Determine Total Float & Free Float (AKA "Slack") of activities in a network diagram



so here's one more quick example on how to calculate free float and total float also known as free slack and total slack the words float in slack are interchangeable they mean the same thing but total float is different from free float so in this video one more example on how to calculate the total float and free float for the various activities in a project on a PDM Network diagram so by now you've been watching these tutorials you already know how to draw the PDM Network diagram for this so let's go ahead and do that it looks just like this then we can do the forward pass to find the early start and early finish of each activity then we can do the backwards pass to find a late start and late finish of each activity now we can calculate the total float of each activity we'll write that above each node just like this and I'm going to drop the TF for each node and I'll just write the value but that's where it'll be located if you recall from the previous videos we can find the total float by subtracting early finish from the late finish or early start from the late start this way would be called the start float and doing it with these numbers will be called the finish float but it's actually the same number regardless of which one you do for example 0 minus 0 is 0 3 minus 3 is 0 so I'll write the total float of activity a just right above it next up we'd have 3 minus 3 is 0 or you could do 7 minus 7 to get 0 total float of activity B is 0 a total float of activity you can see is also 0 total float of activity G is 0 and total float of activity H is 0 you're going to see in a minute that these will form a critical path but let's look at these other three nodes activities see the total float of activity C would be 9 minus 3 that's 6 or conversely get it from here 11 minus 5 is 6 so C is total float 6qt e 12 minus 6 is 6 are all so you can see 11 minus 5 is 6 so it's total float is 6 activity F we have 16 minus 7 so that's a gives us a total float of 9 you can also see that here from the 14 minus 5 that would also give you a 9 and then just by inspection you can look and see with each node that has a total float of 0 we'll more critical paths so let's Atlanta here in red now the last thing to do is find the free float of each activity and we'll write the free float beneath each node just like we've been doing in the previous videos and if you recall the free float is the minimum early start of all of the successes of a given activity minus the early start of the given activity – the given activities duration so for example for activity a the minimum early start of all of its successors we have two options we have three or we have three so we have to pick three so three minus zero minus three it's free float is going to be zero another thing to notice is the free float is always equal to or less than the total float it can never be more than a total float so right away if you're on the critical path you know that the free float will definitely be zero for all of these but I'll just show you a few examples just to prove that for example for B it's free float we would take the earliest start of its successor so would be 7 minus its own early start – its own duration so 7 minus 3 minus 4 that gives us 0 same thing goes for activity D we would have 12 minus 7 minus 5 gives us 0 and the free float of activity G we would have 16 minus 12 minus 4 gives us 0 if you're curious about where that came from now we talked about that whole formula just in a previous video and let's not forget activity H it doesn't have any successors but its total float is 0 so it's free float can't be greater than 0 so it's free float is 0 so let's look at these guys now that aren't on the critical path so activity C it has 2 successors so we have to choose the smallest early start of its successors and it just turns out again that they both have the same early start of 5 so then we must subtract we have 5 minus 3 minus 2 so we find that seize free float is 0 now for activity E will take 12 minus 5 minus 1 so it has a free float of 6 and then for activity F we can go sixteen minus five minus two and we'll get a free float of nine and if you recall so the difference between total float and free float so total float was the amount of time any given activity can be delayed without affecting the end date of the project for example anything on the critical path have you delighted by anything even if you delayed one of these activities just by one day you would extend the entire duration of the project by one day let's look at the activity D for example if we actually delay D so it finished on the on 13 instead of 12 well then we have to bring the 13 here then we'd add four and we'd actually end on 17 with G and then activity age would have to start on 17 and then would end on 20 so we would actually increase the duration of the whole project by one if we increased any of these activities or delayed any of these activities by one day for one of these guys that's not on the critical path for example activity e we have six days that we can delay this by before it would affect the and date of the project imagine if you delete it for less than six days or even up to six days you would if you delayed it by six days instead of having the earliest start of five you would start at 11 and then it would be one day you would end on twelve you'd bring this 12 up here you wouldn't affect the start date of G and then therefore G would still end at 16 and you wouldn't affect the end date of H the free float in this case for node E or activity E is also six because if you recall the free float is the amount of time you can delay an activity by without affecting the earliest start of any of its successors so imagine if you delayed ebuy anything like six days or less as long as you're not finishing e after this twelve here imagine if you finished on eleven or even twelve you bring the twelve up and you're not going to affect the earliest start that G can start on however if you delayed ebuy seven days then you would be affecting the earliest start of G so that's why the free float is only six here so the activity F it has a total float and a free float of nine so same kind of reasoning here but activity C has a total float of six meaning that you could delay activity see by up to six days without increasing the duration of the project but if you delayed see by any amount of time even if it was one day you would affect the earliest start of at least one of its successors you can see here if you maybe started see on day four then you would finish on day six and then you would be effecting these early starts because they wouldn't be able to start on day five they'd have to start on day six again so that's why activity C has a free float of zero

Author Since: Mar 11, 2019

  1. I couldn’t understand this topic in school. But after watched your videos I understood perfectly. Thanks a lot..:)

  2. can you make a sample video on how to make an equipment and manpower utilization schedule? thank you

  3. I was having trouble with my assignment, but after watching the videos I understand it so much better. Thank you so much. YOU ARE AWESOME!

  4. CAN the total float number in one box give different numbers??? or dose that mean a mistake was made in calculating?

  5. Thank you for your informational video, it helped me in my test as i had missed the class of network diagram in my university.

  6. Thank you very much for your videos. I went through all the videos that I could find about this topic and presented by you. I have a firm understanding on this topic now. Again, thanks.

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