How it works
It joins the advertising and the site together: the advertising answers for who came, the site for whether they bought. That is why a falling conversion rate just as often means poor traffic as it means broken tracking.
Formula
Conversions ÷ clicks × 100%
Example
The same shop: 8,000 clicks, 300 orders.
Conversion rate = 300 ÷ 8,000 × 100% = 3.75%.