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27 March 2026
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How to calculate CPM and OOH impressions

Understanding how to measure out-of-home (OOH) advertising is key to planning an effective OOH campaign, reaching the right audiences, and delivering measurable impact. CPM (Cost Per Mille) is one of the most widely used metrics in advertising, particularly for campaigns focused on reach and visibility, and is an effective way to compare performance across marketing channels.

This guide explains how the OOH industry measures impressions and billboard effectiveness using CPM - including how to calculate it and how reliable audience data can support campaign planning and budgeting.

What is CPM in advertising?

CPM stands for Cost Per Mille - the cost of reaching 1,000 people with your advert. It’s a simple way to understand what you’re paying for, making it easier to judge whether a campaign is good value and benchmark performance across other marketing channels, locations or formats.

In OOH advertising, CPM gives businesses a clear and practical way to assess whether a billboard is delivering strong visibility for the budget available - making it especially useful for SMEs deciding which marketing channels offer the best value. By using CPM, businesses can compare different locations, formats, and campaign options more confidently, helping them choose media that maximises reach and impact.

How OOH impressions are measured

In the UK, key OOH metrics including reach and impressions are measured using Route - the industry-agreed audience measurement system. Rather than relying solely on basic traffic counts, Route is designed to reflect how people actually move through real environments and how likely they are to notice an advert.

The likelihood to see model

Route’s methodology is built around a "likelihood to see” approach, which focuses on realistic ad visibility rather than simple exposure. Not all impressions deliver the same value - Route’s methodology assesses how probable it is that someone passing a billboard has genuinely had the opportunity to see it, based on a combination of data sources, including:

  • Real travel and movement behaviour
  • Mobility insights, combining traffic flow and pedestrian footfall
  • Visibility modelling that estimates the probability of an advert being seen

By grounding impressions in real-world behaviour, Route provides a more reliable foundation for calculating CPM and planning OOH campaigns effectively. Its methodology is highly robust, combining research with advanced modelling to give advertisers deeper insight - not just into audience numbers, but into movement patterns and the genuine likelihood of people viewing your OOH ad.

Read more about Route’s likelihood-to-see methodology.

Why Location Matters for CPM in OOH

Visibility and dwell time

If you want your ads to be noticed, location is everything. Choose spots where your audience naturally spends time, and make sure your billboard or screen is clear and easy to see.

At Alight, we carefully select locations with good sightlines for both drivers and pedestrians - near traffic lights, junctions, or busy walkways. We also target places where people tend to have high dwell time, like near shopping centres, transport hubs, pubs and restaurants. The longer someone spends near your ad, the more likely they are to remember it.

Proximity targeting

One of the biggest advantages of OOH advertising is being able to show up in the places that matter most to your customers. Proximity targeting means placing your ads close to where people are likely to make decisions - whether that’s near your shop, a popular event, or a busy transport route.

Audience indexing

Audience indexing helps you look beyond total footfall and understand who is actually passing a site. Instead of choosing locations purely based on volume, indexing shows whether a specific audience - like young professionals or families - are more likely to be there compared to the national average.

This is especially useful for local businesses working with tighter budgets. An OOH site with fewer overall impressions might still be a better investment if it over-indexes for your ideal customers.

Using CPM to Forecast OOH Ad Spend

When purchasing OOH space, advertisers are provided with estimated impressions for a defined campaign period. These estimates are influenced by several factors, including:

  • Location: High-traffic routes typically deliver higher reach, often at a higher cost
  • Seasonality: Travel patterns and footfall fluctuate throughout the year
  • Format: Digital, static, small or large format, delivers different levels of visibility

Get in touch today for a clear estimate of costs and expected results of your next OOH campaign.

FAQs

CPM is calculated using the following formula:

CPM = (Total cost / Total impressions) x 100

For example, if a billboard campaign costs £10,000 and delivers 1,000,000 impressions, the CPM would be £10. This allows advertisers to compare different billboard placements, formats, and campaign durations on a like-for-like basis.

The main metrics used to measure OOH advertising include:

  • CPM (Cost Per Mille): The cost to reach 1,000 people.
  • Impressions: An estimate of how many people see your ad.
  • Reach: How many unique people see your ad at least once.
  • Frequency: How often the same people see your ad.
  • Gross Rating Points: Combines reach and frequency to measure overall campaign impact.
  • Viewability: How likely it is that your ad is actually seen by your audience.

Gross Rating Points are a way to measure the total exposure of your campaign. They combine reach and frequency, providing a simple way to compare the overall impact of different campaigns or media. This metric is useful for large campaigns, where advertising is delivered on multiple platforms.

In digital out-of-home, the number of plays is delivered via structured campaign loops, ensuring ads appear at regular intervals and providing consistent exposure throughout the campaign. Performance is typically measured by share of time - the percentage of total screen time an ad occupies within the loop - alongside total reach and impressions.

OOH impressions estimate how many people are likely to see your ad. In the UK, these are calculated using Route, the industry standard. Route uses a robust methodology combining real-world data on traffic, footfall, mobility patterns and visibility modelling, to show how likely people are to notice your ad.

Viewability measures whether your ad is actually seen. A billboard may be on a busy road, but if it’s obstructed or in a low attention spot, viewability and impression quality will be low.