Clicks + impressions → CTR

Click-through rate
Cost per click (CPC)
Cost per 1,000 impressions (CPM)

Click target + CTR → impressions needed

Impressions needed
Estimated spend

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The CTR formula, and what it does and doesn't tell you

CTR % = (clicks ÷ impressions) × 100

An ad served 40,000 times that earns 900 clicks has a click-through rate of (900 ÷ 40,000) × 100 = 2.25%. The same arithmetic works for an email (clicks divided by messages delivered), a search listing (clicks divided by impressions in Search Console), or a button on a page (clicks divided by views).

Two numbers travel with it. CPC is spend ÷ clicks, and CPM is (spend ÷ impressions) × 1,000. CTR tells you whether the creative earns attention; CPC tells you what that attention costs. A CTR that climbs while CPC climbs faster is not a win.

What CTR never tells you is whether the click was worth having. That's conversion rate's job. The failure mode is a headline that overpromises: clicks spike, the landing page underdelivers, and the campaign reports beautifully while selling nothing.

CTR is one input into share of voice — how much of your market's attention you hold — and its slower cousin share of search.