CPM is the price of attention: what you pay for 1,000 times your ad is shown. The formula is ad spend divided by impressions, multiplied by 1,000. Cost per impression is the same price for a single view, so it drops the thousand. This post gives both formulas and the reversed forms you need for planning, with worked examples. It then covers what each platform counts as an impression, and how CPM turns into the cost of a click and the cost of a result. The free CPM, CPC and CTR calculator runs every formula on this page.
I run Meta and Google accounts every week and I build Adsap, which breaks CPM down on its dashboard. Every definition below has a source at the end.
The CPM formula and the cost per impression formula
CPM (cost per thousand impressions) = ad spend / impressions x 1,000.
Cost per impression = ad spend / impressions. It is CPM divided by 1,000.
Cost per thousand impressions
CPM = ad spend / impressions x 1,000
Cost per impression
CPI = ad spend / impressions
The same price for one view instead of a thousand: CPM / 1,000.
Turned around
impressions = spend / CPM x 1,000
spend = impressions x CPM / 1,000
Meta defines CPM as "the amount spent on an ad campaign, divided by impressions, multiplied by 1,000", with the example "if you spent $50 and got 10,000 impressions, your CPM would be $5". On Google Ads, CPM is also a way to bid on the Display Network: "a way to bid where you pay per one thousand views (impressions)". Google's reports show an average CPM for every campaign either way.
Two reversed forms come up whenever you plan:
- Impressions you can buy = spend / CPM x 1,000.
- Spend needed = impressions x CPM / 1,000.
Worked examples
One ad set, one week
Spend
$200
Impressions
25,000
200 / 25,000 x 1,000
$8 CPM200 / 25,000
$0.008 per impression| Spend | Impressions | CPM | Cost per impression |
|---|---|---|---|
| $200 | 25,000 | 200 / 25,000 x 1,000 = $8.00 | $0.008 |
| $500 | 50,000 | 500 / 50,000 x 1,000 = $10.00 | $0.01 |
| $1,200 | 400,000 | 1,200 / 400,000 x 1,000 = $3.00 | $0.003 |
The reversed forms with the same numbers. How many impressions does $1,000 buy at a $12 CPM? 1,000 / 12 x 1,000 = 83,333. What does it cost to show an ad 200,000 times at a $6.50 CPM? 200,000 x 6.50 / 1,000 = $1,300.
What counts as an impression?
The formula is the same everywhere. The impression is not.
Meta counts "the number of times your ads were on screen". An impression is counted the first time an ad is on screen. If someone scrolls past an ad and back up to it, that is one impression. If the ad is on screen for the same person at two different times in a day, that is two. A video does not have to start playing for the impression to count, except on Audience Network.
Google counts an impression "each time your ad is shown on a search result page or other site on the Google Network". For Display bidding, Google also sells viewable impressions. Its viewable CPM (vCPM) charges only when at least 50% of the ad is on screen for one second or longer, or for two seconds of continuous play for video.
A $6 CPM on Meta and a $6 vCPM on Google are therefore not the same purchase. Compare CPM within one platform and one placement. Across platforms, compare the cost of the result instead.
From CPM to CPC to CPA
CPM is the first step on a ladder. Each step below it multiplies in a rate.
CPM
spend / impressions x 1,000
the price of being seen
CPC
CPM / (1,000 x CTR)
the price of a visit
CPA
CPC / conversion rate
the price of a result
Cost per click = CPM / (1,000 x CTR). At a given CPM, 1,000 impressions cost that CPM and bring 1,000 times the click-through rate in clicks. At a $10 CPM and a 1.2% CTR, 1,000 impressions bring 12 clicks, so a click costs 10 / 12 = $0.83.
Cost per acquisition = CPC / conversion rate. If 2% of those clicks buy, a purchase costs 0.83 / 0.02 = $41.67.
That last figure is the one to hold against your margin. The break-even ROAS calculator returns the most you can pay per purchase from your price and costs. If the ladder lands above it, a cheaper CPM will not save the campaign on its own; the CTR or the conversion rate has to move too.
What moves CPM
CPM is set in an auction, so it moves with the auction.
- Competition and season. More advertisers bid for the same people before the holidays, so CPM climbs every fourth quarter and drops in January.
- Audience size. A narrow audience has fewer cheap impressions to sell. Narrow the targeting and CPM rises.
- Placement. Stories, Reels, Feed, Display and YouTube price differently. A change in the placement mix changes the blended CPM without any change in the auction.
- Creative. Meta's auction rewards ads people engage with. A creative that stops earning clicks and reactions pays more per impression.
- Objective. A campaign optimised for purchases bids for people likely to buy, who cost more to reach than people likely to watch a video.
$200 of spend buys
Double the CPM and the same money reaches half as many screens.
Read CPM next to CTR to find the cause. CPM up and CTR flat: the auction got pricier. CPM up right after you narrowed the audience: the targeting. CPM flat and CPC up: the creative, because the CTR fell.
How to use the CPM calculator
The CPM, CPC and CTR calculator works in two directions.
- Enter spend, impressions and clicks from a campaign report. It returns CPM, CPC and CTR at once, so the three sides of the auction sit on one screen.
- Enter a CPM and a CTR from past campaigns. It returns the estimated CPC and the clicks per 1,000 impressions, which is the planning number before a campaign has spent.
The inputs come from your own account. Any CPM pulled from a benchmark table belongs to someone else's placement, country and season.
Five ways CPM gets misread
- Comparing CPM across platforms or placements. The impression is defined differently, so the price is not comparable.
- Judging a campaign on CPM alone. A cheap CPM on a broad audience that never buys costs more per result than an expensive CPM on the right people.
- Confusing impressions with reach. Impressions count views; reach counts people. Impressions divided by reach is the frequency. A CPM that looks fine can hide a frequency of 6.
- Mixing currencies and dates. Spend in one currency against impressions from a different date range gives a number that means nothing.
- Chasing a low CPM by widening the audience. The CPM drops, the conversion rate drops further, and the cost per result goes up.
CPM in Adsap
The Adsap performance dashboard shows CPM per campaign, ad set and ad, for Meta and Google Ads, from yesterday's numbers. Breakdowns by placement, country, age and gender show where the expensive impressions are, and the metrics guide defines each figure. In the AI copilot the same breakdown is one question:
Break down CPM by placement for act_123 over the last 7 days, sorted by spend.
The CPM glossary entry keeps the short definition. The CPC and CTR entries cover the two rates on the ladder.
Frequently asked questions
CPM = ad spend / impressions x 1,000. Meta's example: $50 of spend for 10,000 impressions is a $5 CPM.
Cost per impression = ad spend / impressions, with no multiplier. It equals CPM divided by 1,000. An $8 CPM is $0.008 per impression.
Impressions = spend / CPM x 1,000. $1,000 at a $12 CPM buys about 83,333 impressions.
The formula is the same; the impression is not. Meta counts an ad the first time it is on screen. Google counts each time the ad is shown, and its viewable CPM on Display charges only when at least half the ad is on screen for one second. Compare CPM inside one platform and one placement.
Sources
- Meta Business Help Center, CPM (cost per 1,000 impressions): the definition and the $50 for 10,000 impressions example. https://www.facebook.com/business/help/753932008002620
- Meta Business Help Center, Impressions: "the number of times your ads were on screen", the scroll-back and same-day counting rules, video not required to play. https://www.facebook.com/business/help/675615482516035
- Google Ads Help, Cost-per-thousand impressions (CPM): "a way to bid where you pay per one thousand views (impressions) on the Google Display Network". https://support.google.com/google-ads/answer/6310
- Google Ads Help, Impressions: "counted each time your ad is shown on a search result page or other site on the Google Network". https://support.google.com/google-ads/answer/6320
- Google Ads Help, Bid on viewable impressions using viewable CPM: 50% of the ad on screen for 1 second, or 2 seconds of continuous play for video. https://support.google.com/google-ads/answer/3499086
- Google Ads Help, Cost-per-click (CPC) and Clickthrough rate (CTR) definitions. https://support.google.com/google-ads/answer/116495 and https://support.google.com/google-ads/answer/2615875
- CPC = CPM / (1,000 x CTR) and CPA = CPC / conversion rate: the formulas behind Adsap's CPM, CPC and CTR calculator. Every worked example uses round numbers chosen for the arithmetic; none is a benchmark.
- Adsap docs, metrics guide.