Formulas used by this calculator
- CPM = ad spend ÷ impressions × 1,000
- Use the same currency, campaign scope, and reporting period for both inputs
Worked example
Illustrative media spend is $500 and delivery is 100,000 impressions.
500 ÷ 100,000 × 1,000 = $5 CPM. Another $500 of spend is not guaranteed to buy the same number of impressions.
If that same spend delivered only 80,000 impressions, CPM would be $6.25. The worksheet makes that arithmetic visible without explaining what caused the change.
Use impressions rather than reach
Google Ads describes average CPM as cost per thousand ad impressions. The multiplier changes the reporting unit; it does not turn impressions into unique people. Google Ads: performance metric definitions.
If one person sees an ad five times, the report can contain five impressions. Entering unique reach in place of impressions answers a different question and can make the calculated CPM look much higher. Keep a note of the exact column you copied.
Compare delivery that serves the same purpose
Separate campaign objective, placement, geography, and reporting period before comparing costs. A small retargeting audience and a broad prospecting campaign may serve different roles. A difference in their CPM does not by itself identify which deserves more budget.
When combining compatible campaigns, add spend and impressions first. Do not average the CPM values: a campaign with a thousand impressions should not receive the same weight as one with a million. Use totals to calculate the blended result.
Connect cheaper exposure to the next outcome
After calculating CPM, inspect click-through rate and the quality of the traffic. Lower-cost exposure can still produce expensive qualified leads if few people respond or the audience cannot buy. Keep reach, response, and acquisition economics as separate checks.
For an unexplained change, compare the delivery mix before editing campaigns. Save both periods as worksheets with dates and account scope. A ratio calculated from two totals cannot distinguish an auction change from a shift in placement or audience mix.
Common questions
Does CPM mean I am paying per impression?
This calculator describes observed cost per thousand impressions. It does not determine the platform's billing event or bidding strategy. A campaign using another bidding method can still have a reportable CPM.
What happens if impressions are zero?
CPM is undefined because there is no delivered-impression denominator. Zero spend with positive impressions produces a zero CPM. Check reporting completeness before interpreting either case.