How Prebid Server Handles Interstitial Ads: What Publishers Need to Know
Interstitial ads promise high viewability and strong CPMs, but managing them in a header bidding setup can be especially complex. Many publishers struggle to maintain effective size targeting and device compatibility, which can undermine both user experience and revenue potential.
This article demystifies how Prebid Server manages interstitial ad requests, focusing on the config options that matter most for publishers. We’ll break down key implementation details, provide real-world examples, and highlight what operational teams should watch out for.
The Mechanics of Interstitial Support in Prebid Server
Prebid Server (PBS) adds robust support for interstitial ads by allowing publishers to define minimum size thresholds as percentages of the device’s screen. These parameters—minwidthperc and minheightperc—allow more granular control over which ad sizes are eligible for interstitial placements across different devices.
How Size Thresholds Work
In your OpenRTB request, you can include device.ext.prebid.interstitial.minwidthperc and device.ext.prebid.interstitial.minheightperc, both given as a percentage value. PBS then interprets these values based on the device width and height it receives. For example, on a device that’s 600px wide, setting minwidthperc to 60 means only ads 360px wide or greater are eligible for interstitial consideration. This ensures that tiny, intrusive ads do not slip through on a large device, maintaining a better user experience and advertiser value.
Practical Example in Request Structure
A typical request for an interstitial placement will set instl:1 and define minwidthperc and minheightperc under device.ext.prebid. If the banner.format array is set to 1×1—or not defined—PBS uses the device’s dimensions as the maximum, then filters against your min thresholds and its internally configured size list. Only the top 10 eligible sizes are sent to bidders, keeping the request efficient.
The Prebid Server Size Selection Logic
A nuanced part of interstitial handling is how PBS generates and filters allowable ad sizes. Unlike simple fixed-banner auctions, interstitials require flexible sizing to fit various devices without overburdening buyers or risking bad ad delivery.
How PBS Determines Eligible Sizes
PBS maintains a configurable list of interstitial sizes, typically ordered with most common and largest sizes first. For every impression, it starts with the device’s screen size (or explicitly provided max dimensions) and then applies the minwidthperc and minheightperc filters. From here, it selects the first 10 sizes that fall within these constraints. There is no attempt to optimize for aspect ratio beyond the size limits, which means some size variety is expected in the output formats.
Implications for Bidders and Auction Performance
Only up to 10 size options are passed to the downstream bidders. This limit prevents bloated requests that could slow down response times or cause bidder errors. The minwidthperc and minheightperc settings are also passed to adapters, so bidders could further fine-tune which sizes they bid on. Publishers should be aware that not all bidders use the size filtering logic identically, which can sometimes create impression mismatches.
Common Pitfalls and Implementation Tips for Publishers
Operational teams often run into issues when rolling out interstitials through Prebid Server—sometimes due to misunderstanding request structure, sometimes due to missing practical settings. A publisher-first approach to setup and troubleshooting can save a lot of time and lost revenue.
Avoiding Silent Failures
The most common mistake is omitting instl:1 or failing to properly structure the device.ext fields. When this happens, PBS either ignores the interstitial parameters or fails to build a suitable format list, resulting in no bids or poorly sized ads. Always double-check your request payloads!
Size List Misalignment and GAM Mapping
Ensure that the allowed interstitial sizes in PBS match the creatives and size mappings in your ad server (e.g., Google Ad Manager). If the two systems expect different sets of sizes, you may see eligible bids dropped or creative misfires—especially as the top 10 formats sent by PBS may not include the ones GAM is ready to accommodate.
What this means for publishers
For publishers, the Prebid Server interstitial handling adds a higher degree of control to what users see, but it also means ad ops teams must carefully align settings between Prebid, ad servers, and bidders. The ability to set minimum size thresholds lets publishers ensure larger, more impactful ads while protecting user experience. However, the capped size list and different bidder filtering add another layer of troubleshooting when issues arise. In practice, this means more operational coordination and ongoing monitoring are required, but the upside is more predictable auction dynamics and potentially higher yield from interstitial placements.
Practical takeaway
If you’re monetizing with interstitials through Prebid Server, always specify minwidthperc and minheightperc to avoid serving undersized or inappropriate ads, especially on varying devices. Confirm that your OpenRTB requests consistently set instl:1, and that all ad server mappings are up-to-date with the relevant interstitial sizes.
Regularly review the actual formats being passed to bidders by logging PBS request payloads, and coordinate with your revenue partners to make sure there’s no disconnect in expected size handling. Treat these settings as a living configuration: minor mismatches or oversight can lead to silent failures, missed revenue, and a degraded user experience. By proactively auditing and aligning your header bidding and ad server practices, you can maximize both control and monetization from interstitial placements.
