CAC (customer acquisition cost)

Marketing and sales spend in a period divided by the new customers acquired in the same period.

FormulaCAC = acquisition spend ÷ new customers (same period)

How we work it through

1
Decide what “new customer” means: a sign-up or a first paid order.
2
Include all the spend of the period — banners and discounts too, if they bought the customer.
3
Ask how many would have come without the spend.

On the practice dataset

The query we run on Meridian, Atterna’s practice marketplace, and what it returns.

-- promo week W38: €10,700 of marketing
SELECT 10700.0 / COUNT(*) AS cac
FROM meridian.buyers
WHERE registered_at >= '2026-09-14'
  AND registered_at <  '2026-09-21';

Result€13.90 per registration (770). Per buyer whose first paid order came that week (494) it is €21.66.

The common trap

Counting customers who would have come anyway, or leaving out part of the spend, so CAC looks lower than it is.

When an AI answers

An AI assistant

“CAC in the promo week was €13.90.”

What it didn’t know: It counted registrations, not buyers, and did not know the promo discount was also a cost.
Ask back: “New sign-ups or new paying customers? Which costs are in the numerator?”

Paste a real AI answer into the checker — it writes these questions for you →