Real-time bidding (RTB)

RTB (real-time bidding) is a way of buying and selling ads in which each impression is auctioned automatically while the web page or app is loading, and the ad from the highest valid bidder is shown.

What is RTB?

IAB Tech Lab defines RTB as a way of transacting ads that lets each individual impression be put up for auction in real time, much like financial markets match orders (IAB Tech Lab, OpenRTB, updated January 2024). What separates it from traditional ad buying is the unit being traded. It is not a banner placement for a month. It is one impression, shown to one specific person, at one specific moment.

So that hundreds of buyers and sellers can talk to each other, the industry shares one protocol, OpenRTB, maintained by IAB Tech Lab. Version 2.6, updated in September 2023, added duration-based floor prices for video and audio ads, plus an attribute that flags deals with guaranteed volume.

How does an RTB auction work?

1. A user opens a page or an app screen that has an ad slot.
2. The publisher’s SSP creates a bid request describing the placement, device and context, and sends it to the exchange or to buyers.
3. Each DSP evaluates the impression against its campaign criteria, then either bids with an ad creative or passes.
4. The winner is picked and its ad loads on the screen.

Each step gets very little time. With Google Authorized Buyers, Google waits 80 to 1,000 milliseconds for buyers to respond, depending on the format and the auction type. The exact limit is in the request’s tmax field, and responses that arrive late are dropped (Google Authorized Buyers, Real-time Bidding, accessed October 2026).

First-price vs. second-price auctions

In a second-price auction, the winner pays just a little more than the second-highest bid. Before 2019, Google Ad Manager ran a second-price auction among Authorized Buyers (including Google Ads, DV360 and other DSPs) and only then compared the result with other sources. In 2019, Google moved to a single first-price auction: the winner pays exactly what it bid, and no buyer’s bid is shared with another buyer before the auction (Google Ad Manager blog, May 2019). The consequence is that DSPs now have to estimate the price that is just enough to win, instead of bidding their ceiling and letting the exchange bring it down.

What changed in 2025 and 2026

RTB on the web relies heavily on third-party cookies to recognize users. In April 2025, Google decided to keep letting Chrome users make their own choice about third-party cookies rather than launching a new prompt (Privacy Sandbox, April 2025). In October 2025, Google retired Protected Audience, the API that let the browser run remarketing auctions on the device, along with Topics and Attribution Reporting (Privacy Sandbox, October 2025). RTB therefore still runs on the old model, with bid requests passing through each party’s servers.

RTB vs. other types of programmatic buying

Frequently asked questions

Are Facebook and TikTok ads RTB?

Meta and TikTok also auction every impression, but the auction runs inside their own systems, and buyers can only get in through the platform’s own Ads Manager or API. When people in the industry say RTB, they usually mean open auctions through exchanges, SSPs and DSPs.

Why is RTB so often linked to ad fraud?

There are a huge number of sellers and every transaction is automated, so fraudsters can set up fake sites or use bots to generate views and sell them. ads.txt and sellers.json help buyers verify who is selling, but you still need to check placement reports.

If you run programmatic display and want to know which links in the chain your money passes through, the Adtek team can talk it through with you via the contact page.