Email Capture Rate
The percentage of website visitors who leave an email address behind, through any form, popup, or footer field, not just one specific popup.
Email capture rate is a site-wide metric, whereas a single popup's conversion rate only measures that one placement. A healthy site is usually collecting emails through several channels at once: a footer form, an exit popup, a scroll-triggered offer, and a checkout opt-in.
Comparing capture rate across traffic sources is often more revealing than the overall number. Visitors arriving from an email newsletter already trust you and convert differently than cold traffic from a paid ad, and a single blended rate hides that.
Raising this number by adding more popups has a ceiling: past a certain point, additional prompts just annoy visitors who already declined once. Better copy and better timing on the popups you already have usually outperforms adding a third one.
See it in practice