"It does not require any portion of the ad to be visible or displayed on screen."
That sentence is from the guideline for counting a served ad impression, written by the industry's own Interactive Advertising Bureau and Media Rating Council. It is the part of CPM most people get wrong, and the metrics built on top of it inherit the confusion. In the basic sense most reports use, an impression is just an ad that was sent to a device and started to render. Nobody has to have seen it.
The three cost measures themselves are simple division. CPM is what you spent divided by impressions, times a thousand — the cost per thousand impressions. CPC is what you spent divided by clicks. CPA is what you spent divided by conversions — the sign-ups, purchases or leads you were paying to get. What makes them slippery is not the division. It is that each platform counts the thing underneath a little differently, and uses the same word for it.
The clearest way through is to read one campaign report line by line, in the order the columns usually appear.
Impressions
The served-impression guideline counts an ad once it has been transmitted to the device and begun to render. Google Ad Manager's own help page is blunter, counting an impression when a creative has "begun to load"—before it is fully downloaded and viewed. In mobile apps it counts one when one or more pixels of the ad are on screen.
A viewable impression is a stricter thing, and the Media Rating Council defines it separately: for a desktop display ad, at least half of the ad's pixels on screen, in a browser tab that has focus, for at least one continuous second after the ad renders. For large ads of 242,500 pixels or more — a 970 by 250 banner — the threshold drops to 30% of the pixels. For video it is half the pixels for two continuous seconds of play.
A practical warning. The document that still ranks first when you search for the viewability guideline, on the IAB's own site, is version 1.0 from June 2014. The current one is version 2.0 from August 2015, and it reverses version 1.0 on how viewability applies inside apps. If you are quoting the rule to someone, check which file you have open.
Meta counts something different again. On Facebook and Instagram, an impression is the number of times an ad is on screen for the first time — no pixel share and no duration. Meta's own help page says that its method of counting video impressions differs from industry standards, and that a video does not have to start playing for the impression to count.
CPM
So CPM is a price per thousand of whatever your platform calls an impression. A Meta CPM and a Google display CPM are prices for two different things; a direct comparison is less revealing than it looks.
The gap that matters most is between a served CPM and a viewable one. If only some of the impressions you paid for were viewable, then the price you actually paid for each viewable thousand is higher than the headline CPM, and the arithmetic is exact: divide the CPM by the viewable rate. Put as a premium over the headline figure, that is:
premium = 1 ÷ viewable rate − 1
At 80% viewable, every viewable thousand costs 25% more than your quoted CPM. At 50% viewable, it costs 100% more — double. The premium climbs faster the lower the rate goes: halving the viewable rate from 50% to 25% does not double the premium, it triples it. CPC and CPA do not move at all, because clicks and conversions are counted the same way regardless.
One refinement for Google Ads users. Google reports a viewable rate as the share of measurable impressions that were viewable — impressions where its Active View technology could tell either way. Some impressions cannot be measured. Its help page says plainly that the separate "viewable impression distribution" figure is not the same thing as the viewability rate. Because the viewable rate leaves out impressions that could not be measured, dividing your CPM by it still understates the real cost of a viewable thousand — by exactly the share of impressions that could not be measured.
As for what a "normal" CPM is: we could not find a figure we would stand behind. The benchmarks that circulate are surveys of a single vendor's own clients, rarely say whether they report an average or a median, and date quickly. Your own campaign history is a better baseline than any of them.
Clicks and CPC
Clicks look like the one unambiguous number in the report, and they are not. Meta's CPC divides what you spent by link clicks — clicks that take someone to a destination — not by all clicks on the ad, which also include likes, expanding the text and tapping the profile. A Meta CPC and a Google search CPC are again dividing by different things.
The one formula Google does publish for these measures is click-through rate: clicks divided by impressions. For CPM and CPC its help pages describe bidding options rather than defining a formula.
Conversions and CPA
CPA is the measure people care about most and the one defined least consistently. Google's own help centre gives two versions: one page divides by "the total cost of conversions", another by "the marketing cost". In a campaign where some of the spend produced no conversions, those are different numbers.
Meta does not report a CPA at all. Its nearest figure is cost per result, which divides spend by whatever "result" you chose as your campaign objective — and part of that count can be modelled rather than directly observed.
As for audits, Meta's accreditation page states that its served display impressions on the Facebook and Instagram feeds have been accredited by the Media Rating Council since 6 March 2018. It also states that metrics other than impressions are not currently accredited. The impression count on those placements has been independently checked. The CPM, CPC and cost per result built on it have not.
Where our calculator fits
Our CPM, CPC and CPA calculator does the division for you, and its arithmetic has always been right: over 4,992 realistic campaigns, it never disagreed with an independent calculation and never divided by zero. Two things around the arithmetic were wrong, and both are fixed. It labelled every result in US dollars, so an advertiser in Malaysia or Indonesia typing a ringgit or rupiah budget read it back as dollars; it now lets you name your currency and converts nothing. And typing five thousand as 5.000, as you would in Indonesia or Vietnam, was read as five; it now asks which you meant.
Everything it prints, though, inherits the definitions above. It divides your spend by your impressions; which impressions those are is decided by the platform that counted them.
The served-impression wording is from the IAB and MRC's desktop display guideline, version 1.1 of October 2017; the viewability thresholds from the MRC viewable impression guideline, version 2.0; Google's and Meta's definitions from their own help pages, and Meta's audit position from its accreditation page — all read on 25 September 2026. We searched all four IAB and MRC documents for the word "billable" and found it in none of them: a "billable impression" is a term in your contract, not an industry standard.
