When you buy online advertising, the pricing model quietly decides what you’re actually paying for — being seen, being clicked, or getting a result. Three acronyms cover almost everything: CPM, CPC, and CPA. Understanding them is the difference between buying attention you can’t use and buying outcomes you can.
CPM — cost per thousand impressions
CPM (the “M” is the Roman numeral for 1,000) means you pay a set price every time your ad is shown 1,000 times, whether or not anyone clicks. If a site charges a $10 CPM and your ad is shown 50,000 times, you pay $500.
You’re buying visibility. CPM makes sense when the goal is awareness — you want a lot of the right people to see your name. The risk is obvious: you pay even if every single viewer ignores the ad.
CPC — cost per click
With CPC you pay only when someone actually clicks and visits you. Impressions are free; the click is the billable event. This is the default for search ads, where a business bids on what it’s willing to pay for a click on a given keyword.
You’re buying visits. CPC shifts the risk of a boring ad onto the advertiser’s creative, not your wallet — you don’t pay for a shrug. It’s usually the right starting point for small businesses because you only spend when you get a visitor. Google explains the mechanics of bidding in its Google Ads Help center.
CPA — cost per action
CPA means you pay when a specific action happens — a lead form submitted, a call placed, a sale completed. It’s the model closest to what you actually care about.
You’re buying outcomes. The catch is that CPA requires accurate conversion tracking — the platform has to know when a result happened. Modern ad platforms increasingly optimize toward a target CPA automatically, but they can only do that once you’ve told them what a “result” is.
Which one should you use?
A simple way to think about it:
- Building awareness for something new → CPM can be fine, but watch it closely.
- Capturing existing demand (people already searching) → CPC is the natural fit.
- Optimizing for a measurable result and you can track conversions → CPA.
Notice these aren’t really competitors — they’re rungs on a ladder. Most advertisers begin on CPC to buy visits, install conversion tracking, then let the platform move them toward CPA once there’s enough data to optimize on. The Interactive Advertising Bureau (IAB) maintains the industry standards these models are built on if you want to go deeper.
Rule of thumb: never buy a pricing model you can’t measure. If you can’t yet track results, start on CPC and buy visits while you build the tracking that makes CPA possible.
Once you know what you’re paying for, the next question is how the whole system fits together. See how online ad networks work, or our full Online Advertising Guide.