Total Float vs Free Float: What Each One Actually Means

Definitions. Total float is how long a task can slip without delaying the project finish date: Late Start minus Early Start (LS − ES). Free float is how long a task can slip without delaying the earliest start of any task that follows it: the successor's Early Start minus the task's Early Finish, less any lag on the link (ES of successor − EF of task − lag), taking the smallest value when there are several successors.

Total float vs free float comes down to what each number measures slack against. Total float is how many days a task can slip without delaying the project. Free float is how many days a task can slip without delaying the very next task. They come from the same schedule, use overlapping inputs, and are frequently confused because most PMs learn "float" as one number instead of two.

The direct answer: total float = Late Start minus Early Start (LS − ES), and it measures slack against the project end date. Free float = the successor's earliest start minus the task's own earliest finish (ES of successor − EF of task), and it measures slack against the very next task only. A task can carry real total float and still have zero free float, because the project end date is not the only thing a delay can threaten; the next task's start date is threatened first.

What Total Float Actually Answers

Total float answers "can this task slip without pushing out the project finish date?" It is the number every critical path calculation produces as a side effect: run a forward pass and a backward pass across the network, and total float falls out as LS minus ES for every task. A task with zero total float is, by definition, on the critical path. A task with 5 days of total float can run 5 days late and the project still finishes on time.

Total float is a project-level number. It does not care what happens to the tasks in between; it only cares whether the finish line moves.

What Free Float Actually Answers

Free float answers a narrower question: "can this task slip without delaying the task right after it?" The formula is the earliest start of the successor minus the earliest finish of the task itself. If a task has more than one successor, free float is the smallest of those values, because the tightest successor is the one that actually constrains the task.

Free float is local. It ignores the project end date entirely and only looks one step ahead. That makes it the earlier warning: a task can lose all of its free float and still not be on the critical path, but the moment it does, its successor's own schedule starts absorbing the delay instead.

The Free Float Formula, Step by Step

The free float formula is Free Float = ES of successor − EF of task − lag, worked out for every link that leaves the task, with the smallest result kept as the task's free float. For a plain finish-to-start link with no lag, it reduces to the successor's earliest start minus the task's earliest finish. It needs only a forward pass through the schedule, which is why free float is quicker to work out by hand than total float, which also needs the backward pass.

1. Run the forward pass. For every task, Early Start (ES) is the latest point its predecessors allow, lag included, and Early Finish (EF) = ES + duration.
2. Take each outgoing link in turn. For a finish-to-start link, subtract the task's EF and the link's lag from the successor's ES.
3. Keep the smallest result. A task with several successors has only as much free float as its tightest successor allows.
4. If the task has no successor, measure against the project finish instead: the project's EF minus the task's EF.

Worked example. Task A starts on day 0 and takes 4 days, so its EF is day 4. It has two successors. Task B is linked finish-to-start (FS) with no lag, and its earliest start is day 6 because a different predecessor holds it until then. Task C is linked finish-to-start with a 1-day lag, so its earliest start is day 5.

Task A's free float is the smaller result: 0 days. B could absorb a 2-day slip in A, but C cannot absorb any, so C is the successor that decides. A can still carry total float at the same time; that depends on how much slack C's own path has before the project finish.

The same rule covers the other dependency types with a different pair of dates: a start-to-start link compares the successor's ES with the task's ES, a finish-to-finish link compares the two EFs, and a start-to-finish link compares the successor's EF with the task's ES, each less the lag.

The Worked Example: Where the Two Numbers Split

Take two non-critical tasks running in series, X then Y, that rejoin a critical task afterward. X starts on day 5 and takes 3 days (finishing day 8). Y starts right after X finishes, on day 8, and also takes 3 days (finishing day 11). The task they both feed into cannot start until day 14, which is where the 3 days of slack in this little chain actually lives.

Run the backward pass and both X and Y come back with 3 days of total float: neither one threatens the downstream task's day-14 start if the whole chain slips by up to 3 days. Free float is where they split. X's free float is the earliest start of its own successor, Y, minus X's earliest finish: day 8 minus day 8, which is zero. X cannot slip at all without immediately pushing Y's start later, even though the project itself would not notice for another 3 days. Y, sitting right before the merge, keeps the full 3 days as free float, because nothing sits between Y and the point where that slack is measured. The full network diagram and forward/backward pass for a complete schedule live in the CPM guide, for a worked example that carries all the way through a critical path calculation.

The same distinction is easiest to see on a drawn schedule; the project network diagram walkthrough shows a task with four days of total float and zero free float.

Total Float vs Free Float at a Glance

Float on non-critical paths is also the natural companion to a visible project buffer; the guide to schedule buffer management shows how to read the two together.

Which One Should You Track Week to Week

Float that shrinks between two status dates is one of the first signs of project schedule slippage, before any milestone moves. Float is only as trustworthy as the durations behind it. When a task's duration is a three-point estimate, see how PERT and CPM fit together before you rely on its float.

Total float tells you whether a task is critical. Free float tells you whether a task is about to start causing trouble for its neighbor, before that trouble has any chance of reaching the project end date. A PM who only watches total float will miss the moment a non-critical task starts squeezing the task after it; a PM who only watches free float will miss slower, chain-wide erosion that eventually removes all the slack from a whole sequence of tasks at once. Track both by hand on a small schedule, or run the free Schedule Health Check against an .mpp file to get both numbers computed directly from the real dependency graph instead of a manual pass. Schedule buffer management covers how to size the reserve that absorbs this erosion before it reaches zero, and dependency types covers how the wrong dependency type can shrink both numbers without anyone changing a single duration. Both posts sit alongside this one in the wider scheduling fundamentals library.

When float drops below zero, the plan can no longer meet a date it has promised; negative float explains the causes and the four fixes in order of cost.

Check your schedule's real float numbers Upload an .mpp or MSPDI XML to the free Schedule Health Check and get total float, free float, and the actual critical path computed from your dependency graph, not a rough manual estimate. No signup, no credit card. → Run the Schedule Health Check