Cost per thousand impressions: what CPM means and how to calculate it

What does CPM mean?

CPM means cost per thousand impressions. It expresses how much spend corresponds, on average, to each group of one thousand recorded impressions. It does not count unique people or confirm that anyone paid attention.

CPM = spend ÷ impressions × 1,000. Google Ads documentation describes CPM as a bid per thousand impressions on its Display Network. Multiplying by 1,000 converts a per-impression rate into a per-thousand rate. With zero impressions, CPM is undefined.

How do you calculate and combine two CPMs?

Imagine two campaign rows with the same currency, period, and impression definition:

Swipe the table to see all columns.

To calculate the combined CPM, add spend and impressions before dividing:

(CLP 60,000 + CLP 30,000) ÷ (30,000 + 10,000) × 1,000 = CLP 2,250

The combined result is CLP 2,250. It is not the simple average of CLP 2,000 and CLP 3,000, which would be CLP 2,500, because each row represents a different number of impressions. This example is fictional and is not a market reference.

Is a lower CPM always better?

Not by itself. A lower CPM means less spend for every thousand impressions counted under that definition. It does not necessarily mean more unique reach, attention, clicks, or conversions. When you compare, keep spend, impressions, and the exact field name alongside the CPM; then check the measure that matches the campaign’s goal.

Reach and impressions are not interchangeable: one person can generate more than one impression. Some platforms also offer viewable-impression metrics. Google Ads distinguishes CPM from viewable CPM, which is limited to impressions considered viewable under its definition. That distinction belongs to the metric and ad environment documented by Google; do not automatically extend it to other networks.

When comparing campaigns across platforms, first check that currency, dates, report level, and impression definition match. To decide whether spend led to visits or actions, also see the social media KPI guide and UTM tracking guide.