Click-through rate(CTR) is calculated by taking the sum of all clicks divided by the sum of all impressions.

sum(clicks) / sum(impressions) x 100 

When looking at CTR totals for an advertiser running multiple campaigns, ensure you are adding up all of the campaign clicks and divide that number by the total of all the campaign impressions.

Example:

Total Clicks = 9,053

Total Impressions = 197,305

9,053/197,305 = .04588 x 100 = 4.59%