The set of services, customers, or regions impacted by a single incident, the operational equivalent of measuring the explosion.
Blast radius is the scope of impact for an incident, expressed in services, customers, regions, or revenue. A regional database outage has a blast radius of every service that talks to that DB, every customer in that region, and every dependent feature. Modern incident response tools auto-compute the blast radius from the service map the moment an incident is declared, so the responder knows in seconds whether they're on a Sev-3 (one service, US-West) or a Sev-1 (every checkout flow, every region).
Triage is mostly a sizing decision: how many people do we wake up, what do we tell customers, do we declare a war room. Wrong sizing burns trust both ways, oversize and the team mocks SRE for crying wolf, undersize and customers find out before status.io does. Auto-computed blast radius collapses that decision from a human guess to a deterministic readout.
See the part of the platform that handles blast radius in production.