Understanding Multi-Format Ad Units in Prebid: A Practical Guide for Publishers
As publisher monetization strategies become increasingly complex, the need for flexible ad units that can maximize demand has never been more urgent. Rigid ad formats often leave valuable revenue on the table, especially as buyers shift budgets between display, video, and native inventory.
Multi-format ad units in Prebid offer a practical solution by enabling a single placement to accept banner, video, and native bids. This not only simplifies setup but also increases competition for each impression, improving fill and potentially CPM. In this guide, we’ll break down exactly how multi-format ad units work, highlight implementation tips, and point out common pitfalls to avoid.
What Are Multi-Format Ad Units and Why Do They Matter?
A multi-format ad unit is a single ad placement configured to accept multiple types of creative (banner, video, and/or native) during the auction. Instead of running distinct auctions for each ad type, Prebid lets you offer these formats concurrently to your demand partners within the same ad slot.
Publisher Challenges Addressed
Traditional configurations force publishers to choose a specific format for each ad slot, limiting buyer competition and fill rates. Multi-format units break this barrier by allowing all compatible bidders—regardless of whether they serve banners, video, or native ads—to compete for the same impression.
Example: How it Works
Suppose you have a 300×250 spot above the fold. If configured as multi-format, a video bidder with outstream creative, a banner DSP, and a native specialist can all bid for the same slot. Prebid automatically determines the highest yielding eligible bid, maximising CPM and fill without manual rotation or complex targeting.
How to Set Up Multi-Format Ad Units in Prebid.js
Configuring a multi-format ad unit in Prebid involves specifying supported media types for a given ad slot, defining format-specific parameters, and handling creative rendering intelligently to display the winning bid.
Step-by-Step Setup
1. Define mediaTypes for your ad unit, listing each supported format. For example, banner with sizes, video with player/SKUs/MIME types, and native with asset requirements.
2. Ensure each participating bidder in your ad unit supports at least one of the declared media types.
3. Implement or select a renderer for video/native as needed—Prebid allows you to plug in renderers like in-renderer.js for outstream or native creative.
4. Use pbjs.addAdUnits to register the ad unit, and pbjs.requestBids to trigger the auction. Display the winning bid with pbjs.renderAd or your GAM integration.
Renderer Handling and Best Practices
Using a renderer is essential when you expect to receive video or native formats in placements primarily designed for banners. InPrebid’s in-renderer.js, for example, handles rendering outstream video within a display slot, abstracting player logic for smoother integration. Always test creative compatibility, especially on mobile and across device/browser types.
Auction Flows, Header Bidding, and Ad Server Integration
When a multi-format ad unit enters the auction, each eligible bidder responds with their best available creative and price. Prebid does the work of comparing apples-to-apples by selecting the highest CPM across all eligible formats, provided that minimum requirements (size, format, player, etc.) are met.
Interactions with Google Ad Manager (GAM)
If passing Prebid results to GAM, ensure that line items or key-values can handle all possible resulting creatives. This may require hybrid line items or checking native/video creative settings within GAM policies. Failing to align settings may cause mismatched creative errors or fallback impressions.
Common Mistakes and Troubleshooting
Common errors include mismatched creative types in GAM line items, forgotten renderers for video/native, or misconfigured size arrays that omit valid bidder responses. Test each format individually and with combined demand to validate proper creative rendering and reporting.
Publisher Advantages and Real-World Considerations
Multi-format ad units drive operational efficiency and yield gains, but require thoughtful setup and QA.
Benefits Worth Noting
• Maximize fill: More buyers can compete on every impression
• Simplify ops: Fewer placements to manage and optimize
• Adaptive monetization: Easily test shifts in buyer demand across formats from a single slot
Potential Pitfalls
Multi-format configurations add complexity for yield analytics and troubleshooting, especially if something breaks in your reporting or creative displays. Make sure your ad server, analytics setup, and QA procedures are equipped to handle the increased variability.
What this means for publishers
By adopting multi-format ad units, publishers open their inventory to all suitable demand—banner, video, and native—without needing to micromanage each slot. This not only unlocks more bids per impression, likely improving CPM and fill, but also reduces the complexity in managing line items. However, it requires diligent testing, thoughtful ad server configuration, and awareness of potential edge cases around creative rendering.
Practical takeaway
For modern publishers looking to optimize every ad impression, multi-format ad units are a smart evolution. They provide the flexibility to attract diverse demand while streamlining ad ops workflows. To get started, audit your top placements and consider where multi-format could drive incremental yield without hurting user experience or overcomplicating ops.
Before deploying, work closely with your ad ops and engineering teams to vet creative rendering, ad server mapping, and reporting. Set up a thorough testing plan for all supported formats across devices. Once you’ve validated, monitor results closely—especially revenue shifts and error rates—and be ready to iterate. With careful rollout and tuning, multi-format ad units in Prebid can unlock new value from your existing inventory.
