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Duration Equation in MS Project

13 March 2007


This is a Free Tutorial about Microsoft Project 2007 and you find more free tutorials here written by Johan Beijar. More tutorials about Microsoft Project 2007 are frequently added. 


Understand the equation above and with some basic mathemathics you will be in controll of your planning in MS project and you will have better possibilities to succeed and evolve as a project manager.


Microsoft Project uses three types of Task Types to calculate the duration of a task; Fixed Unit, Fixed Duration and Fixed Work. To be able to work efficiantly you need to understand how you should use these three Task Types. You are like a mechanic working on a racecar but without knowledge about the engine if you not understand the difference between the Task Types.

For all exampels below the week is 40 hours and fulltime is 8 hours per day.

Exampel of a Task with Fixed Units:

Units are the quantity of a resource assigned to a task.  

You have 1 fulltime resource (Unit=100%) allocated to the task that is estimated to 80 hours of Work. The Duration is then calculated to 80 hours (Work)/1(Units)=80 hours= 2 weeks of Duration.

Let’s say that you find another resource that could work fulltime with the activity, then you have 200% Units. 
The equation: 80 hours Work/2 Units=40 hours= 1 week of Duration.

You once again have 1 fulltime resource (Unit) but you now know that the Duration of the task will be 8 days instead of 10. The Work will then be 8 Hours/day * 8 days=64 hours Work

The equation: 8 days of Duration*1 Unit=64 hours Work  

A Task with Fixed Duration means that the Task must be completed with in the Duration that you specify   

Once again you have 1 fulltime resource (Unit=100%) allocated to the task that is estimated to 80 hours of Work. You have set the Duration to 2 weeks.

You then find another resource which allows you to have 2 resources allocated to this task. When allocating another recource MS Project recalculate the Units. Remeber that the Duration is fixed and the Work still is 80 hours. Each resource will then be allocated 50% to the task during the Duration of 2 weeks.

The equation: 80 hours of Work/(20 hours per week per resource*2 resources*2 weeks (Units))=Duration of 2 weeks (80 hours)

A task with fixed Work will take the amount of work that you specify.

Once again you have 1 fulltime resource (Unit=100%) allocated to the task that is estimated to 80 hours of Work. You have set the Duration to 2 weeks.

As in previous examples you find yet another resources that is able to be allocated to the task, you double the Units. This means, in this exampel, that MS Project will recalculate the Duration.

The equation: 80 hours (Work)/2 (Units)=1 week Duration 

Good Luck!

Johan Beijar

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    One Response to “Duration Equation in MS Project”

  1. Roopak Says:

    In example 1 (for fixed Units), you are changing the Units from 100% to 200%. To me, the example then becomes of ‘Fixed Work’ and not ‘Fixed Units’. Give examples where Units remains fixed.

    My Civil Engineering (Highway construction) projects are effort driven. Is is correct to say that to build a project (which is effort-driven), we must always first calculate the work for each task, assign resources and then let MSP calculate the durations?


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