Total float vs free float
Find the activities with negative, zero and near-critical float in a Primavera P6 .xer or MS Project .xml schedule. The file is read in your browser and never uploaded.
Primavera P6 .xer, or MS Project saved as .xml (File > Save As > XML). It is read on your computer and never uploaded.
Total float
Total float is how long an activity can slip before it delays the finish of the project (or a constrained date it leads to). It is the difference between the activity's late and early dates: late finish minus early finish, the formula DCMA's 14-point assessment uses. An activity is critical when its total float is at or below a limit, zero by default in both P6 and MS Project: any slip then moves the finish date (or the constrained date). P6 can define critical by the longest path instead. In P6 the schedule options choose whether total float is computed from the start dates, the finish dates or the smaller of the two.
Free float
Free float is how long an activity can slip before it delays the early start of any activity after it. It is worked out from early dates only, so it never drops below zero, and it is never more than total float, except when total float is negative. An activity can have plenty of total float and no free float: its slip uses up float that the activities after it share.
Total float vs free float, by example
Walls take 10 days, then Roof takes 4 days. Fence takes 3 days and starts on day 1 too. Handover needs all three and is a milestone fixed at the end of day 20 (a constraint).
- Walls and Roof finish on day 14, so both have 6 days of total float: either can slip 6 days before handover moves.
- Roof holds those 6 days as free float: nothing else waits for it. Walls has no free float: if Walls slips one day, Roof starts one day later.
- Fence finishes on day 3: 17 days of total float and of free float.
What negative float means
Negative float means the schedule cannot meet a date it has been told to meet: a constraint, a deadline or a required finish is earlier than the logic allows. The number is how many working days have to be recovered. It does not mean the activity itself is late today; it means the dates after it cannot all be reached as planned. DCMA's assessment expects none and asks for an explanation and a recovery plan for each.
Float in MS Project
MS Project calls float "slack": Total Slack and Free Slack mean the same as total and free float. A task is critical when its total slack is at or below the "tasks are critical if slack is less than or equal to" setting, 0 days by default (Microsoft: CriticalSlack).
How the finder reads your file
- Float comes from the file as the scheduling software last calculated it; the page does not recalculate the schedule.
- It is shown in working days: of each activity's own calendar in a P6 file, of the project's hours per day in an MS Project file (as MS Project shows slack). Completed activities are left out.
- Near-critical here means more than 0 and up to 10 working days of total float; your project may use another limit.
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