Work out bigquery slot estimate instantly with clear inputs, formula shown and shareable results.
A slot is a unit of query processing capacity, and reservations are purchased in blocks of 100. Peak requirement is concurrency times slots per query, but average utilisation is far lower because peaks are short — which is why on-demand or autoscaling reservations frequently beat a fixed reservation sized for peak.
Slot sizing
peak slots = concurrent queries x slots per query; reservation = round up to the next 100; utilisation = peak x peak hours / 24 / reservation
When utilisation is high and sustained. At 25 percent average utilisation, on-demand scan pricing or autoscaling reservations are usually cheaper.
They queue rather than fail, sharing available slots. Query duration stretches, which is acceptable for batch work and not for interactive dashboards.