Total Float vs Free Float in Construction Scheduling: What They Mean and How to Use Them
Understand the difference between total float and free float in construction scheduling, how to calculate them, and what negative float tells you about project risk.
A client-side project manager receives a monthly programme update. The float on the structural steel activities has dropped from 15 days to three. The completion date hasn’t moved. Is the project on track? Technically, yes. But the near-critical path is days away from becoming critical, and nobody’s flagged it. That’s the problem with float: most people look at the total float number, notice it’s positive, and move on. But total float and free float tell you different things, and if you’re reading only one, you’re missing half the picture.
Float is the flexibility in your programme. It’s the time buffer between when an activity can happen and when it must happen. Understanding the difference between total float and free float isn’t academic. It determines whether you know which activities can absorb delay without consequence, which ones are about to become critical, and whether the programme you’re reviewing is actually reliable. Float interpretation is a core input into a structured construction schedule analysis, and it’s where most schedule misunderstandings begin.
What Is Float in Construction Scheduling?
Float is the amount of time an activity can be delayed without affecting the project completion date or the start of subsequent activities. Every activity that isn’t on the critical path has some float. Activities on the critical path have zero total float. The relationship between float and the critical path is set out in detail in our guide to the critical path method in construction.
Float exists because not all work sequences take the same amount of time. If the structural frame takes 20 weeks and the cladding package takes 14 weeks, and both start at the same time, the cladding has six weeks of float. It can be delayed by up to six weeks without pushing out the project finish.
But float isn’t free time. It’s a shared resource. When one activity consumes float, that float is no longer available for other activities on the same path. Understanding who owns the float, how it’s being consumed, and what happens when it runs out is central to managing programme risk.
Callout box: Float is not padding. Float is the calculated difference between the earliest and latest permissible dates for an activity, based on the network logic. It’s a mathematical output of the CPM calculation, not a subjective estimate.
Total Float Explained
Total float is the amount of time an activity can be delayed without delaying the project completion date. It’s the most commonly referenced float value in scheduling, and it’s the one most people look at first.
Formula
Total Float = Late Start (LS) minus Early Start (ES) = Late Finish (LF) minus Early Finish (EF)
In simpler terms: total float is the gap between when an activity could start (its early date) and when it must start to avoid delaying the project (its late date).
Worked Example
Consider a concrete pour activity:
- Early Start: Day 20
- Early Finish: Day 25 (5-day duration)
- Late Start: Day 30
- Late Finish: Day 35
Total Float = 30 minus 20 = 10 days (or 35 minus 25 = 10 days)
This activity can be delayed by up to 10 days without affecting the project completion date. Once those 10 days are consumed, the activity becomes critical.
What Total Float Tells You About Schedule Risk
Total float is a risk indicator. Activities with high total float (more than 20 working days) have plenty of room for delay. Activities with low total float (less than 10 working days) are near-critical. Activities with zero total float are critical.
But total float has a limitation: it measures impact on the project completion date only. It doesn’t tell you whether delaying this activity will affect the start of the next one. That’s where free float comes in.
Free Float Explained
Free float is the amount of time an activity can be delayed without affecting the early start of any successor activity. It’s a more restrictive measure than total float because it considers the impact on downstream work, not just the project finish.
Formula
Free Float = Early Start of successor (ES_succ) minus Early Finish of current activity (EF_curr)
If an activity finishes on day 25 and its successor can start as early as day 30, the free float is five days. The current activity can be delayed by up to five days without pushing out the next one.
Worked Example
Using the same concrete pour:
- Early Finish of concrete pour: Day 25
- Early Start of successor (curing): Day 28
Free Float = 28 minus 25 = 3 days
Total float is 10 days, but free float is only three days. What does this mean? You can delay the concrete pour by up to 10 days without affecting project completion, but if you delay it by more than three days, you’ll push out the curing activity. The remaining seven days of total float come from float available elsewhere on the path, not from the relationship between these two specific activities.
When Free Float Matters More Than Total Float
Free float matters when you need to coordinate closely sequenced activities. On a typical building project, the structural sequence (formwork, rebar, concrete, curing, strike, next level) is tightly linked. If the rebar activity has 15 days of total float but only one day of free float, any delay to rebar pushes out the concrete pour immediately, even though the project completion date might not be affected yet.
For subcontractors managing their own sequences, free float is the more relevant measure. It tells them whether their delay affects the next trade in the chain.
Total Float vs Free Float: Key Differences
| Feature | Total Float | Free Float |
|---|---|---|
| Measures impact on | Project completion date | Next successor’s early start |
| Formula | LS - ES or LF - EF | ES(successor) - EF(activity) |
| Always less than or equal to total float? | Yes (it’s the larger measure) | Yes, free float is always less than or equal to total float |
| Can be negative? | Yes | Yes |
| Used for | Identifying critical and near-critical activities | Coordinating closely sequenced activities |
| More relevant for | Client PM reviewing programme risk | Subcontractor scheduling and trade coordination |
| Ownership in dispute | Often contested; depends on contract | Generally belongs to the activity’s responsible party |
10 days, float = 0"] S --> B["Activity B
5 days, TF = 10
FF = 3"] A --> C["Activity C
8 days, float = 0"] B --> C C --> F(["Project Finish"]) style S fill:#1b4332,color:#fff style F fill:#1b4332,color:#fff style A fill:#d62828,color:#fff style C fill:#d62828,color:#fff style B fill:#457b9d,color:#fff linkStyle 0 stroke:#d62828,stroke-width:3px linkStyle 2 stroke:#d62828,stroke-width:3px linkStyle 4 stroke:#d62828,stroke-width:3px
Critical path (red): Start → A → C → Finish. Non-critical path (blue): Activity B has 10 days of total float (can slip without delaying the project) but only 3 days of free float (can slip without delaying successor C).
Negative Float: What It Means and Why It Matters
Negative float is a warning sign. It means the schedule is already behind the contractual completion date, or a constraint is impossible to meet.
How Negative Float Occurs
Negative float appears when:
- The programme shows a completion date later than the contractual date
- Hard constraints force activities into impossible time frames
- The critical path has been delayed beyond the available buffer
When total float drops below zero, every additional day of delay compounds the problem. A critical path with negative 10 days of float means the project is running 10 days late.
What we found: Hard constraints can pull a displayed completion date back to the contractual finish even when the network logic, run without those constraints, would produce a later date. A programme update that shows zero or positive float at the contractual milestone but carries Must Finish On constraints on key activities is one to read carefully. The DCMA 14-Point’s hard-constraint threshold (≤5% of activities, metric 5) exists precisely because constraints can override the calculation the rest of the metrics depend on.
What it means: Negative float isn’t a scheduling quirk; it’s a fact about the network. If the logic-driven calculation shows the project finishing late, adding constraints to show an on-time finish doesn’t change reality. It changes the software output. Strip the non-contractual constraints, re-run, and trust the result the network produces.
What Negative Float Tells You About Project Health
| Float Value | Project Status | Action Required |
|---|---|---|
| Positive (>10 days) | On track with buffer | Monitor |
| Positive (1-10 days) | Near-critical; limited buffer | Investigate and closely monitor |
| Zero | Critical; no buffer | Act immediately on any delay |
| Negative (1-30 days) | Behind schedule | Recovery plan required |
| Negative (>30 days) | Well behind schedule | Major re-planning needed; DCMA metric 7 flags any negative float (0% threshold) |
Low risk"] B -- No --> D{"1–10 days?"} D -- Yes --> E["Investigate
Near-critical"] D -- No --> F{"= 0 days?"} F -- Yes --> G["Act Immediately
Critical path"] F -- No --> H["Recovery Plan
Negative float"] style C fill:#2d6a4f,color:#fff style E fill:#e9c46a,color:#000 style G fill:#d62828,color:#fff style H fill:#6a040f,color:#fff
The Defense Contract Management Agency 14-Point Assessment, metric §7, flags any negative float in the schedule. PAM 200.1’s canonical target is zero: any negative float means the network is forecasting completion later than the contractual date, which is why even a single negative-float activity is treated as a flag. The full set of metrics is covered in our guide to the DCMA 14-Point Assessment.
The James Webb Space Telescope launched in December 2021 against an original 2007 launch target, with final development costs in the order of US$10 billion against early-2000s estimates of a few billion dollars. As the schedule slipped further and further behind, the float on remaining activities turned negative and stayed negative. The project kept re-baselining to absorb the overruns, but each re-baseline reset the float without changing the underlying reality that the work was taking far longer than planned. Negative float on a construction programme works the same way: you can re-baseline, but the work still has to get done.
How to Monitor Float in Primavera P6
Viewing Float Columns
In Oracle Primavera P6, total float and free float are displayed as separate columns in the activity table. Add both columns to your default layout so you can see them side by side.
- Right-click the column header, select “Columns,” and add “Total Float” and “Free Float” from the schedule data category
- Sort by total float ascending to see critical and near-critical activities first
- Filter for activities with total float less than or equal to 10 working days to review the near-critical paths
Tracking Float Consumption
Float consumption shows how quickly flexibility is disappearing. Compare total float values between consecutive programme updates for the same set of activities. If the float on a path drops from 20 days to eight between two monthly updates, something is consuming it fast.
P6 doesn’t automatically track float consumption over time, so you need to compare updates manually or use an external tool. ScheduleLens automates this comparison, flagging activities where float has decreased by more than a set threshold between updates.
Interpreting Float Trends
Float trends are a leading indicator of schedule risk:
- Float decreasing steadily: The path is consuming its buffer. If the rate continues, it will become critical.
- Float stable: The path is progressing as planned.
- Float increasing: Logic has changed or activities have been re-sequenced. Verify the change makes sense.
- Float jumping (large sudden change): Logic or constraint changes. Investigate immediately.
| Trend | What It Usually Means | Risk | Action |
|---|---|---|---|
| Steady decrease (5-10 days/month) | Normal progress consuming buffer | Medium | Track rate; project when float hits zero |
| Steady decrease (>15 days/month) | Rapid buffer consumption; potential delay | High | Investigate cause; assess recovery options |
| Stable | On track | Low | Continue monitoring |
| Sudden increase | Logic change or re-sequencing | Medium | Verify the change is genuine and not masking delay |
| Sudden decrease | Delay event or logic change increasing risk | High | Identify the event; check contemporaneous records |
Float Ownership and Delay Claims
Who owns the float? It depends on the contract.
The Three Positions on Float Ownership
- Float belongs to the project. Neither party can exclusively claim float. It’s a shared resource to be used for the benefit of the project. This is the default position in many standard forms and is supported by the SCL Protocol.
- Float belongs to the contractor. The contractor created the programme; the float is a byproduct of their sequencing. An employer can’t use the contractor’s float to offset employer-caused delays.
- Float belongs to the employer. The employer paid for the project; any flexibility in the programme belongs to them. This position is less common but appears in some standard forms.
How Float Consumption Affects EOT Claims
If float belongs to the project, consuming float doesn’t change entitlement. A contractor who uses up float on a non-critical path and then faces an employer-caused delay on that same path may still be entitled to an EOT if the float has been consumed and the path is now critical.
If float belongs to the contractor, the employer can’t argue that float should absorb employer-caused delays.
The SCL Delay and Disruption Protocol, Core Principle 8 (Float as it relates to time), addresses float ownership. Paragraph 8.5 explains: “It has the effect that float is not time for the exclusive use or benefit of either the Employer or the Contractor (unless there is an express provision in the contract).”
Nepal’s school-building programme, referenced in Flyvbjerg and Gardner’s How Big Things Get Done, delivered projects on budget and eight years ahead of schedule. One reason: float was managed as a project resource, not hoarded. When delays occurred, the float was available to absorb them without dispute, because the programme logic was transparent and the float ownership was clear from the start. When float ownership is contested, the disputes consume time and money that could be spent on building.
Float as a Schedule Health Indicator
Float values tell you about programme quality, not just project progress.
High Float Values
Activities or paths with unusually high float (more than 30-40 working days) suggest either:
- Missing logic links (the path isn’t properly connected to the rest of the network)
- Scope that’s been omitted from the schedule
- A placeholder activity that doesn’t reflect real work
The DCMA 14-Point Assessment, metric 1, flags open ends (activities with no predecessor or successor). These often show high float because they’re not properly connected to the completion milestone. Float sequestering, dummy logic, and constraint abuse are all things any reviewer should hunt for: see our walkthrough on how to review a contractor programme.
Float Sequestering
Float sequestering is the practice of hiding float by adding constraints or artificial logic to make non-critical activities appear critical. A contractor who adds a “Must Finish On” constraint to an activity with 20 days of float will show zero float, making it look critical. This prevents the employer from arguing that the activity’s float should absorb an employer-caused delay.
Checking for float sequestering is part of any schedule quality assessment. If you see zero-float activities that aren’t on the main construction sequence, check for constraints.
Recommended Float Thresholds
| Project Type | Minimum Desirable Float (Critical Path) | Minimum Desirable Float (Near-Critical) |
|---|---|---|
| Major infrastructure | 15-20 working days | 10-15 working days |
| Commercial building | 10-15 working days | 5-10 working days |
| Industrial/EPCC | 15-25 working days | 10-15 working days |
| Residential | 5-10 working days | 3-5 working days |
These aren’t standards; they’re practical benchmarks. A critical path with less than 10 days of float on a two-year project has almost no room for error.
Common Float Mistakes in Construction Schedules
Confusing total float with free float. Total float measures impact on the project finish. Free float measures impact on the next activity. They’re different numbers with different implications. If you’re coordinating subcontractor sequences, free float is the relevant measure.
Ignoring float consumption trends. A static float value is less useful than a trend. Float that was 20 days last month and is eight days this month is more concerning than float that’s consistently five days.
Using constraints to hide float. Adding hard constraints to reduce float to zero is float sequestering. It misrepresents the programme’s actual flexibility.
Not checking for negative float after updates. After every schedule update, check for negative float. If it appears, the project is behind the contractual date. Don’t assume the software is wrong; it’s calculating based on the logic you have.
| Mistake | Consequence | Fix |
|---|---|---|
| Confusing total and free float | Misunderstanding downstream impact | Show both columns side by side |
| Ignoring consumption trends | Missing the approach of critical status | Track float between updates |
| Hiding float with constraints | Misrepresenting programme risk | Audit constraints before approving the programme |
| Not checking negative float | Approving a programme that is already late | Set up a negative float filter |
Tools for Float Analysis
Oracle Primavera P6 displays total float and free float as standard columns. Set up a layout that shows both, sorted by total float ascending, with a filter for activities with total float less than or equal to 15 working days.
Microsoft Project shows total slack (its term for total float) by default but doesn’t display free slack as prominently. Right-click the column headers and add the relevant columns.
ScheduleLens automates float monitoring: it tracks float consumption between updates, flags negative float, detects float sequestering through constraint audits, and highlights near-critical paths. For the client-side PM reviewing a monthly update, ScheduleLens answers the question “what’s changed since last month?” without manually comparing two programme files.
| Tool | Float Capability | Best For |
|---|---|---|
| Oracle Primavera P6 | Total float, free float columns; filtering and sorting | Creating and updating the programme |
| Microsoft Project | Total slack, limited free slack display | Smaller projects with simpler float needs |
| ScheduleLens | Automated float tracking, consumption alerts, negative float flags | Reviewing programmes you’ve received |
Standards and References
| Standard | Relevance | Key Section |
|---|---|---|
| DCMA 14-Point Assessment | Float-related quality metrics | Metrics 6 (High Float), 7 (Negative Float), 1 (Logic, relevant because missing logic causes phantom float) |
| SCL Protocol (2nd Edition) | Float ownership and EOT entitlement | Core Principle 8 (paragraphs 8.1–8.8); Core Principle 9 (Identification of float) |
| AACE International RP 29R-03 | Schedule analysis methods referencing float | Section 4.3 (Critical Path and Float, including 4.3.E Ownership of Float); Section 2 (Source Validation Protocols for schedule validity) |
| PMI Practice Standard for Scheduling | Float definitions and calculations | Chapter on float and critical path |
| CIOB Guide to Good Practice | Float management | Chapter on programme management |
What to Do Next
When you review a programme update, don’t just check the completion date. Check the float. Look at total float values for near-critical paths. Track how float is changing between updates. And check for negative float, because if the programme is already showing a late finish, everything else is window dressing.
Float is the space between your programme and your problem. Know where it is, know who owns it, and know when it’s running out.
This article is for educational purposes and does not constitute legal or professional advice. Float ownership and interpretation should be considered in the context of your specific contractual provisions.