Cost per mille (CPM)
Ad cost per thousand impressions. You pay to be shown, whether or not anyone clicks.
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What it is
CPM is the price of one thousand impressions. 'Mille' is Latin for thousand, which is why the M is not 'million' and why nobody spells it right. The math is spend divided by impressions, times 1,000: $60 for 40,000 impressions is a $1.50 CPM.
It wears two hats. It is a way to buy media, and it is also a plain reporting number you can compute on any campaign, including one you bought on clicks or optimized for purchases, just to compare what reaching a thousand people costs across channels. So 'our CPM went up' does not mean anyone changed billing models. It usually means the auction got more expensive.
You buy on CPM when the goal is being seen rather than being clicked: brand campaigns, launches, podcast and newsletter sponsorships, billboards and TV (where it has been the currency for decades). It is also the common yardstick underneath everything else, since platforms convert every bid into an effective CPM (eCPM) to decide which ad wins an auction slot.
Gotcha: CPM says nothing about outcomes, only about price. A $2 CPM against an audience that will never buy is worse value than a $45 CPM against 3,000 of exactly the right people. Cheap CPMs usually mean cheap inventory: autoplay video nobody watched, ads below the fold, app placements with fat fingers. Check viewable impressions before celebrating a low number.
Ask AI for it
Build an interactive CPM vs CPC comparison calculator. Inputs: budget, CPM rate, expected CTR, and CPC rate. Outputs, side by side in two cards: impressions bought, clicks received, and effective cost per click for the CPM buy, versus clicks bought and implied CPM for the CPC buy. Highlight the cheaper route with a colored border and a one line verdict. Include a small formula strip showing spend / impressions x 1000 with live numbers substituted in. Sliders for every input, results updating instantly, currency to two decimals.