CAC and payback: what you can afford to spend
CAC is what it costs to win one customer; payback is how many orders (or days) until you earn it back. Together they decide whether paid growth is fundable or a slow bleed.
AI can compute both from your numbers and tie them to your cash position. You supply real ad spend, orders, and margin, because AI can't see them.
Fastest path: one prompt, end to end
🤖 AI prompt — paste into ChatGPT / Claude
You are a growth-finance analyst. Use MY numbers only (invent nothing).
Ad + acquisition spend last month: [$]
New customers acquired: [#]
Contribution margin per order: [$] Avg orders per customer in first 90 days: [#]
Do this and show the math:
1. CAC = spend / new customers.
2. First-order profit after CAC = contribution margin - CAC. Am I profitable on order one?
3. Payback: how many orders (and roughly how many days, using my repeat rate) until cumulative contribution covers CAC.
4. Verdict: is this CAC affordable given my margin and cash cycle? If payback is long, flag the cash-flow risk (ties to cashflow-basics).
If a number is missing, ask; do not guess.
Output: CAC + first-order math + payback + affordability verdict.
Or do it in 4 steps
- Compute CAC honestly. All acquisition spend (ad spend + agency fees + creative + any tools) ÷ new customers won. Using ad spend alone under-counts real CAC and makes campaigns look better than they are.
- Check first-order economics. Compare CAC to your contribution margin per order (see contribution-margin). If contribution ≥ CAC, you profit on the first order, the safest place to be. If not, you're relying on repeat purchases to break even.
- Know your payback period. How many orders, and how many days, until cumulative contribution from a customer covers their CAC. Short payback (first order, or under ~2-3 months) is fundable from cash; long payback needs a cash cushion because you're underwater until it lands.
- Set your affordable CAC and hold the line. Your max CAC is your first-order contribution (for immediate profit) or a defensible multiple of it if repeat purchases are proven. Scale spend only while CAC stays under that ceiling, rising CAC with long payback is how growth burns cash.
Worked example (labeled): spend $2,000, win 50 customers → CAC $40. Contribution per order $33. First order loses $7 (33 − 40).
If customers average 2.2 orders in 90 days, cumulative contribution ≈ $73 > $40, so payback lands around order two, roughly 6-8 weeks out. Fundable only if your cash can wait that long, otherwise lower CAC or slow spend. Run your own repeat rate.
Track CAC and payback monthly by channel; scale the channels with fast payback, fix or cut the slow ones.