The CAC payback maths every founder should know

By the Imustech growth team · Updated October 2025

If you're spending on Google, Meta or LinkedIn without knowing your CAC payback, you're not doing marketing — you're gambling. The good news: the maths is genuinely simple. Three numbers, one ratio, one afternoon to build it. Here's how.

The three numbers you need

• CAC — Customer acquisition cost. All-in blended spend / new customers.
• Gross margin per customer per month — Not revenue. Margin.
• Retention curve — What % of month 1 revenue is still there in months 3, 6, 12?

If you can't tell me your CAC in the last 30 days, you don't have a paid-media problem. You have an accounting problem.

The single ratio: CAC payback months

Divide CAC by monthly gross margin. That's your payback in months. Under 12 months and you have a growth engine. 12–24 months and you have a business with a plan. Over 24 months and you have a subsidy pretending to be a business.

What good looks like, by category

• Consumer D2C: 3–6 months payback.
• SMB SaaS: 6–12 months payback.
• Mid-market SaaS: 12–18 months payback.
• Enterprise SaaS: 18–24 months payback (with strong retention).

Three levers, three orders of magnitude

When payback is too long, most founders reach for "lower CAC" first. That's usually a mistake. Try — in this order:

• Retention: keeping a customer 30% longer is more valuable than reducing CAC 30%.
• Margin: a 10% price rise passes almost entirely through to margin.
• CAC: hardest, slowest and most easily gamed by measurement.

The single dashboard every founder should have

A weekly view showing CAC, monthly gross margin per customer, month-N retention and payback in one place. Not attribution — payback. If you don't have it, we can build it in a week. Yours to keep afterwards.