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Ad viewability is best and most briefly described as the percentage of ads that are actually viewed by a website visitor. More specifically, ad viewability is typically defined as half of the ad being visible for one second.

Measurements of Ad Viewability

Under the above definition, comScore’s report from 2013 states that a full 54% of ads are not viewable.

But how do they know? How can ad viewability be measured?

The problem with measuring ad viewability is the fact that one doesn’t have a reliable and consistent way of measuring what the user actually sees. Objectively, we can make and combine all kinds of approximations and inferences based on toolsets that report when an ad loads on the page. But whether the user was actually paying attention to the ad is necessarily a subjective measurement.

Here are four useful, though not perfect metrics we can work with.

Ad Clickthroughs – One obvious and direct measurement of ad viewability is ad clickthroughs. One can safely assume that in the vast majority of cases, if the site visitor clicks on an ad, the ad was viewable. While there can be exceptions in terms of an inadvertent click, or a click on an ad that wasn’t fully loaded, statistically speaking an ad click is a good viewability indicator.

Pageviews – An indirect, and not fully-accurate metric is the number of pageviews. By tracking pageviews, you can approximate, roughly, how many ads were viewed. But, this doesn’t address ad viewability itself if, say, the user scrolled (or didn’t scroll) or clicked away too quickly to see the ad. Most viewability happens “above the fold” unless the visitor spends more time moving about the page.

Page Speed – Another indirect metric to track is how long it takes to fully load a page. Ads on fast pages are more likely to be viewable than on slow pages.

Element Tracking – A prior post here on Rigor shows how performance software tools can detect whether an element, such as an iframe or a <div> tag has loaded on the page. Using a large sample size of measurements, one can determine with greater accuracy than clickthroughs, page views, or page speed whether an ad had a chance of being seen on the page.

Causes of Poor Ad Viewability

As noted in the metrics list above, a common thread in troubleshooting ad viewability is the performance of the page both in its design and ad placements likely to be viewed and in the loading speed of the page.

Design and Ad Viewability

Let’s talk about design first. The term “above the fold” has been a watchphrase for many years in website design and development. It is based on the observation that visual cues that appear above the bottom border of the browser window (assumed usually to be fully maximized on the screen) are more likely to be seen and interacted with than those falling “below the fold”, or after the user begins to scroll.

 The importance of above-the-fold design cannot be understated. As far back as 1995, Vince Flanders wrote a fun little book called “Web Pages That Suck” to highlight, in part, the need for great above-the-fold design.

 The phrase Flanders came up with, “the top’s gotta pop or they’re not gonna stop”, helped early Web designers remember to really pour the majority of their efforts into making sure the home page–particularly the top of the home page–was compelling enough to stop people in their tracks long enough to absorb the site’s message.

 The ads that publishers place above the fold are key to viewability.

Page Performance

That same “top’s gotta pop” strategy can also apply to page performance. If a page loads quickly, its elements are more likely to be seen. Let’s see what this looks like on a typical news site like, using Rigor’s waterfall analysis.

 Below is the speed test of the home page from five different locations distributed around the U.S.

ad viewability

waterfall chart boston.comAs you can see, the above waterfall chart displays every file downloaded when loading Moreover, it displays the time it takes for the DNS lookup to resolve, the time to complete the initial connection to the server, how long it takes the SSL protocol (encryption/decryption) to finish, the amount of time to get the first byte of the file, and the time at which the content is deemed as “downloaded” from the server to the client.

The yellow highlighted bars represent all of the ad requests or “files” that are fetched. Notice the sheer size of this page in terms of items that need to be loaded, both individually as line items and collectively in megabytes—1.8MB to be exact.

Most all of this is loading while the user is waiting for the interface to load. That does not bode well for ad viewability, especially on slower mobile devices.

In contrast,, which doesn’t have ads on the home page but does have sidebar sponsored content on the right-hand side of each article, like this one, does much better. It weighs in at a fraction of the homepage as to content size.

ad viewability waterfall chart One very clever thing ViralNova has done is to load their entire ad set for the page in a single graphic. At first blush, this may seem foolish because it’s a bigger file size. However, they make up the savings in the reduced number of round trips to the server for each individual ad. To display individual ads, they simply use a CSS trick to “block out” individual ads from the single, massive graphic and display others.

How to make your ads more viewable

There are several factors at play in making ads viewable, or not viewable as the case may be.

Viewable ads are positioned low above the fold. As it turns out, ads are most view-able just above the fold, as opposed to being at the top or middle of the page when it first loads.

 Viewable ads are tall ads. Whether an ad is “tall” or “short” affects whether it’s viewable. Taller ads show on the page longer as the user scrolls around.

Here are some common scenarios that regularly reduce ad viewability.

Rendering Errors

If the ad creative does not render, and all the visitor sees is a blank space, or just the “ad choices” icon from Google’s AdSense, then the ad is not viewable and there is an obvious need to troubleshoot the ad to make it viewable.

On the other hand, if the ad overlaps other page elements or it is not aligned within its container, that can wreak havoc on the rest of the page and reduce viewability.

For more information on common rendering errors, see Monitoring Ads for Digital Publishers.

Load Time

As you troubleshoot the load time of the entire page the ad sits on, don’t forget to focus also on the load time of the ad itself. If the total aggregate “weight” (file size in kilobytes) of all files for an ad is too much, that can also contribute to a load time problem.

Load Errors

Errors that occur when an ad unit loads can suck away both bandwidth and server processing time. Missing graphics or scripts that result in 404 (not found) and 500 (server timeout) errors as well as iframe blocking at the browser level all conspire to make an ad un-viewable and also slow the page load.


Once you have determined the most accurate (but not perfect) metrics for determining the viewability of ads on your site, the next step is to ensure that you are loading all your resources into the browser in the most efficient manner possible. Reducing the number of server round trips, as well as minifying code and eliminating the “not found” items will put your page speed below the usual attention threshold dictated by the browsing habits of today’s site visitors.

Rigor is the first end-to-end web performance management platform for digital organizations. Our platform programmatically identifies, prioritizes and remediates the root causes of poor site performance and reliability. If you are interested in learning more click here to try free for two weeks.


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