What is bid density?
Bid density is the number of competing bids an auction receives for an impression, usually reported as an average per auction. More bids push the clearing price up and make floors matter less; thin auctions clear near the floor or not at all. It is mostly an exchange and publisher metric, because a single bidder sees only its own bid and whatever price feedback the exchange returns.
Also called: bids per auction, auction density, auction pressure.
How it works
Density depends on how many bidders receive the request and how many of them find it worth a bid. Exchanges send each request to a subset of bidders, shaped by QPS limits and pretargeting, so the same impression can face very different competition on different supply paths. Header bidding raised density for many publishers by putting several exchanges' demand into one decision.
Under first price, density shows up as the gap between the winning bid and the runner-up. An exchange that fills in ${AUCTION_MIN_TO_WIN} gives each bidder a view of that gap without revealing who else bid, and a shrinking gap over time usually means more competition for that inventory. Second-price auctions expose density more directly, because the next bid sets the clearing price.
Low density is a common cause of weak prices and low fill, and it often traces back to the request. Inventory that lacks the identifiers buyers target on, or arrives through a long reseller chain, draws fewer bidders. Raising floors on thin auctions tends to cut fill further rather than lift price.
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Related terms
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