Orginally posted by our esteemed Author Vinay Vasudevan (https://wfmanagement.blogspot.com/p/auth.html) on a different blog The Mechanics of Erlang - SL Model
Buck up! It's time for some mathematics.
If you are from WFM, you would have never missed the word "Erlang". This is so famous amongst the WFM folks that it doesn't need any introduction.
Erlang formulas are such a major contributor in the world of WFM
There are many functions we use in Erlang such as Fractional Agents, SLA, AgentsASA etc.
But what is the workflow behind these formula? How do they work?
In this article, I'm trying to explain the Erlang formula in a simpler way so that we can understand what is the actual mathematics behind it.
Here are some of the inputs required to calculate FTE and other parameters using Grade of Service (GoS).
- Call Volume per Interval
- AHT
- Interval Length
- Service Goal
- Service Time
Here is the formula from Wikipedia
Pw = Probable Wait Time
A = Raw Agents Required
N = Estimated Agents Required
"A" is pure workload which is Calls*AHT/Interval Length
"N" is A+1, but the +1 is subjected to vary depending upon the additional FTE required to meet SL Goal
I've attached the excel calculations at the end of this article which elaborates the formula with worked example.
Make sure you check it, without which this article will be incomplete.
Once we calculate the Probable wait time, other parameters can also be calculated using the below formulas
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  If you closely check the formulas in the sheet, I have rounded up the value of N which is the FTE
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