I have never seen such strong reactions to a software as I have seen for Microsoft Project.

“I changed duration, why did it change the effort?”

“I changed effort, why did it change the dates?”

“I added a resource, why does it not reduce the task duration?”

“Why does it change anything? Why can’t it just behave like Excel? Why all this complexity just for generating Gantt charts?”

I am sure we get these reactions for our own project management software too. All these questions and comments stem from a misunderstanding of the relationship between Duration, Work and Units. In this article, I will try to explain this and much more.

A couple of things before we begin:

  • Work is the same as Effort and is used interchangeably in project management.
  • For this article, we assume that Joe works from 8 AM to 5 PM (with one hour break) every day Monday through Friday i.e 40 hours per week.

Let’s begin.

The relationship between Duration (D), Work (W), and Units (U)

To start with, let’s understand why there is a relationship between the duration (D), the work (W), and its allocation units (U).

If you are still unsure about the difference between duration and work, I suggest you read this duration vs effort blog article first.

Let’s say that Joe is supposed to work full time on a task that starts on Monday, 8 AM and ends Friday at 5 PM.

Duration (D) is 40 hours.

Allocation units (U) is 100% because Joe works full-time on this task.

Now think about it – if Joe works full-time for 40 hours on a task, the effort Joe will incur will be 40 hours i.e. W will have to be 40 hours. It can’t be 20 or 80 or anything else.

Similarly, if Joe works half-time i.e. 50% on the task for 40 hours (D),  W will have to be 20 hours. It can’t be 40 or 10 or 80 or anything else.

The formula is D x U = W

In plain English, it says that if a resource works for D hours but works only U% of that time on a task, then the total work (effort) for that task will be W hours.

It makes sense and is intuitive too.

It also means that if any of D, W or U changes at least one other should change to maintain the equality.

Once this is clear, the rest will be easy. So if you have not grasped it yet, re-read the above section.

Let us demonstrate why Celoxis is the ideal PPM Solution for you.

Effects of changing D, W or U

Now let’s say you create a task for Joe where he works full time for the next week. For this task, U = 100%, D = 40 hours and W = 40 hours.

Joe clarifies and notifies you that the task will take only 20 hours of work to complete, not 40.

If you just change W to 20 without changing anything else, the task will have inconsistent data. U = 100%, D = 40 hours and W = 20 hours. It doesn’t make sense.

You have at least two choices:

  • reduce the duration (D) to 20 hours
  • reduce the allocation (U) to 50%

Both will maintain the consistency and are correct.

If you reduce the duration to 20 hours, the task will finish early i.e. by noon on Wednesday i.e. it will take 2.5 days.

If you reduce the units instead to 50% i.e. make Joe work half-time on this task, the task will continue to finish on Friday at 5 PM. But now Joe is free 50% during that period to work on some other tasks.

When you use a project management tool like Microsoft Project or Celoxis, it will try to maintain the consistency for you based on the formula.  To do this, it will change one of the other two. And that’s where it gets confusing.

Let’s try to understand that now.

When  D, U or W changes, the software could ask you what it should do. So in our earlier example, if you reduce W to 20, it would ask you whether it should reduce D or reduce U.

But that could get irritating and lead of a lot of tired project managers. There is an easier way to solve this problem.

Fixed Work, Fixed Duration, and Fixed Units

Most PM tools allow you to designate a task to one of these three types. As the name suggests, it tells the software what not to change.

Continuing with the above example, if our task was a “Fixed Duration” task and we reduce the effort to 20, the software would reduce U automatically without asking you. i.e. it would change allocation from 100% to 50% for Joe.

If our task was a “Fixed Units” task instead, the software would reduce the duration and automatically recalculate the task’s finish date to Wednesday 12 noon.

If our task was a “Fixed Work” task, then the software would not know which of D and U to change and hence popup a dialog (or something equivalent) to ask you what to change.

So, if you find your PM tool is “misbehaving”, look at this attribute of your task. In Microsoft Project, it is called the “Task Type”. In Celoxis, we call it “Schedule Type”.

When to use what?

It depends but in most cases, when the task is planned, its effort is determined as something that is least likely to change. Hence you would use “Fixed Work” as your default type.


In the article, we have tried to understand the natural relationship between D, W, and U and how changing one affects the other. We have also understood the necessity of having the “Fixed Work”, “Fixed Duration” and “Fixed Units” types of task.


Awesome. thanks for this.

mike hall

Nice work! Please write another article with some practical examples.

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