Erlang Calculator for Call Centres and why they are so important for call centre managers

Call Centre Erlang Calculator

A Call Centre Erlang Calculator is a key tool in helping call centre managers to determine how many call centre agents are needed to meet Service Levels. And more than that, it enables call centre managers to model a range of different scenarios using data to make informed decisions.

But what is an Erlang and is it a friend or foe???

Why you need a Call Centre Erlang Calculator

One of the fundamental objectives of contact centre management is to ensure there are just the right amount of staff to meet the call demand – too many staff and it’s inefficient, too little staff and customers can experience long wait times.

Of course, customers don’t all communicate with each other about the best time to dial in which means calls can and do, arrive randomly throughout the day.

Thankfully, a very smart man called Mr Erlang developed a formula that enables us to model a range of different variables to determine just how many employees are required to meet the Service Level objectives.

It’s this formula, Erlang, that underpins the majority of all contact centre forecast scheduling software across the world today.

Don’t worry though, you don’t need an expensive Workforce Management System to model your contact centre workload – ACXPA Members can download a free call centre Erlang Calculator right here!

Brief History of the Erlang Call Centre Calculator

Back in the early 1900’s, when telephone calls were enabled by manually connecting two circuits together, the Copenhagen Telephone Company needed a way to find out how many circuits they needed to meet the demand, and how many telephone operators they needed to connect the circuits (any of this sounding vaguely familiar?).

Mr. Agner Krarup Erlang

Mr. Agner Krarup Erlang

Well, enter Mr Agner Krarup Erlang, a mathematician, astrologist, physicist and chemist who developed a mathematical formula to provide the answers.

The marketing budget was pretty tight back then (good to see some things never change) so rather than coming up with an elaborate name for the formulas Mr Erlang developed, he decided to go with Erlang A, Erlang B and Erlang C.


OK, this will really blow you away.

Those logarithms Mr. Erlang developed are still the basis for determining the required number of agents to meet a forecasted demand over 100 years later!

In short, Mr. Elang was the original Workforce Optimisation Planner.

And sadly, yes, that was past tense, Mr Erlang passed away following abdominal surgery aged 51 on 3 February 1929.

Common Questions about the Erlang Call Centre Calculator

What is an Erlang?

Erlang is a formula that is used to calculate the number of employees required to answer phone calls, live chat or even retail customers in a given time frame.

In a call centre environment, it is typically used to calculate the number of call centre staff required to meet the Key Performance Indicators set by the business e.g. 80% of all calls answered within 30 seconds.

Which Erlang Formula should you use?

There are actually three different Erlang formulas and when deciding which Erlang formula to use, the key deciding factor is what will happen when all your agents are busy handling calls.

Erlang A

Similar to the most popular formula used (Erlang C),  Erlang A includes abandoned calls whereas Erlang C assumes there are no abandoned calls.

Erlang B

The quick explanation is that Erlang B is normally used to determine how many phone lines (aka trunks) you need.

Erlang C

The most widely used formula in determining how many staff you need to meet the required Service Levels.

What metrics do I need to use an Erlang Call Centre Calculator?

To be able to use an Erlang C Calculator, you will typically need to know the following metrics (that you should be able to get from your call/phone system):

  • Average Handling Time (how long, on average, staff talk to customers on a call and the post-call wrap up)
  • Calls per interval period (most call centre data is recorded in either 15, 30 or 60-minute ‘intervals’
  • The service levels you want to achieve (X percentage of calls answered in Y)
  • More advanced Erlang calculators (like ours!) will enable you to model impacts to your occupancy and shrinkage so if you also know those figures it’s a bonus, otherwise we’ve included industry norms.

Why should you use an Erlang calculator?

For call centre managers, one of the most powerful tools you can have in your toolkit is a Call Centre Erlang Calculator as it takes the emotion out of conversations about whether you have enough agents to achieve Service Levels (you know, the usual “do more with less” conversation with your boss).

It also enables you to create business cases for process/technology improvements with real data and science behind it.

Think about these scenarios:

  • You can clearly show the impact reducing Average Handling Time could have (refer below) if you made system improvements, improved call centre agent training or made some improvements to processes.
  • The reverse is also true, you can model the impact of new processes etc that increases your Average Handling Time.
  • You can also show the impact of more or less call volumes and how that can impact your Service Levels. Lots of things can increase call volumes as you know like product/service issues, media & advertising, new product launches etc whilst things like self-service, better website design, FAQ pages etc can reduce call volumes.
  • You can also model the impact of staffing numbers. If you knew you needed 15 employees to achieve your desired Service Levels (by using the calculator) but only 12 arrived for work, Erlang will show you the impact. Same applies at a macro level – had a budget for 50 FTE that got slashed to 40 FTE? You can show how that will impact your Service Levels and see if your boss is prepared for the fall out of longer wait times etc.

There really are numerous reasons why every contact centre manager should have an Erlang Calculator!

 Erlang calculator example

In the example below, you can see that if interactions (Erlang can be used for calls, live chat – anything with random arrival) and the number of agents you have stay the same (columns 1 & 3), but the Average Handle Time (column 2) changes by 10 seconds, you can see the impact it has on Service Levels (column 4).

If your target was to meet 80% of calls in 3o seconds, an increase in AHT of 40 seconds reduces your Service Levels from 83% to 58% assuming everything else stayed the same.

Thinking about your own call centre, how many things impact AHT? It might be a new product so calls are going longer, less experienced agents, system issues etc.

That’s the power of Erlang!

Erlang Call Centre Calculator
Erlang Call Centre Calculator example showing the impact of changing Average Handle Time (AHT) by 10 seconds on Service Levels using the same number of agents (Available in the PRO version)

What are the limitations of Erlang C?

Whilst the Erlang Call Centre Calculator is a great tool, it does have some limitations as it makes the following assumptions:

  • No abandoned calls (the reality is some callers will hang up prior to their call being answered)
  • No busy signal (so everyone who calls will get through to the queue)
  • Calls will arrive in a random, “steady-state” call arrival pattern (in reality calls may come in big ‘clumps’ throughout the interval period)
  • That all staff will be available to take calls (so all staff will be ready and available to take calls)
  • It won’t work in a Skills-Based Routing environment as it assumes all agents can answer all calls presented.
  • It’s also not great with low call volumes or staffing levels – we typically find anything under around 150 calls a day things can get a bit shaky. If that’s your scenario, don’t worry. We’ve now included a Workload Calculator that can help!

Do you need to be a maths wizard or Workforce Planner to use it?

The good news is NO!

We’ve designed this Erlang Call Centre Calculator to make it as easy as possible to use with some simple-to-follow instructions included.

Courses for Workforce Management and Workforce Optimisation

Of course, having some basic knowledge of Erlang C wouldn’t hurt, and we’ve got a range of online courses through our accredited training partner CX Skills that can bring you up to speed – and as an added bonus, ACXPA members save 25% on all CX Skills courses!

We recommend the following courses:

The courses range from a basic overview of Erlang to an advanced 3-day course for Workforce Planners.

Search the ACXPA Events Calendar to see all the upcoming training courses, webinars, conferences and more.

Example of the Call Centre Erlang Calculator
Example of one of the graphs you can produce in our Call Centre Erlang Calculator PRO that makes it easy to see where you are over/under resourced.

Download our Erlang Call Centre Calculator

We have two Erlang Call Centre Calculators available to download, a free and paid version.  The Free and PRO version both contain five different Erlang calculators that can be used to model different scenarios for your contact centre.

Both versions are the same however the PRO version has all the features unlocked and enables additional modelling and graphs to help you model a range of different scenarios (perfect for building a business case or preparing for a conversation with your manager!).

We’ve made the calculators super easy to use with helpful notes and the sections highlighted where you need to enter your data.

Please note:  Both versions of the calculators are built into an Excel spreadsheet and you must enable the macros for them to work.

The Basic Erlang Calculator enables you to perform basic Erlang calculations to determine how many agents you need to meet your desired Service Levels, along with some limited modelling calculations.

Our Erlang Calculator PRO enables you to perform more powerful modelling and includes some visual charts to make it easier to identify opportunities and has all functionalities enabled (and it’s still super easy to use!).

ACXPA Subscriber PLUS & Members can download our Basic Erlang Calculator for free! 

Click here to find out which option is best for you > Alternatively, you can purchase the PRO version > 

PRO Version – $149.95 AUD ex GST (members can save up to 50%)

The PRO version makes it even easier to model a range of different scenarios, input your specific data and see a visual graph of your peaks, troughs and opportunities and is a critical tool for anyone who works in a contact centre!

Unlike the free version, all functionality is enabled with no restrictions.

ACXPA Members receive discounts of up to 50% on all ACXPA Marketplace items along with a range of other benefits.

View the ACXPA Membership options > or purchase the PRO version now>

Upcoming ACXPA CX Roundtable - Next Guest Announcement
Upcoming ACXPA Contact Centre Roundtable July 2024 - John Stavrakis

Upcoming ACXPA Member Bites Sessions

Short, sharp interviews and presentations on specific topics to fast-track your knowledge!

The ACXPA Member Bites are only available to ACXPA Members! 

ACXPA Members can watch all the ACXPA Member Bites at any time in their Video Library. Discover which membership is right for you >

Nadine Power, ACXPA National Advisory Board Member

IVR Best Practice

Over 90% of contact centres are using an IVR (Press 1 for this, press 2 for that), and we've encountered many contact centres with over four layers of options for customers to select from. In this session, Nadine will be sharing some best practice tips on IVR design for 2024.

Presented by Nadine Power, Client Success Manager, VERSA Connects (and ACXPA National Advisory Board Member)

More sessions announced soon! 

(If you're interested in speaking at one of our events, click here to learn more >)

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