Click-to-open rate (CTOR)
Click-to-open rate (CTOR) is the number of unique clicks divided by the number of unique opens, as a percentage. By using opens rather than deliveries as the denominator, it isolates how compelling the email's content and calls to action were among people who actually opened it. Because the opens denominator is now inflated by privacy-driven image pre-fetching, CTOR has become harder to trust and is read alongside raw click rates.
What this means
CTOR = unique clicks ÷ unique opens, as a percentage. Where click-through rate divides clicks by delivered emails, CTOR divides by opens — so it asks a narrower question: of the people engaged enough to open, how many clicked? That makes it a content-and-offer signal rather than a deliverability or subject-line one.
Why the denominator matters
Switching the denominator from delivered to opened removes subject-line and inbox-placement effects, focusing CTOR on the body of the email. But because Apple's Mail Privacy Protection inflates opens, the denominator is now overstated, which can artificially lower CTOR for audiences with many Apple Mail users.
- CTOR = unique clicks ÷ unique opens
- Isolates content/CTA quality from subject-line pull
- Inflated opens distort the denominator post-MPP
Why it misleads
CTOR inherits all the noise in open tracking. With opens overstated by pre-fetching, CTOR can drift without any change in real content quality. Read it against raw click rate (clicks ÷ delivered), which does not depend on the open pixel, to separate genuine content shifts from open-counting artefacts.
How it appears in analytics and logs
A low CTOR means people who opened did not click — the content, offer, or layout under-delivered relative to the subject line that earned the open; inflated opens can also depress it artificially.
Diagnostic use case
Use CTOR to judge email content and call-to-action effectiveness independently of subject-line pull, while accounting for the open inflation that privacy features introduced into the denominator.
What WebmasterID can help detect
WebmasterID measures the on-site journey after the click, so email content effectiveness can be validated by real first-party behaviour rather than open-based ratios alone.
Common mistakes
- Reading CTOR without accounting for inflated opens.
- Confusing CTOR (clicks ÷ opens) with CTR (clicks ÷ delivered).
- Comparing CTOR across audiences with different mail clients.
Privacy and accuracy notes
CTOR combines pixel opens with click tracking, both recipient-linked; prefer aggregate reporting and treat this as educational, not legal advice.
Related pages
- Email open rate
Email open rate is the number of opens divided by the number of emails delivered, as a percentage. It is measured by a tiny tracking pixel that loads when the message is viewed. Since Apple's Mail Privacy Protection began pre-fetching images regardless of whether a person opened the email, pixel-based opens are inflated and unreliable, so open rate is now read as a soft signal rather than a precise engagement measure.
- Click-through rate (CTR)
Click-through rate is clicks divided by impressions, expressed as a percentage. The catch is what counts as an impression: Google Search Console counts a result appearing in search, while ad platforms count an ad being served or viewed. Because the denominator differs by platform, CTR figures are only comparable within the same system — and a low CTR can mean wrong audience or simply low intent.
- Email deliverability rate
Email deliverability rate is the share of sent emails that were accepted by receiving servers — delivered divided by sent, the inverse of the bounce rate. But 'delivered' only means not bounced; it does not say whether mail reached the inbox or the spam folder. True inbox placement depends on authentication (SPF, DKIM, DMARC), sender reputation, and engagement, which is why deliverability is read with placement and complaint signals.
- Event Explorer
See what clickers did after arriving.
Sources and verification notes
- Apple — Protect Mail activity (Mail Privacy Protection)Open inflation context; CTOR formula is an email-marketing convention.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.