Extended Erlang B calculator

This is an Extended Erlang B calculator. Use it to work out how many lines you need in your trunk group if you want to take into account calls that are blocked and immediately retried.

Extended Erlang B Calculator
Recall factor (retries)

Excel version

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Extended Erlang B Calculator instructions

This calculator estimates the number of voice lines you need. It takes account of calls that are immediately retried when blocked, and you should firstly select the percentage of those calls using the drop-down box.

Three further three variables are involved:

  • Busy Hour Traffic (in Erlangs): the hours of call traffic during the busiest hour of a telephone system’s operation
  • Blocking: the ratio of calls that fail through insufficient lines (e.g. 0.03 mean 3 calls blocked per 100 calls attempted)
  • Lines: is the number of circuits in a trunk group

If you know two of the figures, you can work out the third:

  • Click on the button representing the variable you want to calculate
  • Enter the two known figures into their edit boxes
  • Press Calculate and the unknown value will be calculated and displayed in the remaining box

For example, imagine your target for blocked calls is 3% and you assume that 60% of those blocked calls are immediately retried. If you want to know how many lines will be needed to handle your system’s offered Busy Hour Traffic of 35 Erlangs, you should click the button next to Lines and enter those values into the calculator. When you press the Calculate button, the result will be displayed: 44 lines.

Windows version

Westbay Traffic Calculators is a Windows package of four calculators including this Extended Erlang B Calculator. We offer free upgrades and installation support for the lifetime of the product. The calculator works up to 90,000 Erlangs / Lines and can display and print the results of the previous week.

Westbay Traffic Calculators - Extended Erlang B

Our best-selling Windows software application comprises four calculators for Extended Erlang B and Erlang C. Use them to dimension trunk groups and predict the performance of your help desk or call center agents.

Free upgrades for life on a single computer for a once-only charge of just $99.