How to use the CPM calculator
CPM is the cost of a thousand impressions: ad spend ÷ impressions × 1,000. The 1,000 is part of the definition rather than a convention, so the tool prints that constant inside the result label.
Cost per single impression is simply CPM divided by 1,000 again, so the two numbers on screen always reconcile. Add clicks and the tool also returns CTR (clicks ÷ impressions) and CPC (ad spend ÷ clicks), which shows whether cheap impressions actually turned into clicks.
CPM and eCPM run in opposite directions. CPM divides an advertiser cost by impressions; eCPM divides publisher revenue by impressions. The formulas look alike, so check first whether the figure in front of you is money spent or money earned.
Keep spend and impressions on the same period and the same campaigns. Platforms count an impression differently depending on how much of the ad had to be visible, so CPM from two networks is not directly comparable. Comparing periods within one platform is safer.
Frequently asked questions
The arithmetic matches but the point of view is reversed. CPM is what an advertiser paid for impressions, while eCPM is what a publisher earned from them. Platform fees keep the two from lining up.
It only means impressions were cheap. If the same people see the ad over and over, a low CPM can still produce nothing, so read it alongside CTR and conversion rate.