Total float and free float

Float is the room an activity has to move. Oracle Primavera P6 calculates two kinds every time you schedule: total float, measured against the project's finish, and free float, measured against the very next activities. They answer different questions. Total float tells an owner how close an activity is to delaying completion; free float tells the contractor how much an activity can slip before it disturbs the trade that follows. Read them together and a schedule starts to show where its real slack is, and where it has none.

Learning outcomes

• Define total float and free float as P6 calculates them, and explain why an activity can have one without the other.
• Calculate both by hand from early and late dates for a small network.
• Find the P6 settings that change float: Compute Total Float as, the project Must Finish By date and the float end date.
• Explain negative float, and use float values to target an owner's schedule review.

The lesson

Two definitions

Oracle defines total float as the time an activity can be delayed without delaying the project's finish date, and free float as the time it can be delayed without delaying the activities that immediately follow it [1]. P6 recalculates both every time you schedule, and you cannot type over them [1]. To see them, choose Project, Activities, select an activity, and open the Status tab of Activity Details, or add the Total Float and Free Float columns to the Activity Table [1].

Free float can never be larger than total float. If an activity can slip three days before it touches its successor, and the successor itself has no room, those three days are all the room the activity has. The reverse is common: an activity can have total float and zero free float, because its float is shared with the activities after it on the same chain.

How P6 calculates total float

The forward pass gives each activity an early start and early finish; the backward pass gives a late start and late finish (Lesson 10). Total float is the gap between them, counted in working time on the activity's calendar. The Compute Total Float as setting on the General tab of Schedule Options decides which gap [2]:

• Start Float = Late Start − Early Start.
• Finish Float = Late Finish − Early Finish.
• Smallest of Start Float and Finish Float, which Oracle calls the most critical value.

For a not-started activity on a single calendar the three give the same answer. They can differ when an activity is in progress or when calendars differ along a chain, which is why the owner records the setting with every baseline.

What sets the end of float

Total float needs an end date to measure against. If the project has a Must Finish By date on the Dates tab of Project Details, which Oracle describes as a date constraint on the project end, the backward pass starts from it [3]. If not, it starts from the latest early finish in the network [4]. A Must Finish By date later than the calculated finish gives every activity extra float; one earlier than the calculated finish gives the critical chain negative float. With several projects open, Calculate float based on end date of chooses between each project's own finish and the latest finish of all opened projects; the second option gives open-ended activities more float [2].

Negative float

Negative total float means the activity is already behind a date it has to meet. It comes from a constraint or a Must Finish By date that the logic cannot achieve. It is not an error; it is the schedule telling you, in working days, how far the plan is from the target. P6's Check Schedule report counts activities with negative float, and also flags large float above a threshold you set, because large float usually means missing logic [5].

Free float and driving logic

Free float links directly to the driving relationships in Lesson 9. P6 EPPM Help states that P6 treats a relationship as driving when free float is zero or less, the activity has no Start On or Finish On constraint, and it has no actual finish [6]. A predecessor with a day or two of free float is not driving today but is the first thing to check when its successor starts to slip.

Screenshot to come

Northgate worked example

Northgate Distribution Centre (fictional) is a 12,000 m² tilt-up warehouse in Vaughan, Ontario, with 14 dock doors and 2 drive-in doors, built by Halvorsen Builders Ltd. (fictional) for Northgate Logistics Inc. (fictional) (see the Northgate fact sheet). The extract below is from WBS 3, foundations and slab-on-grade, in Halvorsen's baseline NDC-BL00. Dates are Halvorsen's illustrative baseline dates on the Monday-to-Friday, 8-hour calendar, with Good Friday (Fri Mar 26, 2027) as a holiday. In this extract the footings and the slab are on the longest path, so their total float is zero.

• A3100 Footings and piers, 19d, Mon Mar 15 to Fri Apr 9.
• A3200 Underslab services, 10d, SS 7d from A3100: Wed Mar 24 to Wed Apr 7. FS to the slab.
• A3300 Dock and leveller pits, 8d, SS 5d from A3100: Mon Mar 22 to Thu Apr 1. FS to A3310.
• A3310 Leveller embeds and curb angles, 3d: Fri Apr 2 to Tue Apr 6. FS to the slab.
• A3500 Slab-on-grade, 25d, from Mon Apr 12, driven by the footings.

View the data

Activity	Early start	Early finish	Late start	Late finish	TF	FF
A3100 Footings & piers	Mar 15	Apr 9	Mar 15	Apr 9	0d	0d
A3200 Underslab services	Mar 24	Apr 7	Mar 29	Apr 9	2d	2d
A3300 Dock & leveller pits	Mar 22	Apr 1	Mar 25	Apr 6	3d	0d
A3310 Leveller embeds	Apr 2	Apr 6	Apr 7	Apr 9	3d	3d
A3500 Slab-on-grade	Apr 12	May 14	Apr 12	May 14	0d	0d

Working it out by hand

The slab starts Mon Apr 12, so anything that must finish before it has a late finish of Fri Apr 9. Work backwards (Figure 2):

• A3310 Embeds: late finish Fri Apr 9; three days back gives a late start of Wed Apr 7. Early start was Fri Apr 2. Total float: Apr 2 to Apr 7 is 3 working days. Its successor, the slab, cannot start before Apr 12 anyway, so free float is also 3 days.
• A3300 Pits: late finish is the day before the embeds' late start, Tue Apr 6; eight days back gives a late start of Thu Mar 25. Total float is again 3 days. But the embeds start the day after the pits finish, so free float is 0.
• A3200 Underslab: late finish Fri Apr 9, late start Mon Mar 29 (Good Friday skipped). Early start was Wed Mar 24, so total float and free float are both 2 days.

The lesson in the pits is the important one. A superintendent looking only at total float sees three days on the pits and three on the embeds, and may think there are six. There are three, shared between the two activities. Spend them on the pits and the embeds have none left.

A what-if: when the pits run long

Suppose, as an illustration only, the pits take 12 days instead of 8, finishing Wed Apr 7. The embeds move to Thu Apr 8 to Mon Apr 12, and the slab cannot start until Tue Apr 13, finishing Mon May 17, one working day after M4 (Fri May 14). The pits used their three days of float, and the fourth day went straight to the slab.

What P6 shows next depends on the setup. With no constraint on M4, the slab simply moves and total float on the longest path stays at zero, because float is measured against the new latest finish. If Halvorsen's file carries the M4 contract date as a finish constraint on the milestone, the pits, embeds and slab all show −1d of total float (Figure 3). That negative number is useful: it tells the owner, at a glance, how much recovery the contractor needs to show.

View the data

Activity	TF before	TF after +4d on pits
A3300 Dock & leveller pits	3d	−1d
A3310 Leveller embeds	3d	−1d
A3500 Slab-on-grade	0d	−1d

Screenshot to come

Screenshot to come

Owner's move

• Record the float settings. Compute Total Float as, the Must Finish By date and the float end date setting go in the owner's baseline record. A change at update time can move float without any change on site.
• Read free float before agreeing a re-sequence. When a contractor proposes to "use float" on an activity, check whether that float is its own (free) or borrowed from the activities after it.
• Treat large float as a logic question. An activity with months of float is usually missing a successor. Check Schedule's Large Float check finds them [5].
• Track float on the near-critical paths, not just the critical one. A path that drops from 9 days to 2 over two updates is the next critical path (Lesson 11).
• Decide who owns float in the contract, not in P6. P6 only calculates float. Whether it belongs to the owner, the contractor or the project is set by the contract and its Supplementary Conditions, and should be settled before the first delay discussion.

Common mistakes

1. Adding up total float along a chain. Float on consecutive activities is usually shared. Three days on the pits and three on the embeds is three days, not six.
2. Trusting float in a file with open ends. An activity with no successor takes its float from the project end and looks safer than it is.
3. Ignoring the Must Finish By date. A Must Finish By date set later than the contract date hands every activity float that does not exist under the contract.
4. Treating negative float as a software error. It is a measurement of how far the plan is from a date it must meet.
5. Comparing float across calendars as if it were the same unit. A day of float on a seven-day calendar is not a day on a five-day calendar.

Quick quiz

1. Free float is the time an activity can slip without delaying:

   1. The project finish date
   2. The activities that immediately follow it
   3. The data date
   4. The baseline

2. With Compute Total Float as set to Finish Float, total float equals:

   1. Late Start − Early Start
   2. Late Finish − Early Finish
   3. Early Finish − Early Start
   4. Late Finish − Late Start

3. Northgate's pits show TF 3d and FF 0d; the embeds that follow show TF 3d and FF 3d. How much can the pits slip before the slab moves?

   1. 0 days
   2. 3 days
   3. 6 days
   4. Cannot tell

4. A project's Must Finish By date is earlier than its calculated finish. The critical chain will show:

   1. Zero float
   2. Positive float
   3. Negative float
   4. No float values

5. Which statement is true in P6?

   1. You can type a total float value into an activity
   2. Free float can exceed total float
   3. Float is recalculated every time you schedule
   4. Float is counted in calendar days

Show answers and explanations

1. B. Oracle defines free float against the activities that immediately follow [1].
2. B. Finish Float is Late Finish minus Early Finish [2].
3. B. The three days are shared with the embeds; after three days the slab moves.
4. C. The backward pass starts from a date the logic cannot meet, so the chain shows negative float [3].
5. C. Float is calculated automatically each time you schedule and cannot be edited directly [1].

Key terms

Total float

The time an activity can be delayed without delaying the project finish date.

Free float

The time an activity can be delayed without delaying the activities that immediately follow it.

Start float / finish float

Late start minus early start, and late finish minus early finish; P6 lets you choose either, or the smaller, as total float.

Negative float

Total float below zero, showing that a constraint or Must Finish By date cannot be met by the current logic.

Must Finish By

A date constraint on the project end, set on the Dates tab of Project Details, that the backward pass starts from.

Shared float

Total float that belongs to a chain of activities, so that using it on one reduces it on the others.

References

1. Oracle. View activity float values. Primavera P6 Professional Help, Version 26.
2. Oracle. General tab - Schedule Options dialog box. Primavera P6 Professional Help, Version 26.
3. Oracle. Dates tab - Project Details. Primavera P6 Professional Help, Version 26.
4. Oracle. Scheduling projects. Primavera P6 Professional User Guide, Version 26.
5. Oracle. Dates and Durations tab - Check Schedule dialog box. Primavera P6 Professional Help, Version 26.
6. Oracle. Working with the Activity Network. Primavera P6 EPPM Help, Version 26.

Verified on November 6, 2026.

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