CPT and CPM are two closely related, but not identical, ways of valuing reach across different media channels. To make sound decisions in the media mix, it’s important to understand how these metrics are calculated and why the same figure can mean different things in TV, online and out-of-home. Otherwise, you risk comparing price instead of impact.

In out-of-home, we traditionally start from VAC (visibility-adjusted contacts), reach, frequency and number of exposures, alongside the campaign cost. These are the building blocks that CPM and CPT “translate” into cost per contact and cost per percentage point of reach:

• price + VAC → CPM (cost per thousand visibility-adjusted contacts)
• price + reach (% of target audience) → CPT/CPP (cost per percentage point of reach)

CPM (Cost Per Mille), cost per thousand contacts, is the most widely used metric. CPM is defined as the campaign cost divided by the number of impressions, times 1,000 – a well-established industry standard metric as described, for example, by Wikipedia (“Cost per mille”) and media planning guides from Rochester Institute of Technology (“CPM = cost ÷ impressions × 1000”). In online environments, CPM typically rests on logged impressions; in TV it is based on modelled audiences for a specific programme or time slot; and in OOH it is built on modelled exposure in defined environments. For OOH and DOOH, planners start from traffic flows and location, adjust for visibility and opportunity to see (VAC), and calculate how many contacts a given out-of-home network is expected to generate during the campaign period. The net cost is then divided by these contacts to obtain a CPM aligned with the same format used in other media.

CPT/CPP (Cost Per Point), cost per rating point or per percentage point in the target audience, is used mainly in TV and other broadcast media. One rating point corresponds to one percent of the defined target audience. In industry literature, CPP/CPT is described as a sister metric to CPM, but using rating points (share of the target audience) instead of impressions as the denominator; that is, the cost of buying one rating point, 1% of the audience (“Cost per rating point”, Universal Marketing Dictionary). CPT therefore expresses how much it costs to reach one percent of the target audience once. For cross-media planners, CPT can be a way to compare how quickly different channels build reach in a defined target audience, even though the currency in digital is almost always CPM, while broadcast has traditionally used CPP/CPT (see, for example, “CPP vs CPM” overviews from media consultants such as Inline Media).

In out-of-home, CPM has become the dominant currency, but the same logic can be applied for CPT-type analysis. For instance, you can compare how different formats and packages contribute to percentage reach in a target audience within a specific geographic area: in practice, “what does each additional percentage point of reach cost in this zone?”. A broad package with bus shelters, street furniture and city information panels typically delivers a low CPM and stable reach, whereas select units or stand-alone locations in premium environments generate a higher contact cost but higher relevance and the ability to build frequency against particularly valuable audiences.

For DOOH and programmatic DOOH, CPM is often calculated on verified impressions, meaning exposures where traffic and movement data indicate that people were within an observation zone while the spot was playing. In this context, CPT/CPP is more of an analytical metric than a currency: you can retroactively calculate the cost per percentage point of reach achieved by adding screens in specific environments, and thereby identify where each incremental point of reach is most cost-efficient.

When comparing CPT and CPM across TV, online and (D)OOH, you should always look beyond the headline figure:

• How is a contact/impression defined in each medium?
• What assumptions are made about viewability/visibility and actual opportunity to see (VAC versus “raw impressions”)?
• How often is the same individual counted (frequency versus reach)?

Only when you connect the price (CPM/CPT) with the underlying building blocks – VAC, reach and frequency – can you determine which combinations of channels and networks deliver the best reach and business outcomes, instead of simply chasing the lowest cost per thousand contacts.