Erlang C

Erlang C is a mathematical formula used to calculate the number of agents required to handle a given call volume while meeting a target service level. Named after Danish mathematician A.K. Erlang, the formula models the probability of a caller waiting in queue based on the number of available agents, the call arrival rate, and average handle time. It is the industry-standard staffing model for inbound call centers.

How It Works in VICIdial

Erlang C takes three primary inputs: call volume (number of calls per interval, typically per half-hour), average handle time (the total time per call including talk time and after-call work), and service level target (such as 80% of calls answered within 20 seconds). The formula calculates the minimum number of agents needed to meet the service level target, plus the expected wait time, occupancy rate, and probability of a caller waiting.

While VICIdial does not have a built-in Erlang C calculator, the data needed for Erlang C modeling is readily available from VICIdial’s reporting: historical call volumes by time interval, average handle times by inbound group, and abandon rates. Workforce management teams export this data from VICIdial and run it through Erlang C calculators (available as spreadsheet tools, web applications, or dedicated workforce management software) to generate staffing schedules.

The output of Erlang C is the “raw” agent requirement — the number of agents who must be actively handling calls or available to take calls during each interval. This raw number must then be adjusted upward for shrinkage (breaks, training, meetings, absences) to determine how many total agents need to be scheduled. If Erlang C says you need 20 agents on the phones and your shrinkage rate is 30%, you need to schedule approximately 29 agents.

Why It Matters

Erlang C solves the fundamental staffing challenge: how many agents do you need? Understaffing leads to long wait times, high abandon rates, and poor customer experience. Overstaffing wastes money on idle agents. Erlang C provides a mathematically rigorous answer that balances service quality against cost.

The formula assumes calls arrive randomly (Poisson distribution) and that callers wait indefinitely without abandoning — assumptions that do not perfectly match reality but are close enough for practical staffing. For VICIdial operations running blended campaigns, Erlang C is applied to the inbound component, while outbound dialing fills the gaps between inbound calls. Accurate Erlang C modeling depends on accurate data, so ensure VICIdial’s reporting captures clean average handle time and call volume metrics by maintaining consistent disposition practices and time tracking.