Fill rate is the percentage of ads served based on the total number of video impressions.
For example, if you receive 1,000 impressions where an ad-unit has been placed, and a total of 900 adverts were served per 1,000 impressions, your fill rate would be 90%.
It’s pretty much impossible to maintain a 100% fill rate, even if we do everything right and the video is the greatest video in the world.
Things that affect fill rate:
Ad blocking – Fill rate doesn’t take into account ad blocking software and can sometimes misrepresent your actual eCPM and fill rate. For example, if 50% of your traffic use ad blocking software, your fill rate may be close to 100% but your eCPM will be very low.
Device type – On average, publishers are more likely to serve adverts to users on a mobile device than desktop (and pay more too). Website visitors who use a Blackberry or Windows smartphone may be served fewer ads than traffic using iOS or Android platform, quite simply because they are less popular devices
Location of traffic – Ads are placed with specific targeting options to ensure they are shown to relevant prospects. For example, you may receive a website visitor from Zimbabwe, but if no publishers are targeting prospects from Zimbabwe, there is little chance that an ad will be served.