---
title: "CAC and CAC payback by channel | Cometly Academy"
description: "The single most useful number in B2B SaaS growth is CAC payback by channel. If a channel pays back in under 12 months and your churn is below 3.5% per month, you can probably safely scale it. Above 18 months, you need to fix unit economics first. Cometly calculates this directly so you can stop running it in spreadsheets."
source: "https://www.cometly.com/academy/cac-and-cac-payback-by-channel"
---

# CAC and CAC payback by channel

The decision rule that tells you whether to scale.

- Module 05: Strategy & Reporting
- Lesson: 5.3
- Track: Both
- Format: Report
- Read time: 8 min

The single most useful number in B2B SaaS growth is CAC payback by channel. If a channel pays back in under 12 months and your churn is below 3.5% per month, you can probably safely scale it. Above 18 months, you need to fix unit economics first. Cometly calculates this directly so you can stop running it in spreadsheets.

## Key takeaways

- 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

CAC payback is the most-quoted, least-calculated metric in SaaS. Every founder claims to know theirs; few can produce it broken down by channel with confidence. This report does that calculation directly inside Cometly with the same data your CFO uses.

## Why it matters

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

## Common pitfalls

### 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.