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What is Ad Fill Rate?

What is Ad Fill Rate?


Modern-day websites are attractive digital constructs that relay pertinent information to the visitors while maximizing their user experience. The primary objective of any website is to increase traffic and page ranking.

One of the most common strategies used by website owners (publishers) to help increase their site performance is advertising. Advertisers are always on the lookout for advertising space where they can place their ads to reach the right audience, while publishers are always on the lookout for ads to help fill the space on their website; thus the relationship is formed.

Advertisers pay publishers based on the number of impressions (the number of times an ad is viewed) or the number of clicks an ad receives. In turn, publishers try to ensure that the ad content is somewhat relevant to their website and user, while simultaneously ensuring that the ad does not disrupt the user’s experience in any way.

It is a significant relationship, since many websites today rely on advertising as their primary source of income. This makes the whole process highly competitive as each advertiser wants to reach a specific target audience and needs to choose their publisher space carefully. Likewise, publishers want to increase the effectiveness of their ads while minimizing their costs, thereby maximizing their revenue. And the best way to determine if they are getting the best value for money on an impression or page view, is by looking at the Ad Fill Rate

The Ad Fill Rate refers to the total number of ad impressions in relation to the total number of ad requests sent out by the ad server. It can be depicted by the following formula:

Ad Fill rate = (Total Ad Impressions / Total Ad Requests)* 100

Factors affecting the Ad Fill Rate

This Fill Rate formula suggests that, ideally, one must strive for a 100% Fill Rate, which means that every time an app or website sends a “display request” to the ad network for users to see, the ad network responds by sending back an ad, creating a 1:1 or 100% Ad Fill Rate. Unfortunately, this is not always the case, and several factors come into play that can result in a Fill Rate falling below 100%.


One of the main factors that affect the Ad Fill Rate is compatibility. For the ad to be seen, it needs to load successfully, along with the page, and for that to happen, both the device and the browser being used need to be up to date and support the ad format. Without this, the ad request is sent, but there is no impression, as the ad isn’t viewed by the user, resulting in a lower Fill Rate.

Technological Errors

Building on compatibility, we can delve deeper into the technical aspect of ad functioning and look at errors in technology as another factor that might lead to a lower Ad Fill Rate. From the second a user lands on your page, hundreds of small processes take place behind the scenes, such as scripts running and elements loading. This means that an ad needs to load effectively, in a specific place, within a particular time, along with the rest of the website, and should not, under any circumstances, impair the functioning of the site or the user experience.

The best way to prevent this from happening is to have both ads and websites optimized for different browsers, platforms, and devices. However, this can be a long process and many websites are not fully optimized in more ways than one, preventing them from achieving a high Ad Fill Rate. Additionally, if the ads are not optimized, it could cause users to leave the page before the ad has finished loading, which would lead to advertisers seeking more successful locations for their ads on other websites.

Excess Demand, Short Supply

Fill Rates are also determined by demand and supply. While website ad space may be abundant, advertising networks may not always have a steady supply of relevant ads to display on your site, creating a simple case of high demand and low supply. This can lead to a loss of revenue as well as a lower Fill Rate.


Discrepancies occur when ad requests are sent, as ad servers view an ad request as an impression, whereas ad networks only count an impression when the user views the ad. It causes inaccuracies in the data, which invariably leads to lower Fill Rates. Lastly, we have ad blockers. Very simply, the ad request goes through, but the ad blocker does its job and stops the ad from loading at all, leading to a definitive drop in both Fill Rate and revenue, despite having optimized everything else.

Is 100% Fill Rate Good?

Now that we’ve looked at the factors affecting the Ad Fill Rate, is it absolutely necessary to have a 100% Fill Rate? To answer this question, we need to take a closer look at the primary method of calculating an ad’s performance. This is known as the effective Cost Per Mille, or eCPM. It is calculated by the following formula:

eCPM= (Total Ad Revenue Earned/ Impressions)* 1000

The eCPM tells you how well your ads are performing in terms of profitability and value for money. The higher the eCPM, the better. Does a 100% Ad Fill Rate maximize your eCPM? Not necessarily. A good, or strong Fill Rate, means you have monetized your ad space effectively, and the ad network is displaying the suitable ads at the right time, for the right audience. To achieve this particular outcome, it might benefit you to set your Fill Rate lower than 100%.

This is because it is extremely rare that 100% of ad space can be filled with suitable ads (at the right time, for the right audience). Therefore, while your filters are set to a 100% Fill Rate, and only 70% of ads are pertinent and found to be valuable, the other 30% of ads might be lower quality ads that have a neutral or negative impact on the user. In this scenario, your Fill Rate might be at 100%. However, the extra 30% negatively impacts aggregate ad performance; it causes a drop in revenue, which is depicted by a fall in eCPM.

Alternatively, it might benefit you more to set the Fill Rate at 70%, in which case, all of the ads displayed can be valuable ads that yield a positive response from the user. This ensures that your eCPM is maximized, while having only a 70% Fill Rate, which is a far more favorable outcome, as it represents an increase in revenue.


Ultimately, the best strategyto maximize your revenue is to identify the Fill Rate at which your eCPM begins to fall. If your eCPM begins to drop at an 85% Fill Rate, but still grows at an 80% Fill Rate,maintaining an 80% Fill Rate would be ideal. While several effective strategies can be implemented to help increase the Ad Fill Rate, such as header-bidding and geo-targeting, it is crucial to remember that the best outcome can only be derived from a detailed advertising plan that focuses on optimization, flexibility, and maximizing revenue.

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