Cost per mille:
pricing reach by the thousand impressions

Why advertising is priced per thousand

A single impression, one showing of an ad, is worth far too little to quote in rupees. Advertisers therefore buy and compare reach in blocks of one thousand. The M in CPM is the Roman numeral for thousand, from the Latin mille, so cost per mille means cost per thousand impressions.

CPM is the natural price for awareness work: launches, brand campaigns, local announcements. The aim is to be seen by as many of the right people as possible, and you pay for the showing rather than for any reaction.

It is also a useful common currency. Click-priced and view-priced buys can be restated as an effective CPM, which lets you set a search campaign next to a display one on one scale.

Because the price is per thousand, small numbers can look misleadingly cheap. A ₹60 CPM sounds trivial until a campaign needs 20 million impressions, at which point the bill is ₹1,200,000. Always multiply back to the total before you commit.

The formula and a first example

The figure is a price per thousand showings, not per person. A user who sees the ad four times counts as four impressions, which matters when you compare CPM with a reach number.

Keep the period fixed when you compare. A CPM measured across a quiet month and another across a festival fortnight will differ even for the same ad and audience, so put the dates next to the number in any report you share with a client or a manager.

Planning reach from a budget

The formula can be turned around when you already know the market price. Given a CPM and a budget, impressions come from budget ÷ CPM × 1,000. At a ₹60 CPM, ₹30,000 buys 500,000 impressions, and ₹5,000 buys about 83,333.

Impressions are not people, so divide by the number of times each person is likely to see the ad. If the same 150,000 impressions reached 50,000 distinct people, the average frequency was 3.0 and the cost per thousand people reached, a harder measure, was ₹180.

Frequency is worth planning for rather than leaving to chance. Showing the same ad to one person a handful of times builds recall, while showing it fifteen times tends to waste budget and irritate the viewer. Many platforms let you cap how often one person sees the ad in a day or week.

eCPM: comparing CPC and CPM buys

Suppose one platform sells you clicks and another sells you impressions. To compare them, translate the click-based campaign into the revenue or cost it generates per thousand impressions, known as effective CPM or eCPM.

In that example the click-priced buy is far dearer per thousand impressions, but it also delivers visits, which the ₹60 buy does not guarantee. The right question is what each route costs per outcome, and eCPM is a stepping stone to it.

A useful sanity check is to work out the implied click cost. If a ₹60 CPM campaign delivers a CTR of 0.5 percent, you are paying 60 ÷ (1,000 × 0.005) = ₹12 per click. That tells you whether the awareness buy is also producing visits at a rate that would be acceptable under a click-based plan.

When CPM buying makes sense

• Brand awareness and new product launches, where being seen matters more than immediate clicks.
• Video and display placements where users rarely click but do take in the message.
• Retargeting a small list, where you want controlled frequency.
• Publishers selling their space, since CPM is how most inventory is priced and reported.

Performance goals such as sign-ups and sales usually do better on click or conversion pricing, because you are charged against results. If you must use CPM for a sales goal, track conversions and calculate cost per sale afterwards, so the buying model does not hide poor returns.

Comparing digital reach with outdoor advertising

CPM is not limited to online ads. Outdoor sites such as hoardings and transit panels are often judged the same way, using an estimate of how many people pass and see the site. Suppose a hoarding costs ₹60,000 a month and the media owner estimates 400,000 passing impressions in that period.

The comparison is only as good as the impression estimate, which is usually modelled from traffic counts and not measured per person. Ask how the figure was produced, and prefer sites with documented audience data.

What changes CPM

Prices move with who you target and where you show up. Narrow audiences that many advertisers want are dearer than broad ones, premium sites and prime-time video cost more than remaining inventory, and a festive season sees prices surge as budgets compete for the same attention.

Format matters too. Video and rich media generally have a higher CPM than static banners, and placements that fill the screen cost more than a small corner slot. Look at the CPM beside the quality of what you are buying, not alone.

Traps in CPM reporting

• Impressions are not all seen. A viewable impression has met a minimum standard of being on screen, and not every platform reports on that basis.
• Counting a rotating ad slot shown below the fold as equal to one at the top of the page.
• Treating CPM as cost per person. Repeated exposure to the same users inflates impressions.
• Comparing CPMs across currencies, formats or audiences that are not alike.
• Bot or invalid impressions, which are filtered differently by each vendor.