Float and Slack: Total Float vs Free Float Explained

Float in project management is the amount of time a task can be delayed without pushing back a deadline. It's what separates a schedule that shatters at the first disruption from one that bends and holds. Get it wrong, and you're reacting to every small slip. Get it right, and you know exactly which tasks need protection and which have room to breathe.

What Is Float in Project Management?

Float (also called slack -- the two terms mean exactly the same thing) is the scheduling buffer available to a task. More precisely: the maximum delay an activity can absorb before it causes a deadline to slip -- either the project deadline or the start of a dependent successor task.

Float is calculated after you run the critical path method (CPM) on your network diagram. CPM's forward and backward passes generate four time values for every activity -- early start (ES), early finish (EF), late start (LS), and late finish (LF) -- and those values are the raw material for every float calculation.

Tasks with zero float sit on the critical path. Delay any one of them and the project end date moves.

Key Facts

• PMI's Pulse of the Profession found that only 47% of projects are completed on time, pointing to poor schedule buffer management as a leading cause. (PMI, Pulse of the Profession 2023)
• A study in the International Journal of Project Management found that teams who actively monitored float consumed 20-30% fewer contingency days than those who tracked only milestone dates.
• McKinsey reports that large infrastructure projects run an average of 20% over schedule, largely because project managers fail to distinguish between tasks that have flexibility and those that don't.

Total Float vs Free Float

These are the two float values you'll calculate for every activity. They answer different questions, and confusing them leads to bad decisions.

Total float formula:

Total Float = LS - ES
           = LF - EF

Both give the same answer. Use whichever pair of values you already have on hand.

Free float formula:

Free Float = ES of next activity - EF of current activity - lag

If there's no lag between the two tasks, drop that term.

Which one matters more? Total float is the headline number -- it tells you project risk. Free float is the operational number -- it tells you how much room you have locally without needing to coordinate with the next team.

Float, Slack, and the Critical Path

Float and the critical path are two sides of the same coin. The critical path method produces both: the critical path is the sequence of activities with zero total float, and float is what every other activity has by definition.

A few things follow from this:

• Critical path activities have zero total float -- any delay there is a delay to the project.
• Near-critical tasks have very small total float (often one or two days). They don't sit on the critical path today, but a single overrun can put them on it tomorrow.
• The critical path can shift. If a task burns through its float, it joins the critical path. That's why float isn't just a number you calculate once -- it's something you monitor throughout execution.

When you're working with a network diagram, total float is visible as the gap between the ES/EF box and the LS/LF box of any activity node. On a Gantt chart, it often appears as a shaded bar extending past the task's early finish.

How to Calculate Float

Float calculation follows directly from CPM. You need the network diagram and duration estimates first. Then:

Step 1: Forward Pass (calculate ES and EF)

Work from left to right through the network. The first activity starts at ES = 0 (or day 1, depending on your convention).

For each activity:

• ES = the highest EF among all predecessors
• EF = ES + duration

Continue until you've processed every activity. The EF of the last activity is the project duration.

Step 2: Backward Pass (calculate LS and LF)

Work from right to left. Start by setting the LF of the last activity equal to its EF (assuming you want to finish as early as possible).

For each activity:

• LF = the lowest LS among all successors
• LS = LF - duration

Step 3: Compute Total Float

For every activity:

Total Float = LS - ES    (or LF - EF -- same result)

Activities where Total Float = 0 form the critical path.

Step 4: Compute Free Float

For each activity, look at its immediate successors:

Free Float = ES (successor) - EF (current) - lag

If an activity has multiple successors, use the smallest result. Free float cannot be negative -- if the formula gives a negative number, check your network logic.

Worked Example

Consider a six-activity project. Durations are in days.

Forward pass:

Project duration = 15 days. F's EF = 15, so LF of F = 15.

Backward pass:

Float values:

The critical path runs A -- B -- D -- F (all zero total float). Activities C and E each have one day of total float, but only E has free float (it can slip one day without touching F). C has zero free float because E depends on it.

Other Types of Float

Total float and free float are the most common, but project management literature mentions two others.

Independent float is the most restrictive -- it's the buffer that exists no matter how everything else runs. Project float is the most permissive -- it's the gap between what your network says and what the contract requires.

For most projects, total and free float are the only two you'll need.

Why Float Matters

Float isn't just a scheduling curiosity. It drives real management decisions.

Resource leveling. When you have resource conflicts, you resolve them by delaying tasks that have float. A task with five days of total float can absorb a two-day resource shortage without touching the deadline. A task with zero float cannot.

Risk triage. Not all delays are created equal. Float tells you which delays matter. A two-day slip on a task with ten days of total float is noise. A two-day slip on a near-critical task with two days of float is a project threat.

Scope change impact analysis. When a client asks for a change, you need to know whether it hits the critical path or lands in a float-rich zone. That answer changes your negotiating position completely.

Team communication. Float gives you a factual basis for conversations about pace. Instead of telling a team they're "behind," you can show them exactly how much buffer remains and what happens when it's gone. That's a much more productive conversation, and it's grounded in the triple constraint -- time, cost, and scope -- rather than gut feel.

Schedule compression. When the project is running late and you need to crash the schedule, you focus on critical path tasks. Float tells you which tasks you can skip compressing because they have room to spare.

A common mistake is treating float as free time. It isn't. Float is insurance. Once you consume it, the task joins the critical path and you've lost your buffer. Teams that burn through float early often find themselves scrambling in the final weeks of a project that looked healthy on paper.

Float visibility is also why tools like Gantt charts and PERT charts matter -- they make float visible in a way that a list of tasks never can.

Frequently Asked Questions

Is float the same as slack?

Yes. Float and slack refer to the same concept. The two terms come from different traditions -- float is the term used in the critical path method (CPM), while slack appeared in the Program Evaluation and Review Technique (PERT). Today the terms are interchangeable in practice, and most project management tools use them interchangeably.

Can float be negative?

Yes. Negative float means the project is already behind its target date. It happens when a project has a hard deadline that's earlier than the calculated finish date, or when actual progress has fallen behind the baseline schedule. Negative float on a task means it would need to finish earlier than its current late finish to avoid missing the deadline -- which is only possible by compressing upstream work. Negative float is a red flag that demands immediate attention.

How is free float different from total float in practice?

The distinction shows up most clearly in sequenced handoffs. Suppose your team finishes a deliverable and hands it to another team. If that task has two days of free float, your team can take two extra days without impacting the receiving team's start date -- you don't need to coordinate or warn anyone. If the task has two days of total float but zero free float, any delay immediately pushes the next team's start, even though the project end date isn't yet at risk. Free float governs local coordination; total float governs project-level risk.

Does float change during project execution?

Yes, and it often shrinks faster than teams expect. Float is a calculated value based on current estimates and actuals. As tasks run long, as scope changes get approved, or as dependencies shift, float on downstream activities changes. That's why float should be recalculated (or dynamically updated by your scheduling tool) on a regular cadence -- weekly at minimum on complex projects.

What happens to float when there's a lag or lead between tasks?

Lag adds time between the end of one task and the start of the next -- it reduces the free float of the predecessor. Lead (negative lag) lets a successor start before its predecessor is completely done -- it can increase free float. Both change the free float formula: Free Float = ES (successor) + lead - EF (current) - lag. Your scheduling software handles this automatically once you enter the relationship type and the lag/lead value.

Related Reading

• Critical Path Method (CPM): How to Find the Critical Path
• Network Diagram: How to Map Project Activities
• What Is a PERT Chart?
• What Is a Gantt Chart?
• Triple Constraint in Project Management

Float is one of the most useful numbers in any project schedule -- but only if you track it actively. Calculate it at the start, revisit it each week, and treat near-zero float as the early warning signal it is. A schedule that shows you where the buffers are is a schedule you can actually manage.