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Calcimator

Telephone System Sizing Calculator

PBX trunk and extension count from user and traffic data.

Inputs

%
%

Results

Required trunk lines

17

Extensions needed115
Busy hour traffic (Erlangs)10
Actual blocking (%)0.71
External (PSTN) trunks12
SIP channels (VoIP)17
Bandwidth G.711 (Mbps)1.48
Bandwidth G.729 (Mbps)0.54
Busy hour call volume200
System tier (1-4)2
Internal Capacity30
How to Use This Calculator
  1. Enter the number of employees and the percentage who are simultaneous callers.
  2. Set the average call duration in minutes and the calls per hour per user.
  3. Input the desired blocking probability (standard is 2% or less using Erlang B).
  4. Review the Traffic Intensity in Erlangs and the Number of Lines Required.
  5. Add 20% capacity headroom over the calculated lines to handle traffic spikes.

How the result changes with Total users / stations

Total users / stationsRequired trunk lines
1,001106
3,501338
6,500611
9,000837

What each input means

Total users / stations
Total number of telephone users or desk stations.
Calls per user per hour
Average number of calls each user makes or receives per hour.
Avg call duration (sec)
Average call holding time in seconds. Typical office: 150-240 sec.
Target blocking (%)
Acceptable probability a call is blocked due to all trunks busy. Industry standard: 1% (P.01).
Busy hour traffic fraction
Fraction of daily traffic occurring in the busiest hour. Typical: 0.12-0.18.
Voicemail pickup (%)
Percentage of calls handled by voicemail, reducing trunk holding time.

What each result means

Required trunk lines
Number of trunk lines needed to meet the blocking target (Erlang B).
Extensions needed
Recommended extension count (users + 15% overhead for shared areas).
Busy hour traffic (Erlangs)
Total traffic load during the busiest hour in Erlangs.
Actual blocking (%)
Calculated blocking probability with the determined trunk count.
External (PSTN) trunks
Trunk lines needed for external calls (approx. 70% of total).
SIP channels (VoIP)
Equivalent SIP channel count for IP-based PBX.
Bandwidth G.711 (Mbps)
Network bandwidth needed using G.711 codec (87 kbps/call with overhead).
Bandwidth G.729 (Mbps)
Network bandwidth needed using G.729 codec (32 kbps/call with overhead).
Busy hour call volume
Estimated number of calls during the busy hour.
System tier (1-4)
Recommended PBX tier: 1=Key system, 2=Mid-range, 3=Enterprise, 4=Large/Carrier.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Total users / stations = 100, Calls per user per hour = 2, Avg call duration (sec) = 180, Target blocking (%) = 1 = 6 input(s) provided
  2. Calculate Required trunk lines
    Required trunk lines = ceil(trunks * (1 - voicemailPct / 200))
    17 = 17
  3. Calculate Extensions needed
    Extensions needed
    115 = 115
  4. Calculate Busy hour traffic
    Busy hour traffic = totalErlangs * (busyHourFraction / 0.15)
    10 = 10

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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