Understanding Bid Density

Bid density (number of valid bids per auction) is the primary CPM driver. Learn the five factors that determine it — consent, geo, content category, page speed, and ad sizes — and how to diagnose low density.

Bid density is the number of valid bids received per auction. It is the primary driver of CPM performance — more competing bids push the price up. Understanding what determines bid density helps you identify and address revenue gaps.

Why bid density matters

In an auction where only one DSP bids, the impression clears at that DSP's minimum bid. In an auction where five DSPs bid, each tries to outbid the others, driving the clearing price higher. The mathematical relationship between bidders and CPMs is roughly logarithmic — going from 1 to 3 bidders can double CPMs, while going from 5 to 7 bidders offers a smaller incremental gain.

Factors that influence bid density

1. Consent and user ID availability

DSPs pay significantly more for impressions where they can identify the user (for retargeting, frequency capping, or audience targeting). Visitors who have:

Granted GDPR consent for personalised advertising
A browser cookie that a DSP recognises
A shared user ID (Shared ID, ID5) resolved by the wrapper's user ID modules

...attract more bidders and higher bids. Non-consented, cookieless traffic has lower bid density and lower CPMs — this is expected and unavoidable, not a bug.

2. Geo

US, UK, Canada, Australia, and Western European traffic attract the largest number of active advertiser campaigns. Tier-2 and Tier-3 geos have fewer active campaigns, so fewer DSPs bid on each impression. This is reflected in lower fill rates and CPMs, not a configuration issue.

3. Content category

Some advertiser categories are highly competitive (finance, health, travel, technology) while others attract fewer campaigns (general entertainment, forums). Contextual demand partners like Nexx360 can bridge this gap by targeting on semantic content analysis rather than user profiles.

4. Auction timeout

Advlume's Prebid timeout is 1,500ms. If your page is slow to load, demand partners' bidding infrastructure may not respond in time, reducing bid density. A consistently slow page (Time to Interactive > 4s) can reduce effective bid density by 20–40%.

5. Ad size popularity

The 300×250 medium rectangle has the deepest demand pool of any banner size. Unusual sizes (e.g. 468×60 alone) attract far fewer bidders. Enable multiple sizes per slot to maximise the number of eligible bidders.

Diagnosing low bid density

Open the Live Dashboard and watch the event feed during active traffic. A healthy site should show multiple bidResponse events per page load across different DSPs.
Look at the Bidder breakdown in the Revenue Report. If one DSP is responsible for >70% of revenue, other bidders are either not competing or not winning — investigate why.
Check the browser console for timeout messages: [advlume] Prebid timeout. Frequent timeouts indicate a performance problem.
Compare your consent rate. If most sessions are non-consented and your audience is primarily EEA, low bid density is expected.