Call Minutes Calculator
Blocking target
Busy hour factor (%)
Minutes per day
Lines required
Important: Decimal separator is '.' not ','
Brief instructions

Here is a brief introduction to the calculator.  

  • You can use this calculator to work out how many lines you need to provide in a trunk group if you know the number of minutes of calls which will be offered to that trunk group. The trunk group can be a PSTN link (CO trunks), a group of leased lines between two private network locations or an access group to a Virtual Private Network.
  • As an example, if you wanted to know how many lines would need to be provided for a trunk group which was offered 10000 minutes of calls a day:
    • Click on the Unknown radio button in the Lines Required section to indicate that it is the number of lines you wish to calculate.
    • Enter 10000 in the Minutes per day edit box.
    • Press the Calc. button.
    • The answer is displayed in the Lines edit box (40).
  • Reverse calculations are also possible allowing you to calculate the minutes of calls which can be carried by a trunk group of a particular size. To carry out such a calculation, click on the Unknown radio button in the Minutes per day section, enter the number of lines in the Lines required edit box and press the Calc. button. The answer will be shown in the Minutes per day edit box.
  • This calculator uses the Erlang B traffic model which is discussed in detail in the help system. You have the opportunity to specify two additional parameters relating to this traffic model and to the traffic pattern:
    • The busy hour factor is the percentage of daily minutes which are offered during the busiest hour of the day. 17% (the default) is a reasonable figure for a business which operates an 8 hour working day, but a higher figure could be entered if the business in question operates a shorter working day, or if frequent calls are being made to a different time zone.
    • Blocking target is the ratio of calls which will be blocked because no lines are available. 0.010 (the default) means that 1% of calls would be lost. This is a normal figure for traffic engineering, but other figures can be entered into this edit box.
  • Please note that in order to make a safe judgement of the number of lines required, traffic figures over a significant time period are required. One day's traffic figures are not enough.