Erlang B Calculator
BHT (Erl.)
Important: Decimal separator is '.' not ','
Brief instructions
Here is a brief introduction to the calculator.  For a running record of the results calculated, press the Results button.  This function open new browser window on your desktop.
  • The Erlang B traffic model is used by telephone system designers to estimate the number of lines required for PSTN connections (CO trunks) or private wire connections.  The three variables involved are Busy Hour Traffic (BHT), Blocking and Lines:
    • Busy Hour Traffic (in Erlangs) is the number of hours of call traffic there are during the busiest hour of operation of a telephone system.
    • Blocking is the failure of calls due to an insufficient number of lines being available.  E.g. 0.03 mean 3 calls blocked per 100 calls attempted.
    • Lines is the number of lines in a trunk group.
  • If you know two of the figures, you can work out the third as follows:
    • Click on the Unknown radio button representing the unknown variable.
    • Enter the 2 known figures into their edit boxes.
    • Press Calc and the third figure will be calculated and displayed in the remaining edit box.
  • For example, if from your call logger, you know that the Busy Hour Traffic is 10 Erlangs, and you want to know how many lines are required in this trunk group if you are prepared to tolerate 2 calls being blocked in every 100 calls attempted then:
    • Press the Lines Unknown radio button (this is selected by default).
    • Enter 10 in the BHT edit box.
    • Enter 0.02 in the Blocking edit box.
    • When you press the Calc button, 17 will be displayed in the Lines edit box indicating that 17 lines would be required in this trunk group.