Float In Project Management – How To Calculate Float Of An Activity?

Float in project management express the flexibility that the project manager has in order to deal with delays in activities, and still be able to complete the project on time.

In order to understand the float in project management, you need fist understand the following concepts.

  1. What type of dependencies exist the project management?
  2. What is project schedule network diagram and how to create it?
  3. The critical path method (CPM) – How to use critical path method to find out the critical path in the project?

In case you have not gone through these concepts, please read through the below blog posts, before you start understanding about the float in project management.

Float In Project Management

Float In Project Management

Slack is another name for float in project management terms.

Float in project management terms tells you how much luxury (extra time) you have to complete an activity, so that you stay on project schedule always.

Once you create the project schedule network diagram, and figuring out the critical path in the project, it is time to find out the float using the critical path.

The float of an activity is the amount of time you can slip without delaying the overall project timelines or schedule.

How To Calculate Float In Project Management

Fortunately calculating float for an activity in a network diagram is not so difficult.

To keep this easy to understand let us take a simple example to understand how to calculate the float in project management.

For example let us use the following project activities information to find the float.

Activity Duration (in hours) Predecessor
A 4 Start
B 3 A
C 7 B
D 4 Start
E 1 D
F 2 D
G 3 F

Step 1 –Create the project schedule network diagram

Draw the network diagram as we learned this in the blog post on how to create network diagram using arrow on node (AON) or precedence diagraming method (PDM).

Step 2 – Find out the critical path.

Identify all the paths along with the length of each path in the project schedule network diagram. Based on the total duration of each path you figure out the longest path among all the paths in the project.

As we know by now that the longest path in the project is critical path of the project.

Observe that the float for each activity on the critical path is zero. This is not surprise as it is the longest path in the project. And by definition, critical path does not have any flexibility of delays. If any activity or the critical path delays, that delays the whole project.

Path 1 : START => A => B => C => FINISH

Path 2 : START => D => E => FINISH

Path 3 : START => D => F => G => FINISH

Now find out the length or duration of each path.

To start with let us take Path 1 duration = 4+3+7 = 14.

Then Path 2 duration = 4+1 = 5.

Then Path 3 duration = 4+2+3 = 9.

So in the mentioned paths above, we can see that path1 is the longest path in the project compared to the other paths path 2 and path 3. Hence path 1 is the critical path in the project.

Step 3 – Find the next longest path and find the float

After you find the critical path (step1) in the project, you need find out the next longest path in the project.

The next longest path in the project is path 3.

Now subtract the duration of path3 from the critical path and that is going to be the float of an activity on path 3.

Float of an activity on path3 = 14 – 9 = 5

So for the activities that are there on path3, the float is 5.

Meaning that any activities in path3 can slip its schedule by 5 units. In this specific example we have taken hours as units. Hence 5 hours.

Step 4 – Continue Step 2 until traversing through all the paths in the project.

Continue the step to find out the float for each and every path in the project.

For this example, we are left with only one path, which is path 2.

Now find out the float for path 2 activities using the same above mentioned method.

Float of activities on path2 = 14 – 5 = 9

But the tricky part here is for the activity D we have already calculated the float (float=5) in the path3. And hence we keep it as it is saying that the activity D can maximum slip 5 hours, without delaying the whole project timelines. This is because activity D is predecessor for both activities E and F.

For the rest of the activities in path 2, the float is 9 hours.

Conclusion

To conclude with, in this post we have seen what is a float in project management. Then we have gone through a detailed step by step process of calculating the float.

Calculating the float in the project will help the project manager to understand how much time the activities can slip, without introducing delays to the overall project.

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