Total Float

Once the early and late dates have been derived, the schedule can be assessed for flexibility. The difference between the time an activity may start or finish and the time it must start or finish in order for the project to be completed on time is known as total float (TF). Total float is calculated as the difference between an activity’s early and late dates:

 TF = LS - ES

or

 TF = LF - EF

Total float is noted in figure 57 as “TF.” Activities that have no total float are highlighted in red.

Figure 57: Total Float in a Start-Up and Testing Network
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All activities but one in figure 57 have 0 total float. A total float value of 0 indicates that an activity has no flexibility between the date when it may start and the date when it must start or between the dates when it may finish and must finish. Any delay in its start or finish dates transfers directly to the end milestone.

Three paths through the set-up and testing network have no total float and thus no flexibility:

  1. “Inspections complete” → “set electricity meter” → “start up and test electrical system” → “start up and test complete”;

  2. “Inspections complete” → “set gas meter” → “start up and test plumbing fixtures” → “start up and test complete”;

  3. “Inspections complete” → “set water meter” → “start up and test plumbing fixtures” → “start up and test complete.”

Any delay along the activities on these paths is transferred directly, day for day, to the finish date of the “start up and test complete” milestone, unless the delay is mitigated.

Two paths through the set-up and testing network have total float available and thus have flexibility:

  1. “Inspections complete” → “set electricity meter” → “start up and test furnace and AC” → “start up and test complete”;

  2. “Inspections complete” → “set gas meter” → “start up and test furnace and AC” → “start up and test complete.”

Practically speaking, only one activity in the network can slip a day. None of the meter-setting activities can slip because they are all on the critical path: a slip in any one of the three will cause either the electrical system or the plumbing fixtures test to slip a day, causing the finish milestone to slip a day. Only the furnace and AC test activity can safely slip 1 day without pushing the completion milestone out. To introduce more flexibility into the schedule, the general contractor may want to assign an extra electrician to the electrical system testing. An extra resource can reduce the duration by 1 day and therefore introduce an extra day of total float for that activity. Because total float is shared along a path, the test’s predecessor, “set electricity meter,” would also gain a day of total float.