Conversion rate (CVR) is the share of an audience that completed the action you were optimizing for, calculated as conversions divided by the base (clicks, sessions, or recipients). Because it is a percentage, it is only meaningful alongside the volume it was calculated on: a high rate on a small base can represent almost no conversions.
The metric that hides the number
CVR is genuinely useful for comparing like with like, one ad against another at similar volume, or a page before and after a change. Where it misleads is when it is used to rank things of very different sizes. A 10% conversion rate on 20 clicks is 2 conversions; a 3% rate on 2,000 clicks is 60. Sorted by rate, the first looks like the winner, and it produced thirty times fewer customers.
Turn the rate into a count
Before ranking campaigns by conversion rate, multiply the rate by the base to see the actual number, then rank by that and by revenue or pipeline. Small samples also make the rate jumpy in both directions, so treat early or low-traffic rates as noise. We work through the trap in your best-converting campaign might have made almost no sales.