Click-to-open rate (CTOR) is the percentage of people who opened an email and then clicked at least one link in it. It shows how well the content persuaded the people who saw it, separately from how many people opened the email in the first place.
How click-to-open rate works
The formula is unique clicks divided by unique opens, multiplied by 100. Unique means each person is counted once, however many times they open the email or click.
Take a send that reaches 10,000 inboxes. 4,000 people open it and 300 click a link. The CTOR is 300 ÷ 4,000 = 7.5%. The click rate for the same email, unique clicks divided by emails delivered, is 300 ÷ 10,000 = 3%. The two figures answer different questions: click rate describes how the whole send performed, while CTOR describes how the email itself performed among the people who opened it.
That split helps with diagnosis. The subject line, sender name and preheader text mostly decide opens. Once someone is reading, the layout, offer, copy and call to action decide the click. A falling open rate with a steady CTOR points at the inbox; steady opens with a falling CTOR point at the content.
Why it matters for a UK business
CTOR used to be the cleanest way to judge an email’s content. Since Apple introduced Mail Privacy Protection in 2021, many opens are logged automatically when Apple’s servers load the email, whether or not anyone read it. Those phantom opens swell the bottom half of the sum, so CTOR now reads lower than the truth and moves with the share of Apple Mail users on your list as much as with the quality of the email.
It still earns its place if you calculate it with care. When you compare two emails sent to the same list a week apart, or two versions of a newsletter in a test, CTOR shows which content held attention better. For a small UK business with a modest list, it is one of the quicker ways to learn which kinds of content customers act on: a how-to piece, a product launch, an event invitation or a seasonal offer.
Common mistakes
- Comparing CTOR across lists with different proportions of Apple Mail readers, or against figures from before 2021.
- Using total clicks instead of unique clicks, so one enthusiastic reader inflates the result.
- Counting clicks on the unsubscribe link or the “view in browser” link as engagement. Check how your platform counts clicks before you trust the figure.
- Pushing CTOR up by mailing only recent openers, which improves the ratio while reducing sales.
- Treating a published industry benchmark as a target without knowing how its opens were counted.
How to act on it
Check whether your email platform can exclude machine or Apple privacy opens, and calculate CTOR on the remaining human opens where it can. Track it alongside click rate and the result you actually want, such as orders or enquiries, and never on its own.
When CTOR drops, look at the content. Was there one clear call to action? Did the link text say what would happen next? Did the email display properly on a phone? Test one change at a time on the same list so you know what made the difference. If you want help deciding which email measures belong in your reporting alongside search and ads, that is part of my digital marketing strategy and consulting work.