Scheduling & Three Types of Tasks : MS Project

If you are new Project Manager or new to using MS Project,  when putting workplan together one of the question you may ask is – what type of task should I choose?

MS Project has 3 options –

  1. Fixed Duration;
  2. Fixed Units;
  3. Fixed Work. 

The choice will decide how MS Project schedules the work, in other words, which one of three remains fixed and other two change.  To make things easier to remember, Duration is in days, Work is in hours and Units are resources.  And formula to keep in mind is ‘wud’ i.e.   w(ork) = u(nit) * d(uration).

Summary table – Left green column is task type in MS Project; top red column is what we change; blue colums are rescheduled by MS project.


Change Duration

Change Unit

Change Work

Fixed Duration


Reschedules Work

Reschedules Units

Fixed Units

Reschedules Work


Reschedules Duration

Fixed Work

Reschedules Units

Reschedules Duration


Basically, 1day duration has 8 hrs of work with 1 unit or resource working 100%.

Fixed Duration (d) = Fixed duration is chosen when we know task is resource independent like cement-concrete mix requires 21 days for curing; a laboratory test requires 24 hrs for culture to grow; paint requires 4 hrs to dry, etc.

In this case, duration of the task (in days) is fixed.  If you put more resources (Units) to work on the same task; still duration will not change.  

For a fixed duration task of 1 day and 8 hrs of work with 1 resource 100% dedicated; if resource is now allocated to work 50% ; work hrs will change to 4 hrs. 

Instead of reducing resources (or units), if you allocate 2 resources working 100% (equal to 200% for the same duration of 1 day); work hrs will go to 16 hrs.

The formula is: d = w / u  (Note – MS Project will calculate ‘w’ when ‘u’ is changed to keep ‘d’ constant.) In our case d=1day = 8 hrs

if one unit is working; d = 8/1 = 8
if unit is reduced to half; d = 4/0.5 = 8
if 2 units are working 100%; d = 16/2 = 8

 Fixed Unit (u):This is default task that MS Project assigns.  When task duration (d) is changed; work (w) is calculated as resource unit is fixed.  Formula assumes that no overtime is allowed.  If work is increasing; duration also increases.

Unit (u) = work(w) / duration (d)

if work is 8 hrs; duration will be 1day (equal to 8 hr in a day); u = 8 /8  = 1
if work is 12 hrs; duration will increase to 1.5; u = 12 / (8 * 1.5) = 1

Fixed Work (w):As name implies, it is used when you have firm estimate that particular task takes given number of hours and if you put more resources; task can be completed sooner.  Such as painting a room takes 8 hours with one painter; if we have two painters working with 100%; task will be completed in half a day.  If both painter are working 50% on this project and spending 50% on some other task; it will take 1 whole day.

work(w) = unit (u) * duration (d)

if fixed work is 40 hrs and 1 resource 100%; w  = 1 * (5*8) = 40
if work is still 40 hrs and duration is 2 days with one resource; w = 2.5  * (2*8) = 40
This means; we need 2.5 resources working 100% to complete task in 2 days, else resource is working 250% (impossible amount of overtime)
Note – It requires that you have entered your resources, working hours and calendar information into MS project.

I hope it helps those who are new to MS Project.  Let me know if there are any corrections/additions to this will make it more useful.  Thanks for reading.


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