Free float
Last reviewed 9 September 20261,569 words7 min read
The one people confuse with total float
🟩 In one line Free float is how long an activity can slip before it disturbs the next activity in line. Total float is about the project finish. Free float is about your neighbour.
👤 Who this is for Junior planners who keep getting asked "can I move this?" and reaching for the wrong column. Site engineers and supervisors, who — honestly — are the people free float was invented for. Mid-level planners doing look-aheads and crew sequencing.
First, let's be honest about why this page exists
Most planners can recite the definition of free float. Almost none of them use it.
Ask a planner for the float on an activity and they will read you total float, every time. It's the column P6 shows by default, it's the one the standards talk about, it's the one that matters for the finish date. Free float sits quietly next to it, mostly ignored, mostly zero.
And yet — when a foreman walks up and says "I want to start the blockwork on Level 3 next week instead of this week, is that a problem?" — the answer he needs is free float, not total float. He doesn't care about the project finish. He cares whether the plasterer behind him is going to be standing around.
That's the gap this page closes. Free float is the local question. Total float is the global one. You need both, and you need to know which one you've been asked.
🟨 The standard — what "good" looks like
The definitions line up across the board:
Free Float = the amount of time an activity can be delayed without delaying the early start of any successor.
- PMI Practice Standard for Scheduling and AACE RP 10S-90 (Cost Engineering Terminology) both define it this way.
- The DCMA 14-Point Assessment doesn't test free float directly — its float metrics are all total float. That tells you something about how the industry ranks it.
- No contract form (FIDIC, NEC, JCT) mentions free float at all. Entitlement is decided on completion, so total float governs. Free float is a management tool, not a contractual one.
Two rules that always hold:
- Free float can never be more than total float. If it looks like it is, something is broken.
- Free float lives at the end of a chain. In a simple finish-to-start sequence, only the last activity before a merge point has any free float. Everything upstream has zero — because delaying it delays the next one directly.
How it actually works
Think of a queue at a site gate. Five trucks, bumper to bumper. If the first truck waits, every truck behind it waits. None of those trucks has free float — any delay passes straight down the line.
Now the last truck in the queue: behind it, nothing for twenty minutes. That truck can hang back, take a phone call, whatever — nobody behind it is affected. That gap is free float. It belongs to the last truck alone.
That's the whole concept. Free float appears where a path arrives somewhere — at a merge with another path, at a milestone, at a successor that's waiting on something else anyway. Everywhere else in a tight chain, it's zero.
Why that matters on site
When you're building a three-week look-ahead, activities with free float are the ones you can legitimately shuffle to smooth out crew movements, share a crane, or avoid two trades in one room. Nobody downstream notices. Activities with total float but no free float are different — moving them is allowed by the project finish, but the next trade will feel it immediately, and they'll want to know why.
Why it matters in P6
The Free Float column is there; add it to your update layout next to Total Float. When they're equal, you're at the end of a path. When free float is zero and total float is large, you're mid-chain in a path that has room overall, but no room between you and the next activity. That's useful to know before you promise anything.
🟢 A handy mental check: Total float answers "will the project be late?" Free float answers "will someone be angry with me on Monday?" Different questions, different columns.
🟥 Where people go wrong
1. Quoting total float when someone asked a free float question. "You've got 12 days, no problem." Twelve days of total float. Free float: zero. The successor trade turns up on Monday to nothing. This is the single most common way planners lose the trust of site teams — not by being wrong about the project, but by being wrong about next week.
2. Thinking zero free float means critical. It doesn't. Most activities in a healthy schedule have zero free float, because most activities are followed directly by something else. Criticality is a total float / longest path question. (See critical-path-vs-longest-path.)
3. Trying to use free float in a delay claim. Doesn't work. Completion is what the contract cares about. An activity with free float that's delayed by the Employer still may or may not affect completion — and that is decided by total float. Free float is irrelevant to entitlement, and an assessor will tell you so.
4. Forgetting that lags and relationship types change it. With finish-to-start and no lag, free float is easy to read. Add a start-to-start with a lag, or a finish-to-finish, and free float is measured against a different point on the successor. It's still correct — it's just not where your eye expects it. If free float looks odd, look at the relationship type first.
5. Ignoring the calendar. Free float is calculated on the predecessor's calendar in P6. If your activity is on a 6-day calendar and its successor is on a 5-day calendar, the number can look off by a day or two. It isn't wrong; it's counting different days from you.
⚖️ When you're challenged
"Can I push this back a week without affecting anyone?" Look at free float, not total float. If FF ≥ 6 working days: "Yes — nobody behind you moves." If FF is zero and TF is 20: "The project's fine, but the next trade starts a week later. Let me check with them before you commit." That second sentence is what makes site people trust you.
"Why is free float zero when total float is 20?" "Because the next activity starts the moment this one finishes. There's 20 days spare on the whole chain, but none of it sits between you and the next job — it's all at the far end of the path." Draw it if you have to. Two boxes, an arrow, a gap at the end. Takes ten seconds.
"So who owns the free float?" Practically — the activity that has it, because using it hurts nobody. Contractually — the question doesn't arise; the contract only tests completion. This is one of the few float arguments you'll never have to have.
📄 Related pages
- Total float — the shared one, and who owns it
- Critical path vs longest path — why zero float isn't automatically critical
- Leads and lags — what happens to float when relationships aren't simple FS
- Weekly Report and Look-Ahead — where free float earns its keep
- Multiple calendars and float — when the two numbers stop agreeing
✏️ Worked example
A small finishes sequence in a villa. 6-day calendar. Two paths meet at a joint inspection.
`` Path 1: Screed (4d) → Tiling (6d) ─────────────┐ ├→ Joint inspection Path 2: First-fix MEP (8d) → Second-fix MEP (5d)┘ ``
Both paths start on day 1. Path 1 needs 10 days. Path 2 needs 13 days. Inspection can start on day 14.
| Activity | Dur | ES | EF | Successor ES | Free float | Total float |
|---|---|---|---|---|---|---|
| Screed | 4 | 1 | 4 | 5 (Tiling) | 0 | 3 |
| Tiling | 6 | 5 | 10 | 14 (Inspection) | 3 | 3 |
| First-fix MEP | 8 | 1 | 8 | 9 (Second-fix) | 0 | 0 |
| Second-fix MEP | 5 | 9 | 13 | 14 (Inspection) | 0 | 0 |
Read the two float columns together:
- Screed has 3 days total float (the whole tiling path has 3 days spare) but zero free float. Delay the screed one day and the tiler starts one day late. He'll notice.
- Tiling has 3 days of both. It's the last activity on its path before the merge, so the path's spare time sits with it. Tiling can slip 3 days and the inspection still happens on day 14.
- The MEP path is driving. Zero everywhere. Nothing to give.
If the screed foreman asks to start a day late, the honest answer is: "The project won't move, but the tiler will start a day late — ask him." If the tiler asks for a day, the honest answer is: "Yes, go ahead, nobody's affected." Same total float. Completely different conversation.
📖 References
- PMI, Practice Standard for Scheduling, 3rd ed. — §4.4
- AACE International, RP 10S-90, Cost Engineering Terminology — "Free Float"
- Oracle Primavera P6 Professional, User's Guide — "Free Float" field definition
From the field
Experience from working planners. Unreviewed — read it as experience, not guidance.
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