What Is Erlang, and Why Does It Matter?

Anwesha Roy

The age-old call distribution formula still in use today

What Is Erlang, and Why Does It Matter?

Modern contact centers have changed significantly over the years, evolving into intelligent omnichannel environments, brimming with powerful analytics, tools, and transformative features.

But some components of the contact center environment have survived over the decades. The Erlang C Formula is one of them.

What Is Erlang? A Definition

An Erlang is a unit used in telephony to measure the load carried by a contact center or physical telephony service system in a given period.

Typically, this period is one hour. When this is the case, one Erlang equals 60 minutes’ worth of calls.

So, if the customer service team handles 6,000 minutes of calls in an hour, the contact center has processed 100 Erlangs of traffic.

In the past, this measure was mission-critical for contact centers, helping determine how many circuits the operation needed to meet voice demand.

Yet, nowadays, it is mainly used as an input into the Erlang C formula.

Why Does It Matter?

Some contact centers still use the Erlang C formula to calculate how many staff they need to meet forecasted call traffic.

First introduced in 1917 by Danish mathematician A.K. Erlang, modern adaptions of the equation also factor in several other contact center metrics, including:

  • Desired service level
  • Target occupancy
  • Average handling time

While many modern WFM applications provide more accurate staffing calculations, Erlang C is still the preferred alternative for smaller operations that want to avoid investing in the technology. Typically, these contact centers are smaller than 30 seats.

There are also the Erlang A and B equations, yet contact centers do not use these.

With that said, Erlang B once did have a contact center application, helping operations calculate the number of lines into the contact center required for queuing.

Understanding the Erlang C Formula

The Erlang C formula allows companies to calculate their staffing requirements while it also allows businesses to estimate how long they may have to wait.

This formula makes a few key assumptions. For instance, it assumes customer requests in the contact center will follow a “Poisson” call arrival process. It also assumes:

  • There are a large number of customers to address
  • A single customer can impact overall system performance
  • All customers use the contact center system independently
  • Service times are distributed exponentially
  • Customers will not abandon service requests when waiting for an agent
  • Lost calls are “delayed” rather than abandoned
  • Call center agents only handle one customer at a time
  • The number of support agents is lower than the volume of customers

The Erlang C formula doesn’t work if customer requests are not independent or triggered by a common event. Such scenarios include many incidences, from marketing campaigns to natural disasters.

Additionally, the formula will usually only deliver accurate results when the number of customers is at least ten times higher than the number of available agents.

Yet, Erlang C issues extend beyond this alone…

Why is Erlang C a Problem for Today’s Contact Centres?

While Erlang C still has value to offer contact centers, it does have some issues. One drawback is that it assumes things that may no longer be true in the modern calling landscape.

For example, the formula disregards lost calls. It also assumes contact center agents will never handle more than one conversation at once.

As such, call center managers often end up overestimating their staffing requirements.

The equation also doesn’t accommodate contact fluctuations within a particular reporting period. This can lead to inaccuracies in forecasting data.

Generally, Erlang C is useful for environments where there are minimal abandoned calls and busy signals, but it is not accurate in all situations.

Getting Started with Erlang Calculators

Fortunately, there are more intelligent alternatives to complex Erlang C, Excel-based calculations. Instead, contact center managers can generate a quick staffing estimate for their upcoming targets using Erlang C.

There are free web-based tools and downloadable apps where users can input operational variables to calculate staffing needs.

Thanks to these tools’ cost-effectiveness and accuracy, it is still too soon to dismiss Erlang as an outdated, legacy concept.

Discover lots more advice for staffing the contact center in our article: What Is Workforce Management? An Introductory Guide


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