What is CPM?
Cost per mille, the amount you pay for one thousand ad impressions.
CPM = (Ad Spend / Impressions) x 1000
A worked example
A Meta campaign spends Rs 30,000 and delivers 1,20,000 impressions. Your CPM is Rs 250, meaning a thousand people saw the ad for Rs 250. Whether any of them noticed it is a separate question that CPM cannot answer.
Why it matters
CPM is the price of attention before anything happens with it, which makes it the cleanest available measure of how expensive your audience is to reach. Two brands bidding for the same women aged 25 to 40 in Mumbai will pay similar CPMs, so a CPM well above category norms usually means your targeting is narrow, your creative is being served poorly, or you are competing in a seasonal auction spike.
For anyone buying awareness, CPM is the working number. For anyone buying sales, it is a diagnostic rather than a goal. A rising CPM alongside stable cost per purchase is fine. A rising CPM alongside falling click-through is the early signal that creative is wearing out, usually two to three weeks before it shows up in your revenue.
CPM also explains a lot of seasonal pain. Indian ad auctions get expensive around Diwali, wedding season and major sale events, so a campaign that worked in August can look broken in October without anything changing on your side.
The nuance most people miss
Impressions and reach are different, and confusing them makes CPM meaningless. Impressions count every time the ad was served, including the fourth time to the same person. Reach counts unique people. A campaign with a CPM of Rs 200 and a frequency of 6 is showing your ad repeatedly to a small group, which inflates impressions and flatters CPM while genuinely reaching very few new people. Read CPM alongside frequency, always.
Common mistakes
- Optimising for the lowest CPM, which usually just means reaching cheaper and less relevant people
- Comparing CPM across placements. Reels, feed, Stories and Audience Network price very differently
- Ignoring frequency, so a low CPM hides the fact that you are showing one small group the same ad repeatedly
- Treating a seasonal CPM rise as an account problem when the whole auction moved