Waterfall

A waterfall is a way of selling impressions in which the ad server calls demand sources one at a time, in a fixed order usually based on expected price, and stops at the first taker.

The order comes from history, not from live bids. A source ranked third never sees the impression if the first or second takes it, even when it would have paid more. Each call also adds delay. Google describes the difference in its own products. Ad Manager's mediation for mobile apps calls yield partners one by one, in sequence. Open Bidding calls all partners at once so they compete in real time (Google Ad Manager Help, accessed 9 November 2026).

The UK CMA described a related effect on the web. With a non-Google ad server, Google's exchange did not take part in header bidding, so it competed with an "expected" bid in a waterfall-like way. The CMA called that inefficient for the publisher (CMA, Appendix M, paragraph 434, 1 July 2020). Header bidding replaced most web waterfalls. Waterfalls persist in app mediation and some video set-ups, often mixed with bidding.

Example: An app's mediation stack calls three networks in turn, each with a 300 ms timeout. When the first two pass, the third fills the ad after almost a second, and the user has already scrolled past the slot. Moving the three networks into one bidding auction cuts the wait to a single timeout.

Related terms: header bidding, Open Bidding, ad network, header bidding wrapper, fill rate, last-look.

Authoritative source: Google Ad Manager Help, "Introduction to Open Bidding" (accessed 9 November 2026).

Read the full guide: Header bidding explained: wrappers, Prebid.js, client-side vs server-side.