CAC (customer acquisition cost)
What one new customer actually costs you: all your marketing and sales spend divided by the customers it brought in.
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What it is
CAC is total sales and marketing spend for a period divided by the number of new customers that period produced. Spend 10,000 dollars, get 200 customers, CAC is 50 dollars. The number only means something next to two others: LTV (what a customer is worth over their lifetime) and payback period (how many months of revenue it takes to earn the 50 dollars back). A rough healthy target for subscription products is LTV at least 3x CAC with payback under 12 months.
Reach for it when you are deciding whether to pour more money into a channel. CAC per channel is the useful cut: paid search, paid social, and affiliates almost never cost the same, and the average hides which one is quietly broken.
Gotcha: blended CAC (all spend divided by all customers, including the ones who found you organically) always looks better than paid CAC (ad spend divided by ad-attributed customers). People quote blended when fundraising and paid when budgeting, then compare the two and panic. Pick one definition, write it down, and include salaries and tool costs, not just ad spend.
Ask AI for it
Build a CAC calculator panel. Inputs: the measurement period, total sales and marketing spend for that period (ads, salaries, tools), new paying customers won, monthly ARPU, contribution margin percent, and either monthly churn or expected customer lifetime in months. Outputs, shown as three large stat tiles: blended CAC, LTV to CAC ratio, and payback period in months. Print every formula and every assumption on screen, including exactly how LTV was derived, and flag any input left blank instead of filling it with a guess. Color the ratio tile green above 3x, amber between 1x and 3x, red below 1x, and add a small per-channel table underneath (channel, spend, customers, CAC) sorted cheapest first. Keep it one screen, no charts, numbers big enough to read from across the room.