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MS Project: Understanding How Microsoft Project Thinks
Posted on Thursday, August 21 @ 06:47:34 EDT by webadmin

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There are a number of factors that dictate how Microsoft Project “thinks”.

The two main factors are:

  • The Scheduling Formula
  • The Task Type
The Scheduling Formula leverages three variables:

  • Units - The percentage of time required by a resource or resources to complete the task
  • Duration - The number of working days required to complete the task
  • Work - The effort required to complete the task
Many people say that the scheduling formula is: Units x Duration = Work

When you initially assign a Resource to a Task, this is the formula that is leveraged. Let’s demonstrate:

We will create Task 1 that is 5 days in Duration


We will now assign a Resource to this task



As soon as we assign this Resource to this task, Work changes to 40 hours.

Here is the how that number was calculated:

Units x Duration = Work
100% x (5*8) = 40
100% x 40 = 40 hours

But using simple math, we can re-write our equation to solve for a different variable.

Our original formula solves for Work:
Units x Duration = Work

But we could re-write the formula to solve for Units:
Work / Duration = Units

Or we could re-write the formula to solve for Duration:
Work / Units = Duration

So we have shown that The Scheduling Formula can actually be written three different ways:

Units x Duration = Work
Work / Duration = Units
Work / Units = Duration

Now let’s prove that Microsoft Project also thinks this way.

In order to demonstrate this, we must introduce The Task Type or the Type field:


By default, Type is set to Fixed Units. But there are actually three different Task Type variables:

  • Fixed Duration
  • Fixed Units
  • Fixed Work

Now to test Microsoft Project

Using our original scenario, let’s force Microsoft Project to solve for Duration.


We will leave Type set to Fixed Units and change Work to 80 hrs


Here is how Duration was calculated:

Work / Units = Duration
80 / 100% = 80
80 / 100% = (80 / 8)
80 / 100% = 10 days


Returning to our original scenario, let’s force Microsoft Project to solve for Work.


Leave Type set to Fixed Units and enter 10 days Duration


Here is how Work was calculated:

Units x Duration = Work
100% * 10 days = 80
100% * 10 days = (10 * 8)
100% * 10 days = 80 hours Work


Returning to our original scenario, let’s force Microsoft Project to solve for Units


Change Type to Fixed Duration and enter 20 hours Work


Here is how Units was calculated:

Work / Duration = Units
20 / 5 days = 50%
20 / 5 days = 20 / (8 * 5)
20 / 5 days = 20 / 40
20 / 5 days = 0.5


To Review:

There are three different Task Types:

  • Fixed Units
  • Fixed Work
  • Fixed Duration
There are three different variables of the Scheduling Formula:

  • Units
  • Work
  • Duration
Depending upon how the Task Type is set in combination with which Scheduling Formula variable is altered dictates which variable Microsoft Project solves for.

The Task Type and Scheduling Formula Cheat Sheet will help you control how Microsoft Project “thinks”:


See The Microsoft Project Task Type and Scheduling Formula Cheat Sheet here.

See this article in Slide format here.

See this article in Video format here.

Purchase “The Cheat Sheet” Mouse Pad here.

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