Definition
Erlang A is a staffing formula that extends Erlang C by explicitly modelling caller abandonment, callers who hang up while waiting in queue, rather than assuming every caller waits indefinitely until answered. Erlang C's core weakness is exactly this assumption: it treats abandonment as if it does not exist, which means it tends to overstate the agents needed in operations where a meaningful share of callers actually give up before being answered.
Erlang A adds a caller patience parameter, how long callers typically wait before abandoning, to the calculation, producing more realistic staffing numbers for operations with a real, measurable abandon rate.
Erlang A is more complex to calculate and less commonly implemented in free or basic staffing tools than Erlang C, and Erlang C remains a reasonable, more conservative approximation when abandon rate is low.
Why it matters
- If your abandon rate is above a few percent, Erlang C is quietly costing you headcount you may not actually need.
- Caller patience is not fixed: it shortens when callers have alternatives and lengthens when they do not, so the parameter needs measuring, not guessing.
- Running Erlang C and Erlang A side by side shows exactly what abandonment is worth in agents.
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