The click-to-open rate, or CTOR, is calculated by dividing the total number of unique clickers by the total number of opens.

To put this in a mathematical format, it would look like: Unique Clickers / Opens = CTOR %

Why not divide by clicks? Clicks is a higher number because it not only includes unique clicks, but repeat clicks as well.

In the message activity report, Total Clicks represents the number of unique clickers. The unique clickers are broken down into Repeat, contacts who have clicked a link in a previous send, and New, contacts who clicked a link for the first time in this send.

The conversation analytics report displays a different breakdown of data, however, unique clickers is still reported. Here it is represented in the Clickers.