New-customer CAC
New-customer CAC is ad spend divided by the number of first-time customers acquired in the same period. It is the acquisition cost growth actually depends on, because it excludes repeat buyers who inflate every blended cost figure.
Cost per purchase and new-customer CAC are often confused because both divide spend by a count of buyers. The difference is who is in the denominator. Cost per purchase counts every order. New-customer CAC counts only people buying for the first time, which is the only group that grows the customer base.
A worked example, as arithmetic. $30,000 of spend, 600 attributed purchases, and a reported cost per purchase of $50. Matched against the customer list, 240 of those buyers were new. New-customer CAC is $125. If the first order carries $60 of contribution margin, each new customer starts $65 underwater, and the business is relying on repeat purchase to recover it. Whether that is a good bet depends on measured repeat behavior by cohort, not on hope.
The denominator is only as good as the customer history behind it. A list that starts two years ago will misclassify a customer who last bought three years ago as new. That is why the date the history begins should be stated alongside the figure, and why purchases before that horizon should be treated as unknown rather than new.
Report new-customer CAC by channel and by source rather than as one average. Prospecting on paid social and brand search will sit at very different levels, and the average hides which one is actually growing the base.
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