The maths
Budget needed = (target impressions ÷ 1,000) × CPM
The formula is trivial; the definitions are where people come unstuck. An impression is an ad being served, not a person seeing it, and certainly not a person noticing it. One individual scrolling past your ad six times generates six impressions and one reach.
That distinction matters because CPM is the denominator of most efficiency arguments. If two placements report the same CPM but one shows your ad to the same small audience repeatedly, they are not the same purchase at all.
Reading it properly
Low CPMs are not automatically good news. Inventory is cheap for a reason — it may be below the fold, in an app nobody looks at, or served to an audience with no interest in you. I have seen media plans optimised so hard towards a low CPM that the campaign bought vast quantities of attention from people who would never buy anything.
The useful way to read CPM is comparatively and within a like-for-like context: same audience, same placement, same time of year. Across those boundaries the comparison quietly stops being meaningful, because you are pricing different things.
Common mistakes
- Treating CPM as a performance metric. It measures cost, not outcome. A campaign can win on CPM and lose on every metric that pays the bills.
- Confusing impressions with reach. Impressions count deliveries; reach counts people. Frequency is the ratio between them, and it is where a great deal of budget quietly disappears.
- Comparing CPMs across formats. A skippable video impression, a small banner and a full-screen interstitial are not equivalent units, so their prices are not comparable.
- Forgetting seasonality. Auction prices climb steeply during retail peaks and Ramadan in the markets I work in. A CPM rise is often the market, not your campaign.
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Questions people actually ask
Does CPM mean cost per million?
No — the M is the Roman numeral for one thousand. CPM is cost per thousand impressions. It is one of the most common misreadings in the industry, and it makes budget calculations wrong by a factor of a thousand.
Is a low CPM always better?
No. CPM is a price, and cheap inventory is usually cheap because fewer advertisers want it. What matters is the cost of the outcome, not the cost of the impression. A higher CPM that reaches genuinely interested people is frequently the better purchase.
What is the difference between CPM and vCPM?
CPM charges for every impression served. vCPM charges only for impressions that met a viewability standard — typically a set portion of the ad in view for a minimum time. vCPM prices look higher because you are paying for a smaller, better-defined thing.
Why did my CPM suddenly increase?
Usually competition. Auction prices respond to demand, so retail peaks, holidays and major events push them up regardless of what you changed. Narrow audience targeting, small budgets and creative fatigue also raise the price you pay to keep reaching the same people.
How do I lower my CPM?
Broaden narrow audiences, refresh creative that has been running long enough to fatigue, and check whether you are competing with yourself across ad sets. But do not pursue a lower CPM as a goal in itself — it is easy to buy cheaper attention that converts worse.
Go deeper: How I run performance marketing · Measurement & analytics · Field notes