In refining, capacity is typically measured in terms of the volume of the primary liquid feed that goes into the unit, though there are some exceptions (such as alkylation) where product output is the more common measure.
A given refinery or unit will typically have a number of potentially different measures of its capacity:
- Design capacity (name plate) - The capacity that the unit was originally designed for, which can be higher or lower than actual maximum capacity under real operating conditions
- Stream day capacity - Maximum throughput for a single day of operation
- Calendar day capacity - Maximum sustainable throughput over an extended period of time, recognizing the need for some downtime
- Mechanical availability - Maximum throughput given the mechanical capability of the hardware at a moment in time
- Operational availability - Maximum throughput factoring in unit availability and other operational limits on throughput
Capacity for a refinery as a whole is typically defined by the volume of crude oil it can process into atmospheric distillation. Refinery sizes range from a few thousand barrels/day up to 1.2 million barrels/day, but most refineries are in the range of 100-250 thousand barrels/day. At this size a refinery is able to capture most of the benefits that come with scale without exceeding the demand for product in its local market (as long as it is near a large city).