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CAC Payback Period

Measure and manage the payback period — how many months of gross margin it takes to recover the CAC — to ensure paid growth is profitable.

Why

CAC payback period is the single metric that tells you whether paid growth is sustainable — a 6-month payback means every dollar spent is recovered in 6 months and everything after is profit, a 24-month payback means you're financing 2 years of growth before breaking even. The target depends on your gross margin and cost of capital: SaaS at 80% margin can afford 12-18 months, consumer at 30% margin needs <6 months. A short payback compounds (recovered CAC can be re-spent on acquisition), while a long payback drains cash (you're spending next quarter's revenue on this quarter's growth). The discipline is to measure payback per channel and per cohort, because blended payback hides that one channel is profitable (4 months) and another is bleeding (30 months). Managing payback, not just CAC, is what separates growth that scales from growth that bankrupts.

When

When paid acquisition is live. Measure monthly; the payback period is the constraint on how fast you can scale paid spend.

How

1. Calculate CAC per channel: total ad spend / attributed signups (use channel-specific attribution, not blended)
2. Calculate gross margin per user: (revenue - COGS) / revenue; for SaaS, margin is usually 70-85%
3. Calculate monthly gross profit per user: ARPU × gross margin; e.g. $20/mo × 80% = $16/mo gross profit
4. Payback period = CAC / monthly gross profit; e.g. $48 CAC / $16/mo = 3 months (excellent)
5. Set a payback target based on margin and capital: SaaS (80% margin) target <12 months; consumer (30% margin) target <6 months
6. Measure payback per channel and per cohort; a blended 8-month payback may hide a 3-month channel and a 20-month channel
7. Scale spend on channels with payback < target; cap or cut channels with payback > target — they drain cash that the profitable channels could use
8. Monitor the trend: if payback is lengthening, CAC is rising or retention is falling; investigate before it crosses the target

Example

# CAC payback per channel (monthly)
Gross margin: 80%  |  ARPU: $25/mo  |  Monthly gross profit: $20/user

Channel     | CAC   | Payback  | vs target (12mo) | Action
Google Search| $34  | 1.7 mo   | ✓ excellent      | scale +20%/wk
Reddit       | $26  | 1.3 mo   | ✓ excellent      | scale +20%/wk
LinkedIn     | $95  | 4.8 mo   | ✓ ok             | hold, optimize
Display      | $180 | 9.0 mo   | ✗ above target   | cut, reallocate
TikTok       | $240 | 12.0 mo  | ✗ at target      | kill (no margin for error)

# Blended (misleading if not decomposed)
Blended CAC: $42  |  Blended payback: 2.1 mo  (looks great!)
# But display + TikTok are burning cash that Google+Reddit could use to scale

# Trend (monthly payback, Google Search)
Jan: 1.5mo  Feb: 1.6mo  Mar: 1.7mo  → slowly rising (CAC creeping up)
# Action: optimize landing page before payback crosses 2mo

Common Problems

  • ⚠Measuring blended CAC and blended payback, which hides that one channel is profitable and another is bankrupting the company.
  • ⚠Targeting a CAC number without considering payback — a $30 CAC is great at $5/mo margin (6mo payback) and fatal at $2/mo margin (15mo).
  • ⚠Scaling spend on a channel before measuring its payback; volume goes up but cash flow goes negative.
  • ⚠Ignoring the payback trend; a channel that was 3-month payback in Q1 may be 8-month in Q3 as audience saturates.

Best Practices

  • ✓Measure payback per channel and per cohort, not blended; the blended number hides the channels that are draining cash.
  • ✓Set the payback target from your gross margin and cost of capital; SaaS can afford 12-18 months, consumer needs <6.
  • ✓Scale spend only on channels with payback below target; cap or cut the rest and reallocate to the profitable ones.
  • ✓Monitor the payback trend monthly; a lengthening payback is the early warning that CAC is rising or retention is falling.