CAC is the number every board asks for and most companies compute wrong. Payback is the one that actually tells you whether growth is funding itself or quietly eating the balance sheet.
CAC equals sales and marketing cost in the period, divided by closed-won in the period. Payback equals CAC divided by monthly gross profit per customer.
If your sales cycle is 90 days, this quarter's wins were paid for last quarter. Lag the cost by one cycle length, or the number will lie to you in both directions and you will make decisions on the noise.
Under 12 months for SMB, under 18 for mid-market. Above 24 months you are financing your own growth, and every acceleration makes the cash position worse before it makes it better. That is survivable if you know it. It is fatal if you do not.
Using revenue instead of gross profit in the payback denominator. It makes the number look roughly a third better than reality, which is exactly the margin of error that turns a bad quarter into a surprise.
Trust lands in both halves of this fraction. Belief built before the call shortens the cycle, so the same team closes more in the same period, which lifts the denominator. And the win rate rises, because the buyer arrives already convinced rather than needing to be talked into it. This is the number to watch after a Build, and it is the one that takes a quarter to show up.
Worked example
| Sales and marketing cost, lagged one cycle | $180,000 |
| Closed-won in the period | 12 |
| Gross profit per customer, monthly | $2,300 |
CAC $15,000, payback 6.5 months
Lift the win rate five points on the same meeting volume and you close roughly 14 instead of 12. CAC falls to about $12,860 and payback drops under six months, with no change in spend.
No single number tells you much. Together they tell you where the trust is leaking and what it costs.
The Trust Diagnostic takes four minutes and costs nothing. The calculator on the finance page prices your own leak. Neither one asks for an email.
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