Calendars and Total Float values are crucial in project scheduling in the context of defining the efficiency of the working process and the possibility of delays. The calendars define the working and non-working days of each activity, and the total float is used to determine the amount of flexibility available before an activity pushes the entire project.
Total Float and Calendars work together to ensure that project managers develop reasonable schedules, determine critical activities, and effectively manage resources. The knowledge of these two aspects and their interactions will enable the scheduler to anticipate bottlenecks, optimize workflows, and have control over deadlines, which will make them vital to effective project planning, scheduling and controlling.
A typical project schedule for renovation of a library is shown above with multiple task calendars and the total float/slack values. With the multiple activity calendars in a project schedule, it is usual to observe the difference in Total Float values of different activities in the same network path. This is easy to disorient a reviewer of a printed or PDF version of the schedule even when the relationship lines are present. This is also in case of such discrepancies in schedules, which contain resource-based activities having various working schedules.
How Calendars Affect Total Float?
After the creation and proper assignment of the activity and resource calendars, the schedule determines the duration of the activities using the valid workdays. Once relationships are built, the activity network paths are modelled by the scheduling tool by the duration and dependencies. It is through this process that the critical path, near-critical paths and all other work paths are created as of the current data date.
Why Total Float Values May Differ?
When multiple calendars are in use, the forward and backward pass calculations can yield different Total Float values for activities along the same driven path. This difference arises from the variations in workdays defined by each calendar.
This is one reason why many professionals avoid defining the Critical Path strictly as “Total Float = 0.”
Multiple calendars improve schedule accuracy and reflect realistic work patterns. They also make it harder to interpret float values when viewing a static Gantt chart in PDF format.
How Different Calendars Affect Float Values?
To understand this, we can assume a general example from a typical schedule which includes a project’s procurement and installation activities as follows:
The Material Submission follows a 5-day workweek calendar with holidays.
The review and approval process uses a 7-day workweek calendar, as the contract specifies 15 calendar days for submittal reviews.
The Material Fabrication and Delivery activity also follows a 7-day calendar.
The Equipment Installation activity is based on a 5-day workweek (possibly adjusted for holidays or weather delays).
Even though these activities are part of the same logical sequence, their Total Float values will differ. Activities on a 7-day calendar will generally show higher Total Float, as they include more working days within the same calendar duration.
We can take a look at the MS Project practical example with relevant calculations as follows:
Suppose a Network (logical sequence) of Activities A – B – D, A – C – D (B and C meet at D)
Activity A = 5 workdays (5-day calendar)
Activity B = 4 workdays (5-day calendar)
Activity C = 3 working days (two cases, 5-day calendar and 7-day calendar)
Activity D = 6 workdays (5-day calendar)
This example has been further explained through a simple MS Project schedule as shown above.
Now, let’s discuss scenarios of different calendars used in the same project.
Scenario 1: All activities based on the use of a 5-day calendar
In first scenario we are assuming all activities including Activity C has 5-day work calendar as follows:
In this snapshot from MS Project, a separate column for Task Calendar is added and we can notice that all activities have the same 5-day calendar. Now let’s have a look at their forward pass, backward pass and Float calculations:
Forward Pass:
ESA = 0 – EFA = ESA + 5 = 5
ESB = EFA = 5 – EFB = 5 + 4 = 9
ESC = EFA = 5 – EFC = 5 + 3 = 8
ESD = max(EFB, EFC) = max(9, 8) = 9 – EFD = 9 + 6 = 15
Backward Pass:
LFD = 15 – LSD = 15 – 6 = 9
LFB = LSD = 9 – LSB = 9 – 4 = 5
LFC = LSD = 9 – LSC = 9 – 3 = 6
LFA = min(LSB, LSC) = min(5, 6) = 5 – LSA = 5 – 5 = 0
Total Float (TF):
TFA = 5 – 5 = 0
TFB = 9 – 9 = 0
TFC = 9 – 8 = 1
TFD = 15 – 15 = 0
These calculations can be verified through the MS Project. As shown above a separate column for Total Slack is added and we can verify that the same Total Slack/Float values are being shown as of these calculations.
The critical path in the case of one 5-day calendar is A -B -D while Activity C has TF=1
Scenario 2: Activity C has a 7 day calendar
Now, as Activity C can take weekends, its EF will be earlier on elapsed calendar days of the same number of working days.
This image shows how we have changed the Task Calendar for the Task C from 5d/wk to 7d/wk.
In the next very simple approach to counting, EFC reduces to 7 rather than 8 and the float is changed significantly as follows:
MS Project is also verifying that the Total Slack in this case will be 3 days for the Task C as shown in above snapshot. So, it is obvious that changing Activity C’s calendar increased its Total Float from 1 to 3. So, Calendars change how durations map to calendar dates and therefore change float values.
Wrapping Up:
The basis of sound and reliable project scheduling is calendars and total float values. The knowledge of the impact of various calendars on float calculations helps project managers to read schedule logic properly, recognize genuine driving activities and anticipate possible delays. The built-in calendar design would make sure that every activity replicates the real work patterns in the world whereas the float analysis would make sure that one does not run out of time and focus on deadlines and critical paths. The projects become smoother when both of them are handled with accuracy, the risks are reduced, and resources are utilized effectively.
In Leopard Project Controls, our main focus is to solve the complex scheduling issues that relate to the use of more than one calendar, float differences, and critical path analysis. Our consultants have more than 20 years of experience in project controls, construction scheduling, and delay analysis and assist teams to verify schedule logic, conduct extensive float studies, and ensure overall schedule stability. No matter which software you are working with (Primavera P6 or Microsoft Project), our specialists will be able to make your project schedules more transparent and reliable.
