The Erlang B Calculator tells you how many lines, SIP trunk channels or IVR ports you need so that only a chosen share of busy-hour calls find every line busy.

How to Use#

Under "How do you want to enter traffic?", pick Calls and duration or Erlangs.
For calls and duration, enter Calls in the busy hour and Average call duration in minutes. For erlangs, enter Busy-hour traffic.
Choose a Target blocking (grade of service): 0.1%, 0.5%, 1%, 2% or 5%.
Optionally enter Lines you have now to check your current setup. Leave it at 0 to skip.
Read the results and the table of lines needed at every blocking target.

Inputs#

Calls in the busy hour: inbound and outbound calls that need a line during your single busiest hour.
Average call duration: minutes per call. Decimals are fine (2.5 is two and a half minutes).
Busy-hour traffic (erlangs): one erlang is one line busy for the whole hour. Use this if your phone system already reports it.
Target blocking: the share of calls you accept will get a busy signal or fail to connect.
Lines you have now: your current line or channel count. Decimals are rounded down.

Reading the results#

Traffic (erlangs) = Calls in the busy hour x Average minutes / 60
Lines needed = smallest line count where Erlang B blocking <= target
Blocked calls per busy hour = Calls x Blocking at that count

Lines needed: the smallest count that meets your target.
Blocking at that count: the actual blocking with that many lines, usually a little under the target.
Blocked calls per busy hour (calls mode) or Traffic lost to blocking (erlangs mode).
If you entered current lines, you also see the blocking with those lines, how many lines short or spare you are, and a warning if you are short.

The table lists lines needed and actual blocking at all five targets, so you can see what a stricter or looser target costs in lines.

Things to know#

Size for the busiest hour, not a daily average.
Erlang B assumes blocked calls are lost, not queued or retried. Arrivals are random and independent, and a line is either busy or free.
If callers wait in a queue for an agent instead of hearing a busy tone, that is Erlang C. Size agents with Erlang C first, then use this tool to size the lines they need.