The total cost to acquire one paying customer: all sales and marketing spend over a period, divided by the number of new customers it brought in. CAC is the single number that decides whether growth makes you money or just makes you busy. The mistake most founders make is measuring CAC at the platform level (what Meta or Google reports) instead of blended across every dollar of acquisition spend, which is the only version that ties back to the P&L.
Benchmark. There is no universal 'good' CAC. The only test that matters is CAC against contribution margin and payback: under 3 months is excellent, 3-6 is the healthy standard, 6-12 is workable, and over 12 is fragile.