The Erlang B Calculator is a fundamental tool in telecommunications traffic engineering. It is used to determine the number of trunk lines or channels required to handle a known traffic load with a specific "Grade of Service" (GoS). While many engineers search for an "Erlang calculator Excel" sheet, this web-based tool provides instant, accurate results without needing to construct complex VBA scripts or data tables.
What is an Erlang?
An Erlang is a dimensionless unit of traffic intensity. One Erlang represents the continuous use of one voice circuit (or channel) for one hour. For example:
If a single phone line is busy for 60 minutes in an hour, that is 1 Erlang of traffic.
If a group of agents handles 30 calls that last 2 minutes each within an hour (60 minutes total load), that is also 1 Erlang.
Formula: Traffic (Erlangs) = (Calls per Hour × Average Call Duration in Seconds) / 3600.
How to Calculate Erlang B in Excel
If you are looking to replicate this calculation in Microsoft Excel, there isn't a native =ERLANGB() function. However, you can derive it mathematically using the Poisson distribution functions available in Excel.
You can use the following formula where A1 contains your Traffic in Erlangs and B1 contains your Number of Lines:
This formula leverages the mathematical relationship between the Poisson distribution and the Erlang B loss formula.
Understanding the Inputs
Traffic Load: The total volume of traffic trying to access the system, measured in Erlangs. This is often calculated based on the Busy Hour Traffic (BHT).
Number of Lines/Channels: The capacity of your system. In a VoIP trunk, this is the number of concurrent call paths available. In a call center, this might represent the number of receiving lines.
Interpreting the Results
Grade of Service (GoS): This is the probability that a call will be blocked (receive a busy signal) because all lines are occupied. A GoS of 0.01 means there is a 1% chance of blocking.
Efficiency: This percentage indicates how effectively the lines are being utilized. Very low blocking usually results in lower utilization (many idle lines), while high utilization often leads to higher blocking probabilities.
Why Use Erlang B?
The Erlang B model assumes that blocked calls are cleared—meaning if a caller gets a busy signal, they hang up and do not immediately retry (or the system does not queue them). This makes it the standard model for dimensioning trunk lines between switches or PBX trunking capacity. For scenarios where calls wait in a queue (like a customer service center), the Erlang C model is used instead.