Facebook Ads CPM: what it measures, and why it isn't enough

CPM is the cost per thousand impressions of your ad. Spend €10, get your ad shown 1,000 times, and your CPM is €10. It is the first price you pay on Facebook and Instagram, before anyone even clicks.

The formula: CPM = spend ÷ impressions × 1,000.

Why everyone talks about CPM

Because it is the easiest number to compare. A CPM that doubles from one month to the next is visible straight away, and worrying. But it says nothing about what the ad brings in: a campaign can have a high CPM and an excellent cost per customer, or the reverse.

What makes CPM move

Meta sells its impressions at auction. The price depends mostly on who else wants to reach the same people, at the same time.

• The season. November and December are the most expensive months: every brand wants to be seen before Christmas and Black Friday. A tradesperson who sells all year round then pays the price set by online shops.
• The audience. The smaller the area and the tighter the targeting, the more advertisers compete for the same people. A 15 km radius around a showroom often costs more per thousand than a whole region.
• The creative. Meta favours ads that people watch. A video that stops the scroll pays less for its impressions than an image everyone skips.
• The campaign objective. Asking Meta to find people ready to fill in a form targets a rarer, so more expensive, audience than an awareness campaign. Choosing the right objective matters more than the price per thousand.

A low CPM is not good news in itself

It is the classic trap. You try to lower the CPM, widen the audience or switch to a "reach" objective, and impressions become very cheap. They are also shown to people who will never buy.

What counts is the rest of the chain: out of a thousand impressions, how many clicks, then how many enquiries, then how many customers. A CPM twice as high that brings enquiries three times better qualified is a much better deal.

In the accounts I run, CPM is never the first number I look at. The first is cost per lead, then the cost of a signed customer, compared with the margin. CPM then helps explain why those numbers move.

When CPM deserves your attention

• It jumps for no seasonal reason. Often, the creatives are wearing out: the same people see the same ad too many times. Ad fatigue shows up there first.
• It is far above the rest of your account. An audience that is too narrow, or a creative nobody watches.
• You are planning a November budget. Better to know that the same amount will buy fewer impressions, and decide accordingly.

What it means for your budget

You can't negotiate CPM. Part of it you simply pay, and the part you can work on goes through the creative. The only useful question: at this price, do I make money on each customer? To answer it, you need a budget built from the cost of a customer, not from the price per thousand.

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Further reading: How much a Facebook ad costs and the Facebook advertising glossary.

Has your CPM gone up and you're not sure whether it matters? Book a 30-minute call, and we'll look at your account together.