Size Matters:

File Load Size Significantly Affects Ad Viewability

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Working Paper


October 11, 2016



Viewability is one of the most important issues in ad tech and is changing the way that inventory is bought, sold and measured.


The industry standard for viewability was developed under the leadership of the Media Rating Council (MRC) and endorsed by the IAB. It defines desktop and mobile display ads to be viewable if 50% of their pixels are in view for a minimum of one continuous second. For larger desktop ad units, 30% of pixels in view for one second constitutes a viewable ad.


There are many factors that impact whether an impression is likely to be viewed that are outside an advertiser's control. These include the quality of the publisher, the user’s device screen size and internet connection speed, and their general behavior toward ads.


One factor that advertisers do have control over is the file load size of their ad. Load size correlates with load speed, and faster load speeds give an ad a greater chance of being in view.


To find out how much advertisers should be investing in optimizing file loads, we studied how file load size impacts viewability in display ads by looking for trends in the correlation between ad load size and view rate.


Materials and Methods

In conjunction with our global DSP and programmatic exchange partners, Thunder analyzed the viewability of billions of impressions measured against their file load size. We studied a trend analysis of the mean viewability in display ad inventory up to 200KB as a representation of the baseline correlation between load size and viewability.



File Load Size

Interpolated Viewability

Approximate view rate fall off per 10KB











We found that on average the lightest ads have the best viewability, interpolated at 52% but reaching all the way up to 70%. A line of best fit analysis shows that in ads under 50KB there is a 3% drop in view rates for each 10KB after the initial 10KB. That is followed by a 0.5% to 1% drop in view rate per extra 10KB beyond that. Average viewability bottoms out at around 25% in the 150-200KB range — the IAB recommended maximum file load size for a display ad.




The recent founding of the Coalition for Better Ads and the update to the IAB LEAN standards announced on September 26th, 2016 has created a call to action for advertisers to produce consumer-friendly ads with lighter file load sizes and less server calls. Adopting those standards may be more important than advertisers realize because of the significant correlation to viewability.


The shift to a mobile-first world is one of the driving factors in this trend. On mobile devices download speeds can be slow, therefore heavier ads more easily miss views. Conversely, lighter weight ads with faster load times are more in view, easier on mobile data plans and provide a better experience for consumers.


As HTML5 has replaced Flash as the dominant format for display ads on the desktop, new challenges in controlling file size have arisen. Factors like animation, type and custom fonts can bloat the size of an HTML5 ad’s size. But often the custom type and brand appropriate animations are required for the ad to represent the advertiser effectively.


There are several ways to reduce the file load size of an HTML5 ad:

  1. Leverage the web browser’s native animation capabilities instead of using heavy JavaScript where possible
  2. When custom fonts are used, rasterize your type (convert the text to images)
  3. Always compress images for the optimal balance of file size and aesthetics
  4. Get attention by tailoring your ad content to be more relevant to the audience. Use targeted messages instead of heavier rich media ads


Further Research

Although billions of impressions are studied, the ramifications of these findings surface a need for further investigation. Thunder is working to expand this research and is seeking additional ad exchange and viewability partners to participate.


About Thunder

Thunder enables creative relevancy for programmatic advertising. Our Creative Management Platform allows publishers, agencies and advertisers to tailor display and Facebook ad creatives to the increasing variety of targeting segments and ad formats. Thunder’s proprietary J2H ad framework automatically optimizes ads for file load size across animations, type, images and code.


Learn more about Thunder Creative Management Platform at