Definition:
CTOR (Click-to-Open Rate) is the percentage of recipients who clicked at least once in an email among those who registered an open. It is an email marketing metric that relates interaction with the content to the audience that opened the message.
CTOR does not measure the performance of the entire send. Its denominator excludes delivered emails that were not opened, so it helps examine what happened after the open. It also does not show whether a click led to a purchase, registration or another conversion.
Table of contents
What CTOR measures
CTOR shows whether an email’s content generates interaction among people who open it. It can provide context about the clarity of the proposition, the relevance of the message, the visibility of links and the calls to action.
The metric is most useful when equivalent campaigns are compared. The audience, type of email, objective, frequency and measurement method need to be similar. A change can guide an investigation into the content, but it does not identify the cause on its own.
The reading needs to be completed with link-level clicks and subsequent behaviour. Visits, conversions, unsubscribes, bounces or revenue can show whether the interaction produced the intended result.
How CTOR is calculated
The usual formula relates recipients with at least one click to recipients with a recorded open:
CTOR = (unique clicks / unique opens) × 100
If an email records 4,000 unique opens and 500 recipients click at least once, its CTOR is 12.5%. Counting unique users prevents several opens or clicks by the same person from multiplying the result.
Platforms may use different names or deduplication criteria. Before comparing reports, check whether the tool displays unique clicks, total clicks, unique opens and filtered automated activity.
CTOR, CTR and open rate
The three metrics describe different stages of a campaign. For the same send, they can be expressed as follows:
- Open rate: Divides unique opens by delivered emails. It focuses on the recorded open, although this measurement has technical limitations.
- CTR: Divides unique clicks by delivered emails. It reflects the ability of the complete send to generate clicks.
- CTOR: Divides unique clicks by unique opens. It focuses on the response from people who opened the message.
In the previous example, if 10,000 emails were delivered, the open rate would be 40%, CTR 5% and CTOR 12.5%. All three percentages are compatible because they use different denominators.
Measurement limitations
CTOR depends on reliable open and click tracking. An open is usually detected when a tracking image is downloaded. Image blocking can prevent that record, while privacy features such as Apple Mail Privacy Protection can generate automated opens. Security systems and link previews may also open messages or visit links before the recipient does.
Mailchimp explains that automated activity can inflate opens, clicks and clicks per unique open. An increase in the denominator can artificially reduce CTOR, while automated clicks can raise it. Bot filters and the provider’s measurement rules therefore need to remain consistent when periods are compared.
There is no ideal CTOR that applies to every campaign. The result depends on the industry, audience, objective and platform methodology. It becomes more useful when compared with an organisation’s own history and post-click metrics, rather than treated as an isolated score.