Cost per click (CPC) is the average price an advertiser pays per ad click, computed as ad spend / clicks over the window in view. A “click” on modern auction platforms means any billable interaction with the ad unit, not only clicks that load the destination URL, and the window matters: CPC over a day, a week, and a quarter can look very different on the same campaign.
The label hides two meanings. CPC originated as a billing model: the advertiser bids and is charged per click, the historical Google Ads default. It is also a derived efficiency metric: the advertiser bought impressions on a CPM basis, and CPC is the resulting per-click cost after the auction settled. Modern platforms report CPC either way, even when the bid wasn’t click-based, which is where most operator confusion starts.
CPC is most useful as the upstream half of a CPA diagnosis. The identity CPA = CPC / conversion rate says a worsening CPA traces either to a CPC problem (the auction got more expensive, ad relevance is falling, the audience is saturating) or a conversion rate problem (landing page, offer, fit). Reading the two together tells you which side moved.
CPC has no view into post-click quality. A $0.20 click that bounces is worse than a $2.00 click that converts, and CPC varies sharply by placement and audience: branded-search CPC tends to run structurally lower than cold-prospecting CPC. Chasing low CPC by bidding cheaper keywords or broadening audiences routinely raises CPA. Treat it as a diagnostic alongside CTR and conversion rate, not a goal in isolation.