Click-to-Open Rate (CTOR)
CTOR doesn't measure how many people received the email. It measures, among those who opened it, how many found something inside worth clicking — the question open rate alone can no longer answer.
“If the subject line keeps getting opens but CTOR collapses, the problem is no longer convincing people to open the email. It's giving them a reason to do something after opening it.”
Otto GTM ObservatorySeparating the subject line problem from the content problem
CTOR allows isolating two often-confused variables: the subject line's ability to generate opens and the internal content's ability to generate action. An email with high open rate but low CTOR has an effective subject line and weak internal content; an email with low open rate but high CTOR probably has a deliverability or list targeting problem, not a message quality issue.
This distinction is particularly useful in B2B, where internal content often needs to communicate complex value in a few lines: monitoring CTOR over time by segment helps understand whether a performance decline depends on message fatigue, audience saturation, or delivery problems.
Anti-patterns
Common mistake: reacting to a low CTOR by further optimizing the email subject line, when the real problem lies in the content or internal call-to-action, not the ability to generate opens.
Second mistake: monitoring CTOR only in aggregate without segmenting by content type (educational, promotional, transactional), mixing very different behavior patterns into a single, poorly actionable number.
Practical Application
A company notices a series of promotional emails maintains a stable 38% open rate for three months, but CTOR drops from 14% to 5%. After testing a clearer, more specific call-to-action instead of a generic 'Learn more', CTOR climbs back to 12% with no change to the email subject lines, confirming the problem was in the content, not the opening.