Erlang C Call Centre Calculator
Calculate the agents needed to hit a service level target using the Erlang C model, with occupancy, average speed of answer, abandonment estimate and shrinkage adjusted headcount.
Last reviewed by the Radiatus Cloud team
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Queues do not scale linearly
Doubling the number of calls does not double the agents needed, and adding one agent to a busy queue can cut the average wait by more than half. Queueing behaviour is non linear, which is why staffing a contact centre by dividing total call minutes by agent minutes produces a plan that fails badly at exactly the times it matters. The Erlang C formula, derived by Agner Erlang for telephone exchanges in 1917, models the queue properly and is still the standard used by workforce management systems.
Occupancy is the trap
Running agents at ninety five percent occupancy looks efficient and is unsustainable. Above roughly eighty five percent, waiting times rise steeply for a small change in volume, agents get no recovery time between contacts, and attrition climbs. The model shows occupancy alongside the agent count precisely so that a plan which meets the service level by working people flat out is visible as such rather than being presented as an optimisation.
Shrinkage turns agents into headcount
The Erlang calculation returns agents required on the phones at a given moment. Actual headcount is higher, because people take breaks, holiday, sick leave, training and team meetings. Shrinkage of thirty to thirty five percent is typical, so a requirement of twenty agents on the phones means around thirty employees rostered. Omitting shrinkage is the single most common reason a staffing model that looked correct produces long queues in practice.
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Frequently Asked Questions
What does Erlang C assume?
That calls arrive randomly following a Poisson process, that no caller abandons the queue, and that agents are interchangeable. Real callers do abandon, which makes Erlang C slightly conservative, and real arrivals are burstier than Poisson, which makes it slightly optimistic.
What service level should I target?
The traditional standard is 80 percent of calls answered within 20 seconds. It is a convention rather than a finding, and the right target depends on your customers and cost. The calculator shows what different targets cost in agents so the trade off is explicit.
What is a safe occupancy level?
Between 75 and 85 percent for a sustained operation. Above 85 percent, waiting times become very sensitive to small volume changes and agent burnout rises sharply. Below 70 percent, you are paying for idle capacity.
How much shrinkage should I allow?
Thirty to thirty five percent is typical once breaks, holiday, sickness, training, coaching and meetings are counted. Measure your own rather than assuming: it varies enormously with how much coaching and training the operation does.
Does this work for chat and email?
Not directly. Erlang C assumes an agent handles one contact at a time. Chat agents handle several concurrently and email is not a real time queue, so both need different models. Use it for voice and for any channel handled strictly one at a time.
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How to Use
Enter call volume, handling time and your service level target to get the required agent count.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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