tmax

tmax is the OpenRTB bid request field that states the maximum time, in milliseconds, the exchange will wait for bids to arrive, including the time they spend crossing the internet.

The field sits at the top of the BidRequest object in OpenRTB 2.6. Its value overrides any timeout the exchange agreed with the bidder in advance (IAB Tech Lab, OpenRTB 2.6, section 3.2.1). A bidder must parse the request, decide, price and answer inside that window. Late bids are thrown away. The IAB Tech Lab's auction primer lists "received after timeout expired" first among the technical reasons a seller rejects a bid (IAB Tech Lab, Programmatic Auction Definitions).

The deadline covers the whole round trip, not just the bidder's compute time. Distance to the exchange, TLS handshakes and queues all count against it. Bidders therefore plan for their slowest common responses, not their average. In a chain of resellers, each hop sends a shorter tmax downstream, because it must keep time to run its own auction and reply upstream.

Example: One SSP told the UK CMA in 2020 that server-side header bidding timeouts typically run from 300 to 500 ms (CMA, Appendix M, paragraph 455, 1 July 2020). A bidder whose network path to the exchange takes 40 ms each way has about 40 ms left to compute when the request says "tmax": 120. You can model your own budget with the RTB latency calculator.

Related terms: timeout, real-time bidding, bid request, p99 latency, QPS, OpenRTB.

Authoritative source: IAB Tech Lab, OpenRTB 2.6, section 3.2.1 "Object: BidRequest" (accessed 9 November 2026).

Read the full guide: OpenRTB 2.6 explained: bid request anatomy, field by field.