Forecasting Staff Requirements


May 01, 2004

What would be good software to forecast short-term (1 to 3 months out) and long-term (4 to 6 months out) staffing requirements/calculating call center required agents in a large call center with 80-600 FTEs based on anticipated call volumes during these months? What are the key metrics and variables that should be factored into calculations? Thank you. -- Natasha Culiuc, GEICO


  • Mohanad elmasry Posted at 10:50PM on Jun 13, 2010

    After calculating the required staff using erlang C based on half-hour intervals, how to you turn this into monthly figure? is it the sum of the required staff among all intervals divid by the shift lenght (number of hours)?

  • Posted at 12:00AM on Jul 1, 2004

    I’m going to make some assumptions about what you are trying to do here. It sounds like you are trying to forecast staffing needs at the interval, daily and monthly level and do not want to purchase a full blown workforce management solution. I’m also assuming you are comfortable with your volume and handle time forecast accuracy at these levels.

    You need to complete two steps to accurately forecast your staffing requirements. The first step is to calculate the number of agents you need answering the phones at the interval level. The second step is to apply shrinkage factors to your interval staffing requirements to take non-phone work, absenteeism, vacations and other factors into account.

    The Erlang C formula is a good choice for calculating staffing requirements by interval. There are several commercially available Erlang calculators that are either stand alone programs or MS Excel add-ins. An internet search for “Erlang C” will uncover a number of choices. A word of caution, most calculators only use calls per hour so make sure you find one that accepts calls by the half hour if your switch reports out at that interval. You will need the following metrics to calculate your staffing requirements using Erlang:

    • Call Volume Offered
    • Average Handle Time
    • Service Level Goal

    The calculator will return the number of staff required to answer the phones and the occupancy of the staff. There are two issues you need to be aware of when using Erlang. The calculation assumes that agents do nothing other than handle calls or wait for calls. Therefore it defines occupancy as the amount of time agents are handling calls divided by the amount of time they handle calls plus the amount of time they are waiting for calls. This is important because it means you have to account for any off phone time not included in AHT separately (We’ll tackle that in our second step). The second issue is that the formula looks at each interval in a vacuum, so it does not take into account calls that roll over from a previous interval. You will have to manually adjust volumes if you are anticipating a long wait time in the prior interval.

    You now know how many agents you need to have signed in and waiting for calls on an interval by interval basis. Our next step will be to add in all the other factors that take agents away from the phones. We will call them shrinkage factors. If you are not using workforce management software then you will need to create something in a spreadsheet. You follow three steps to create a shrinkage spreadsheet:

    Calculate your workload-Use total work time (calls answered times handle time) multiplied by occupancy (weighted average from your Erlang calculations) to get “on phone hours” for an entire month.

    Create a design factor working from the on phone hours up. First create a factor for all the different reasons agents are off the phones but in the building (off phone work, adherence, meetings, trainings, breaks etc.). Then create factors for all the reasons agents are not in the building (vacations, holidays, disability etc.) Keep in mind that the factors must be applied in order to make sure that a meeting factor is not applied to vacationing agents.
    Apply your design factor to your workload and then convert to FTEs.

    We created this at the monthly level to make sure we included all the factors. On a daily or interval basis you will have to adjust your design factor to ignore some factors. For example, even though your January monthly staffing forecast includes a holiday factor, when you forecast staff for January 12th, you will remove that factor because the 12th isn’t a holiday. You will also need to adjust for interval by interval and daily changes in factors like absenteeism by day or the fact that you open at 7 AM but have no team meetings or trainings scheduled until 9 AM.

    I hope this points you in the right direction,

    -- Dan Rickwalder, Forecasting Consultant, Incoming Calls Management Institute, Tel. (443) 451-4301, Fax: (443)-451-3364,

  • Posted at 12:00AM on Aug 1, 2004

    If you have historical data for the volume that came in, you may not need any calculators or Workforce Management software to build a forecast. The steps that you would need to follow are as follows:

    1. Use data for the past two years. Take, for example, 2002 and 2003.
    2. For 2003, calculate percentage of calls coming in January, February, March and so on. For example, if the total number of calls received in 2003 were 12000 and the number of calls received in January was 1000, then you can ascertain that 10% of your annual volume comes in in January.
    3. Now, for every month, divide the month into five weeks, with the 1st of the month being the first day of the week. Then, from the data you have for 2003, calculate the percentage of calls recieved in weeks 1, 2, 3, 4 and 5. Note that the 5th week will have only 2 - 3 days.
    4. Now, for the entire 2003, calculate the percentage of calls received on each day of the week.
    5. After the above is done, calculate the annual percentage difference (calls in 2002 - calls in 2003)/calls in 2002 * 100.
    6. From the above step you get to know exactly how many calls you would receive in the year 2004.
    7. Then divide the calls into monthly calls as per percentage calculated in step 2.
    8. Now, for every month, calculate the segregation of weeks from percentage calculated in step 3.
    9. Then for every week, calculate daily calls depending on the weekday percentage calculated in step 4.
    10. For hourly division of calls, you again need to take six months data and calculate percentage of calls for every hour.
    11. Apply the same percentage for calculating calls for every day for every hour.

    Your forecast is DONE!!! Using spreadsheets makes it extremely easy and you can get an accuracy of 95 - 98%.

    Now, to calculate staffing required for every hour... use the following formula:

    Staff Required: (number of calls * AHT / 60)

    The above formula would give you staffing with 100% utilization, meaning all the agents would always be talking or in after call work, so you can also take shrinkage and instead of 60, use a number like 40 or 50.

    This would also help you plan your breaks. -- Tarun Mathur, Tecnovate PLC.

  • Posted at 12:00AM on Sep 1, 2004

    Tarun, since you cannot staff to 100% utilization ever, I suggest you use Erlang C or its variants when calculation staff from the workload as calls bunch up. -- Ajay Gahlaut, Daksh

  • bhushibaby Posted at 12:00AM on Sep 6, 2007

    Hi, While calculating the staff we consider the following things for the accurate forecasting like, absences, breaks, shrinkage, and for processes with inbound and outbound, the inbound to outbound ratio. For example, consider a case for an interval of 30 minutes: Rostered 100 Present 90 Abs 10% Break 5 Outbound 5 So your total shrinkage will be 20% .... You need to calculate and study the trend. Once you get the trend you will get an idea like you have 30% shrinkage on Saturday and Sunday and 20% on weekdays. Similarly, towards the month end it will be less as agents might be out of pocket and more at the start of the month (mostly sitting in a pub). Once you get this idea, get the day-wise break up of the call volume and on that study the trend of percentages where your volumes are high. Staff accordingly using the following formula: Agents required = Forecast * (approximate AHT /30 time of interval)* Shrinkage. The most important thing that you need to consider here is the behaviour of the queue. If you understand that then there is no one in this world to beat you. Make use of Erlang calculator, too.

  • Ivo D'silva Posted at 12:00AM on Dec 14, 2007

    Could you please suggest a formula for staffing an outbound call centre? Specifically I have a task of setting up an outbound call centre to launch a new hotel loyalty programme. There are 8000 existing customers that need to be converted and 20,000 customers who have stayed in the hotel sometime or the other. Thanks -- Ivo D'silva

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