What is click-to-open rate?

Unique clicks divided by unique opens — how well the content converted the audience that actually saw the email.

How does CTOR differ from CTR?

CTR divides clicks by delivered (whole-funnel); CTOR divides by opens, isolating content effectiveness.

How did Apple MPP affect CTOR?

Machine-generated opens inflate the denominator, deflating measured CTOR — trend comparisons within a list survive; absolutes don't.

Growth Marketing Glossary

Click-to-Open Rate (CTOR)

click-to-o·pen rate/klɪk tɪ ˈoʊpən ɹeɪt/noun

Of the people who opened, how many cared? — the content-quality metric opens can't fake. Mostly.

Schematic — clicks divided by opens

Term: Click-to-Open Rate
Abbreviation: CTOR
Formula: Unique clicks ÷ unique opens
Post-2021 caveat: Denominator inflated by Apple MPP

Forms & parts of speech

CTOR · acronym

Clicks over opens.

"CTR is flat but CTOR fell by a third — the list still opens, the content stopped landing."

Definition in plain terms

Click-to-open rate divides unique clicks by unique opens: of the people who (apparently) saw the email, what share clicked? Where click-through rate (clicks ÷ delivered) blends list quality, subject line, and content into one number, CTOR isolates the content question — did the message earn action from its actual audience? An email delivered to 10,000, opened by 3,000, clicked by 450 has a 15% CTOR.

The mechanics

CTOR's diagnostic power is the separation: low open rate with high CTOR means a targeting/subject problem wrapped around good content; high opens with low CTOR means the envelope writes checks the letter can't cash. The 2021 caveat is structural — Apple's Mail Privacy Protection inflates the open denominator with machine opens, DEFLATING measured CTOR for Apple-heavy lists. It remains useful for trend comparison within a stable list and for relative judgments between sends; absolute cross-list benchmarks died with honest opens.

When it matters

CTOR matters as the email program's content-quality meter — the number that arbitrates between 'our newsletter is boring' and 'nobody sees it.' Segment-level CTOR finds where relevance decays (engaged-cohort CTOR triple the tail's = a list-hygiene mandate). Post-MPP, sophisticated teams either filter machine opens where their ESP supports it or quietly migrate CTOR's job to click-per-delivered trends — keeping the QUESTION while replacing the compromised denominator.

Worked example. A media company's newsletter holds a steady 42% open rate while revenue per send sinks. The CTOR cut tells the story opens hid: 18% to 7% over two quarters — readers still open from habit, but the content stopped earning clicks. The rebuild runs on CTOR as judge: subject lines stop overpromising (closing the envelope-letter gap), the link hierarchy drops from fourteen to four, and each section gets a CTOR target. Two months later CTOR recovers to 14% and revenue follows — the audience was never lost, just bored.

Failure modes to watch. Benchmarking absolute CTOR across lists post-MPP; reading low CTOR as a list problem when it's a content problem; stuffing more links in to chase clicks; and ignoring the machine-open denominator drift.

Synonyms & antonyms

Synonyms

CTORclick-to-open rate

Antonyms

CTR (clicks per delivered)unclicked opens

Origin & history

*No verifiable first use is documented; the account is built from how the field adopted the term. CTOR emerged from 2000s email-marketing analytics as ESPs sought a content metric independent of list size — the clicks-over-opens ratio became a standard dashboard line as the industry matured, and survives (with caveats) the 2021 machine-open era.

Etymology: source.

Usage trends

Search interest for this term over the last five years:

View interest-over-time on Google Trends →

Common questions

What is click-to-open rate?: Unique clicks divided by unique opens — how well the content converted the audience that actually saw the email.
How does CTOR differ from CTR?: CTR divides clicks by delivered (whole-funnel); CTOR divides by opens, isolating content effectiveness.
How did Apple MPP affect CTOR?: Machine-generated opens inflate the denominator, deflating measured CTOR — trend comparisons within a list survive; absolutes don't.

Related tools & calculators

toolFunnel drop-off analyzer

Resources & people to follow

referenceLitmus — MPP-era email analytics guidance
referenceMailchimp/Campaign Monitor — CTOR benchmark methodology
referenceRGM analysis — migrate CTOR's job to click-per-delivered trends post-MPP

Curated, non-competitor resources verified per term.

Related training

moduleMarketing analytics

Disciplines

Areas of marketing where click-to-open rate (ctor) is a core concern:

MeasurementGrowth strategy

Sources

trendsGoogle Trends — "click to open rate"