Definition
Erlang C is a queueing model that calculates the probability an arriving contact must wait, given a number of agents and a workload measured in erlangs. Traffic intensity (erlangs) = contacts × AHT ÷ interval length in seconds.
From that probability it produces service level, average speed of answer and occupancy. It requires more agents than erlangs of workload, otherwise the queue grows without limit and no finite answer exists.
The model assumes random Poisson arrivals, no abandonment, interchangeable agents and a steady state within the interval. Because it assumes callers wait forever, it is slightly conservative, real service level is usually a little better than predicted.
Why it matters
- Almost every commercial WFM staffing engine is built on Erlang C, so understanding it means understanding what your tool is doing.
- It shows why staffing cannot be scaled linearly with volume, and why pooled queues outperform many small skill groups.
- Knowing its assumptions tells you when to reach for something else: Erlang A for abandonment, concurrency models for chat, simulation for complex skill routing.
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