Click Through Rate (CTR) is a measure of how many links were clicked in relation to how many emails were opened.

It is a very useful measure of a recipient's engagement with an email because it requires them to do something, that is, to click. 

CTR is calculated for every email sent and can be seen on the Email Performance (tracking) page.

CTR = (total link clicks) / (unique email opens) x 100%

CTR is shown on the Clicks graph on the Email Performance page as a percentage,such as:

12.62%
         18

Where 12.62% is the CTR and (18) is the total number of clicks for that email.