How to Use Client-Side Rendering to Increase Demand

BiddingStack Client-Side Rendering lets a placement accept creative sizes beyond its physical slot size and render the winner scaled to fit. BiddingStack widens the auction with extra sizes, then converts the winning ad, whether it comes from Prebid or from your ad server (Google Ad Manager, AdX, direct), so it always fits your slot. Larger creatives are scaled down responsively while keeping their aspect ratio.

This is configured at the placement level and works entirely in the browser through the BiddingStack Web SDK; no Google Ad Manager creative or line item changes are needed.

Looking for server-side conversion of Prebid Server bids (native to banner, video, templates)? See How to Convert and Render Prebid Server Bids. Both features can be used together: Prebid Server conversion applies per bidder, Client-Side Rendering applies to the whole placement.

Why Use It

A slot with an uncommon size (for example 320x250) often has thin demand. By also requesting popular sizes such as 300x250, 336x280, or 300x600 and scaling the winner into the slot, more buyers compete for the same impression, typically lifting fill rate and CPM.

Where to Find It

  1. In your project settings, under Demand Settings, turn on Enable Client-Side Rendering (Beta). This is the project-level master switch; while it is off, no placement conversion runs.
  2. Go to Placements, edit a placement, and scroll to Client-Side Rendering (Beta).
  3. Enable one or both conversion paths and configure the options below them.

Conversion Paths

  • Convert Prebid Winners: winning Prebid bids render through the BiddingStack ad-tag with responsive scale-to-fit, instead of the standard Prebid renderer.
  • Convert Ad Server Winners: creatives served by Google Ad Manager, AdX, or direct campaigns are scaled to fit the slot. Compatible with SafeFrame.

You can enable either path independently. When both are on, every winner, programmatic or direct, renders at your slot size.

Additional Auction Sizes

Enter the extra sizes to request, for example 300x250, 336x280, 300x600. These sizes are added to:

  • the Prebid auction (all bidders on the placement),
  • the Google Ad Manager ad request,
  • Amazon APS (if enabled).

Make sure your Google Ad Manager ad unit and line items accept the added sizes, otherwise GAM will not serve them.

The slot on the page keeps its original size; oversized winners are scaled down (centered, aspect ratio preserved) to fit it.

Render Options

The same optional render options as Prebid Server conversion are available; leave fields on Default to keep the built-in behavior:

  • Placement Override: render the winner in a high-impact format (floating footer, prestitial, etc.) instead of in the slot. Applies to the Prebid path.
  • Close Button, Countdown, Corner Position, Mobile Width / Height: same behavior as described in Prebid Server Render Options.
  • Slot Width / Height Override (px): use a different target size than the placement's primary size.

Practical Example: a 320x250 Slot

  1. Enable Convert Prebid Winners and Convert Ad Server Winners.
  2. Add Additional Auction Sizes: 300x250, 336x280.

A winning 336x280 creative is scaled to about 93% and centered in the 320x250 slot; a 300x250 winner displays with small side margins. Nothing changes for exact-size winners.

Use Cases

Proven BiddingStack setups that widen demand without hurting the reading experience:

Large banners in a mobile sticky footer

A mobile footer slot (320x50 or 320x100) has far less demand than 728x90 or 468x60. Add those as Additional Auction Sizes and set Placement Override to Floating Footer; the winner is scaled to the screen width and pinned to the bottom, with a Minimize Button so users can collapse it. Bigger, better-paying creatives; same small footprint.

Vertical banners floated on mobile

Skyscrapers (160x600, 300x600) rarely have a home on mobile. Enable the prebid path with Placement Override set to Dynamic Footer, Corner Position bottom right, and Mobile Width / Height around 120x240. The vertical creative shrinks into a corner unit with a close button; desktop demand now monetizes mobile traffic without covering the content.

Near-size substitution in content slots

An in-content 320x250 or 336x250 slot converts cleanly from 300x250 and 336x280; scale factors stay above 90%, so the creative remains crisp and the page layout never shifts. This is the safest, highest-impact starting point.

One direct-sold creative, every slot

With Convert Ad Server Winners on, a direct campaign trafficked with a single 300x250 creative can serve into 320x250, 336x280, or responsive slots, with no extra creative production, no layout breakage, and SafeFrame compatibility.

Leaderboard demand in medium rectangles (desktop)

A below-the-fold 300x250 can additionally request 468x60 and 320x100; wide winners are centered with letterboxing. Fill improves on remnant inventory while exact-size winners still render untouched.

Whatever the setup, BiddingStack keeps the slot's original footprint: scaling happens inside the existing box (or in a dismissible floating unit), so content never jumps and users stay in control.

Tips

  • Only add sizes with a similar aspect ratio to your slot; a 300x600 winner in a 320x250 slot shrinks to 42% and may be hard to read. Stick to near-size substitutes for the best look.
  • Ad server conversion is compatible with SafeFrame and does not modify the creative.
  • If an ad does not render as expected, check the browser console and see How to Debug Your BiddingStack Integration.

Need help choosing sizes for your inventory? Contact the BiddingStack team and we'll recommend a configuration.