How many impressions does my budget buy?

Divide the budget by the CPM, then multiply by 1,000. A $5,000 budget at a $2.50 CPM buys two million impressions. Set the solver to 'Impressions' to do it directly.

What is the difference between CPM and eCPM?

CPM is a rate you agreed before delivery. eCPM is what the inventory actually earned per thousand impressions after everything that happened in between. A contracted CPM is an input; an eCPM is a result, and the two rarely match.

Why is the CPM I pay higher than what the publisher reports?

Because fees sit between the two. The DSP, the exchange, any curation or data layer and the SSP each take a share, so the buyer's gross CPM and the publisher's net CPM describe the same impression at different points in the chain. A gap of 30% or more between the two ends is unremarkable.

Should CPM be calculated on served or measured impressions?

On whichever the contract specifies, and state it. Served impressions come from the ad server, measured impressions from a verification vendor, and they differ by a few percent. Billing on one while reporting the other is a recurring source of reconciliation disputes.

CPM Calculator

Solve any one of cost, impressions or CPM from the other two. CPM is the unit almost everything in programmatic is priced in, and the arithmetic is a single multiplication — but the direction you need changes constantly during a working day, so this calculator runs all three ways.

Total cost

Impressions

CPM

$—

Enter cost and impressions.

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Formula

CPM = (Total Cost ÷ Impressions) × 1,000    ·    Cost = (CPM × Impressions) ÷ 1,000    ·    Impressions = (Cost ÷ CPM) × 1,000

All three are rearrangements of the same relationship. The factor of 1,000 is what the M in CPM means — mille, not million — and forgetting it produces answers off by three orders of magnitude, which is the single most common error in ad ops spreadsheets.

The CPM you pay is not the CPM the publisher receives

A single impression carries several different CPMs depending on where you stand in the supply chain. The advertiser's gross CPM includes the DSP fee. What reaches the exchange is lower. What reaches the SSP is lower again, and the publisher's net CPM sits at the bottom after the sell-side takes its share. Add a curation or data layer and there is another slice. None of these numbers is wrong — they are measurements at different points — but comparing a buyer's CPM to a publisher's reported CPM without accounting for the chain between them will always look like a discrepancy when it is arithmetic working as designed.

Where CPM stops being the right unit

CPM prices impressions, so it answers questions about reach and cost efficiency well and questions about outcomes badly. Two campaigns at identical CPMs can deliver completely different results if one runs on viewable, brand-safe, in-geo inventory and the other does not. Before comparing CPMs across partners, confirm you are comparing the same quality of impression: viewability rate, measurement rate and invalid-traffic filtering all change what a thousand impressions is actually worth. A $1.50 CPM at 40% viewability is more expensive per viewable impression than a $2.50 CPM at 75%.

Frequently asked questions

How many impressions does my budget buy?: Divide the budget by the CPM, then multiply by 1,000. A $5,000 budget at a $2.50 CPM buys two million impressions. Set the solver to 'Impressions' to do it directly.
What is the difference between CPM and eCPM?: CPM is a rate you agreed before delivery. eCPM is what the inventory actually earned per thousand impressions after everything that happened in between. A contracted CPM is an input; an eCPM is a result, and the two rarely match.
Why is the CPM I pay higher than what the publisher reports?: Because fees sit between the two. The DSP, the exchange, any curation or data layer and the SSP each take a share, so the buyer's gross CPM and the publisher's net CPM describe the same impression at different points in the chain. A gap of 30% or more between the two ends is unremarkable.
Should CPM be calculated on served or measured impressions?: On whichever the contract specifies, and state it. Served impressions come from the ad server, measured impressions from a verification vendor, and they differ by a few percent. Billing on one while reporting the other is a recurring source of reconciliation disputes.