RTB: definition, mechanics and real-time auctions
Updated on February 22, 2026
Quick definition
RTB (Real-Time Bidding) is an automated real-time auction mechanism that allows online advertising space to be bought and sold individually — impression by impression — in a few tens of milliseconds. RTB is the backbone of modern programmatic advertising and connects DSP (buyers) and SSP (sellers) via ad exchanges.
How it works
The RTB process unfolds in less than 100 milliseconds, while a web page is loading:
- 1A user loads a page containing an ad slot
- 2The publisher's SSP sends a bid request to the ad exchange (format, placement, URL, user data)
- 3The ad exchange broadcasts the bid request to several connected DSPs
- 4Each DSP evaluates the request, calculates a maximum bid and replies with a bid response
- 5The ad exchange selects the highest bidder according to the auction model (first or second price)
- 6The winning ad is served on the page
RTB exists in several forms:
- Open Auction: auctions open to all
- Private Marketplace (PMP): restricted access for selected buyers
- Preferred Deal: negotiated fixed CPM, with no volume guarantee
- Programmatic Guaranteed: guaranteed volume and CPM
RTB's accuracy relies on third-party cookies, making it vulnerable to their gradual disappearance.
Why it matters
RTB has transformed the advertising industry by making media buying more efficient, transparent and personalised. It allows advertisers to buy only the impressions matching their target, reducing budget waste.
For publishers, it ensures permanent competition for their inventory, maximising RPM.
- Globally, the programmatic market represents the vast majority of display advertising spend
- RTB creates tensions with GDPR due to the massive transmission of personal data
- The rise of first-party data and cookieless approaches is reshaping auction models
How to improve or use it
- 1Segment your audiences finely by combining CRM data, analytics behaviours and contextual data.
- 2Test different bidding strategies: fixed CPM for awareness, optimised CPC or CPA for performance.
- 3Set strict frequency capping to avoid over-exposure and brand-image degradation.
- 4Use suppression lists to exclude already-converted audiences.
- 5Analyse brand-safety reports to avoid inappropriate placements.
With Sublim
Sublim helps you identify your site's most valuable audience segments (high-engagement visitors, pre-conversion behaviours) to feed your RTB strategies with first-party data. These audiences are more durable and accurate than RTB segments based on third-party cookies, and remain usable in a post-cookie environment compliant with GDPR.
Frequently asked questions
What is the difference between RTB and programmatic?
Programmatic advertising designates the entire field of automated buying of digital advertising. RTB is a subset of programmatic that corresponds specifically to real-time impression-by-impression auctions. Programmatic also includes Programmatic Guaranteed and Preferred Deal, which do not involve real-time auctions.
How long does an RTB auction take?
An RTB auction takes less than 100 milliseconds, i.e. while the web page is loading. The entire process — from bid request to ad display — is invisible to the user and takes place before the page is fully rendered in the browser.
Is RTB GDPR-compliant?
RTB involves the transmission of personal data (via cookies, location data) to many third-party players in a few milliseconds, which makes strict GDPR compliance difficult. The CNIL has issued recommendations on consent in programmatic, and IAB Europe's TCF framework attempts to govern these practices, without resolving every compliance question.
Related terms
A DSP (Demand-Side Platform) is a technology platform that allows adve…
An SSP (Supply-Side Platform) is a technology platform used by publish…
CPM (Cost Per Mille) is the cost per thousand impressions that represe…
First-party data is all the data collected directly by an organisation…