÷ new paying customers from that source. Use the same attribution model as your CFO-ready ROAS report (lesson 5.2) and the same window.
Payback months = CAC ÷ first-month ARPA (average revenue per account). For PLG: use first-month MRR. For SLG: use either the average monthly contract value (ACV ÷ 12 for annual contracts) or the first-month booking value, depending on how your finance team books revenue.
Target ratios for venture-backed B2B SaaS: CAC ≤ 1/6 of LTV, payback ≤ 12 months, monthly churn < 3.5%. For bootstrapped or capital-efficient companies, payback target is closer to 6 months.
Acting on it
Channels under target on both CAC and payback are scaling candidates. Channels above target on either are audit candidates — usually one of three problems: poor creative-audience fit, mismatched optimization event, or saturation in the targetable audience.
Run the report monthly. Payback drifts with churn, ARPA, and channel saturation. A channel that paid back in 10 months last quarter might pay back in 16 months this quarter without any change in spend — usually because of audience saturation or churn changes.
- CAC by source = spend ÷ new customers, attribution-model-consistent
- Payback = CAC ÷ first-month ARPA (or ACV ÷ 12)
- Target: payback ≤ 12 months for VC-backed; ≤ 6 for capital-efficient
- Cross-check against churn and LTV — payback alone hides quality issues
What to watch for.
- Calculating CAC against all customers, not new customers
Existing customer revenue isn’t marketing’s. Use only new customers in the denominator.
- Comparing channels with different attribution models
If two channels are calculated against different models, their CAC isn’t comparable. Use one model.
- Treating payback as static
Payback drifts with churn and ARPA. Recalculate monthly.
Recap.
- CAC = paid spend ÷ new paying customers, by source
- Payback months = CAC ÷ ARPA (average revenue per account per month)
- Target CAC ≤ 1/6 of LTV for sustainable scale
- Target CAC payback ≤ 12 months for venture-backed growth
- Run this report monthly — payback drifts with churn, ARPA, and channel saturation