CTR (click-through rate)

The formula

• CTR = 100 · k / I

k is the number of clicks and I the number of impressions. The 100 turns the share into a percentage: 2,500 clicks on 100,000 impressions is a share of 0.025 and a CTR of 2.5%.

A worked example

An ad is shown 100,000 times and clicked 2,500 times: CTR = 100 · 2,500 / 100,000 = 2.5%. The same number of clicks on 500,000 impressions would be 0.5%.

From a reach to the clicks

Turned round, k = CTR · I / 100. A newsletter sent to 8,000 readers with a link that is clicked by 4% gives 4 · 8,000 / 100 = 320 clicks. The third rearrangement gives the impressions a click target needs: 500 clicks at 2.5% need 20,000 impressions.

CTR between CPM and CPC

A campaign priced per thousand views has a cost per click only through the CTR. At a CPM of 12.50, a thousand impressions cost 12.50 and, at 2.5%, give 25 clicks, so the CPC is 0.50. Double the CTR and the same views give twice the clicks for the same money.

Common mistakes

• Dividing by clicks instead of impressions. CTR is clicks over views, a share that is less than 1 for any ad.
• Comparing CTRs across placements as if they measured the same thing. A search ad, a banner and a link in an e-mail are seen in different ways, and their CTRs live on different scales.
• Counting the same person twice. Impressions count showings, not people. A CTR per unique viewer is a different figure.

Frequently asked questions