The Free Float Calculator computes the time each activity may be delayed without impacting successor early starts in construction programmes.
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About the Free Float Calculator
Free float is the amount of time an activity can be delayed without affecting the earliest start of its next dependent task. It is calculated within a precedence network after you run a forward pass. Unlike total float, free float looks only at the activity and its successors. It does not compare against the entire project deadline.
This calculator focuses on the practical needs of construction teams. You can enter durations, relationships, and calendars to match field conditions. The result helps you decide where to place crews, when to order materials, and how to stage work areas. It also supports quick what-if checks while you refine a schedule estimate.
Use it to see which tasks have breathing room and which ones cannot slip. When decisions are tight, knowing free float lets you choose the least risky delay. It gives foremen and planners a clear metric they can act on during weekly coordination meetings.
The Mechanics Behind Free Float
Free float comes from the logic of a precedence diagram. Each activity has early dates from the forward pass. Its successors also get early dates. Free float compares the earliest finish of the activity against the earliest start of its successors. The difference is the available slack that does not push the next task.
- Network logic: Finish‑to‑Start, Start‑to‑Start, Finish‑to‑Finish, and Start‑to‑Finish links, with optional leads or lags.
- Forward pass: Computes earliest start (ES) and earliest finish (EF) for each activity.
- Successor gate: Free float is bound by the tightest (minimum) earliest start among direct successors, adjusted for lags.
- Calendars: Workdays, shifts, and weather calendars shape ES and EF, which flow into free float.
- Terminal tasks: If an activity has no successors, its free float often equals its total float under most methods.
Because free float checks only the next step, it is a local measure. It is valuable when sequencing crews between trades. It is also helpful when material deliveries shift. If a slab cure has float but rebar install does not, you know where to protect time. Pair free float with total float for a full picture.
Formulas for Free Float
Free float is rooted in the standard dates of the Critical Path Method (CPM). The forward pass creates earliest dates. The backward pass creates latest dates. While total float compares early and late dates, free float compares an activity’s finish to the earliest start of its successors.
- Free Float for activity i: FF(i) = min[ES(successors of i)] − EF(i)
- With lags: FF(i) = min[ES(successor j) − Lag(i→j)] − EF(i)
- Earliest Finish: EF(i) = ES(i) + D(i) where D is the activity duration in chosen units
- Total Float context: TF(i) = LS(i) − ES(i) = LF(i) − EF(i)
- Terminal activity rule of thumb: If no successors, set FF(i) = TF(i) unless your method defines a project-level successor.
Free float will never exceed total float. It can be zero when an activity’s earliest finish equals the earliest start of its tightest successor. Under certain links with leads, it may be negative, which signals an immediate logic conflict. The calculator highlights these cases so you can adjust durations, sequencing, or delivery dates.
Inputs and Assumptions for Free Float
The calculator focuses on the inputs a scheduler controls. It uses your activity list, durations, logic ties, and calendars to compute early dates. It then applies the formulas to produce free float for each activity. You can adjust units to match your plan window and crew rhythm.
- Activity list: Unique IDs and names representing scope (e.g., “A102 Form Footings”).
- Durations: Estimated lengths of work in consistent units, such as days or hours.
- Logic relationships: FS, SS, FF, or SF with optional leads/lags (e.g., FS+2d).
- Calendars: Working days, shifts, holidays, and weather allowances by activity or global calendar.
- Constraints (optional): Earliest start, must finish by, or delivery windows for critical materials.
- Project boundary (optional): Data date and target completion, used for total float context.
Assume that durations are deterministic estimates unless you run probabilistic analysis elsewhere. If an activity has multiple successors, the minimum earliest start governs. If an activity has no successors, the tool uses its total float as free float by default. Leads can create negative values when overlap is required. Mixed calendars may reduce apparent float due to nonworking time. Very short units can exaggerate rounding; choose units that fit site practice.
How to Use the Free Float Calculator (Steps)
Here’s a concise overview before we dive into the key points:
- Define the activity list and assign a clear, measurable scope for each item.
- Enter durations and select consistent units, like days or hours, for the whole schedule.
- Add logic ties between activities, including any necessary lags related to materials or cure times.
- Choose the working calendar, including weekends, holidays, and shift patterns.
- Run the forward pass to compute earliest start and earliest finish for all activities.
- Review free float results per activity and note any zero or negative values.
These points provide quick orientation—use them alongside the full explanations in this page.
Worked Examples
Residential footing sequence: Activity A “Form Footings” has D=3 days, ES=Day 0, EF=Day 3. Its successor, Activity B “Place Rebar,” cannot start until materials arrive on Day 5, so ES(B)=Day 5. Free Float(A)=min[ES(B)]−EF(A)=5−3=2 days. Interpretation: You can delay formwork by up to 2 days without delaying rebar installation, assuming the materials delivery holds. What this means: If rain pushes formwork by a day, the next crew stays on schedule.
Commercial build-out parallel trades: Activity C “Electrical Rough-In” has D=4 days, ES=Day 5, EF=Day 9. The successor inspection (Activity D) has ES(D)=Day 10. Free Float(C)=10−9=1 day. Total float for this branch is 4 days due to slack in a different path, but free float is only 1 day because inspection can start as soon as electrical is ready. Interpretation: Electrical can slip a day without moving inspection, but slips beyond a day will push D, even though the overall project still has buffer. What this means: Protect one day for electrical; use any extra slack elsewhere.
Limits of the Free Float Approach
Free float is a precise but narrow measure. It focuses on the immediate successor, not the full network. It does not account for resource limits or crew availability unless you model them as logic or calendars. When you level resources later, free float can change.
- Local scope: Ignores downstream paths beyond the next successor.
- Resource blindness: Does not see labor or equipment constraints unless modeled as logic.
- Calendar sensitivity: Results can shift when calendars differ across activities.
- Lead/lag complexity: Overlaps can create negative free float that needs special review.
Use free float together with total float, path analysis, and resource checks. For complex jobs, run a full critical path review after each major estimate update. Then confirm that key materials and inspections align with float values. This helps keep site decisions tied to reality.
Units and Symbols
Scheduling math depends on consistent units. In construction, many teams plan in working days, but some crews work in hours or shifts. The calculator accepts either, as long as durations and calendars match. Symbols are used for standard CPM dates and floats.
| Symbol | Meaning | Typical Units |
|---|---|---|
| FF | Slack available without delaying the earliest start of direct successors | Days or hours |
| TF | Slack before delaying project completion or a target milestone | Days or hours |
| ES | The soonest an activity can begin from the forward pass | Calendar date or day number |
| EF | The soonest an activity can finish from the forward pass | Calendar date or day number |
| D | Planned time to perform the activity under its calendar | Days, hours, or shifts |
Read the table as a quick legend while reviewing results. If you switch from days to hours, keep all durations and lags in the same units. When a delivery lag is in calendar days, adjust the calendar or convert it so it aligns with work time.
Troubleshooting
If results look off, start with units and logic. Mixed calendars, unintentional leads, or missing successors can distort free float. Check that every activity has the right relationships and that materials delivery lags reflect real-world waiting time, not work time, unless you intend that.
- Zero free float everywhere: Look for tight FS chains without gaps or for a constraint pinning early dates.
- Negative free float: Inspect leads and overlapping links; confirm the successor cannot truly start that early.
- Strange spikes in float: Verify calendars and ensure nonworking days are correctly set.
After corrections, rerun the forward pass and compare changes. If resource leveling will occur later, expect free float to change. Recheck any tasks near critical trades or key inspections, and confirm your estimate reflects crew productivity and delivery promises.
FAQ about Free Float Calculator
How is free float different from total float?
Free float looks only at the next dependent activity. Total float compares against the project’s late dates or finish target. Free float is always less than or equal to total float.
Can free float be negative?
Yes, with leads or overlap requirements, a successor can start before its predecessor finishes. That creates negative free float, signaling an immediate scheduling risk.
Do materials deliveries change free float?
They do if modeled as lags or constraints. A later delivery pushes the earliest start of the successor, which can increase free float on the predecessor.
What units should I use for small jobs?
Use hours or shifts if tasks are short and change daily. Use days for longer tasks. Keep all durations, lags, and calendars in consistent units.
Free Float Terms & Definitions
Free Float
The time an activity can be delayed without delaying the earliest start of any of its direct successors.
Total Float
The time an activity can be delayed without delaying project completion or a defined target milestone.
Successor
An activity that logically follows another. Its earliest start can limit the predecessor’s free float.
Forward Pass
The CPM calculation that finds earliest start and earliest finish dates based on durations and logic.
Lag
A waiting period between activities, often used for curing, inspections, or materials deliveries.
Calendar
A set of working and nonworking days or hours applied to activities to reflect real crew and site time.
Constraint
A date or rule that limits when an activity can start or finish, such as “must finish by.”
Critical Path
The sequence of activities with zero total float that determines the earliest possible project completion date.
References
Here’s a concise overview before we dive into the key points:
- Critical Path Method overview on Wikipedia
- U.S. GAO Schedule Assessment Guide (Best Practices)
- PMI: The Critical Path Method: Uses and Abuses
- NASA Schedule Management Handbook
- AACE International recommended practices (Scheduling)
These points provide quick orientation—use them alongside the full explanations in this page.