Open auction

An open auction is a real-time auction in which any buyer connected to an exchange can bid on a publisher's impression, limited only by the publisher's floor price and block lists.

Open auctions, also called the open exchange or open marketplace, are the default way programmatic inventory is sold. The UK CMA lists them first among transaction types and defines them as auctions in which any advertiser can bid (CMA, Appendix M, paragraph 48, 1 July 2020). In OpenRTB, a pmp.private_auction value of 0 means all bids are accepted, while 1 restricts bidding to the listed deals (IAB Tech Lab, OpenRTB 2.6, section 3.2.11).

The open auction offers reach and low prices. It also carries the most risk. Buyers can land on thousands of domains and apps, some of them made-for-advertising sites, and they have less control over placement. Many advertisers therefore buy through private marketplaces and curated deals, and keep open auctions for reach and testing.

Sellers set the floor price and decide which buyers and ad categories to block. Buyers decide how much to bid. Since most exchanges now run first-price auctions, many buyers use bid shading so they do not pay more than needed.

Example: In the ANA's Q2 2025 benchmark, private marketplace deals took 87.8% of programmatic spend, up from 64.5%. Average PMP CPMs were $7.15, against $4.41 in the open marketplace (ANA, 14 August 2025).

Related terms: PMP, preferred deal, first-price auction, floor price, MFA site, curation.

Authoritative source: UK CMA, market study Appendix M, paragraph 48 (1 July 2020).

Read the full guide: Programmatic guaranteed vs PMP vs preferred deal vs open auction.