Guide
What is the critical path method, in plain English?
The critical path method (CPM) is a way to find which chain of work controls your finish date. You list the activities, tie them together in the order they must happen, give each one a duration, then add up every path. The longest path is the critical path: slip anything on it and the job finishes late.
A simple example
Here's a small electrical scope. Durations are workdays. Day 0 is the start.
| ID | Activity | Days | Predecessors | Early start / finish | Late start / finish | Total float | Free float |
|---|---|---|---|---|---|---|---|
| A | Pour equipment pad | 2 | Start | 0 / 2 | 3 / 5 | 3 | 0 |
| B | Set switchgear | 1 | A | 2 / 3 | 5 / 6 | 3 | 3 |
| C | Install feeder conduit | 6 | Start | 0 / 6 | 0 / 6 | 0 | 0 |
| D | Pull feeders | 3 | B, C | 6 / 9 | 6 / 9 | 0 | 0 |
| E | Terminate and test | 2 | D | 9 / 11 | 9 / 11 | 0 | 0 |
Two paths run through the job:
- A → B → D → E = 2 + 1 + 3 + 2 = 8 days
- C → D → E = 6 + 3 + 2 = 11 days
The longest path is C → D → E, so the job takes 11 days and the conduit is on the critical path. The pad and the gear have 3 days of float between them.
Forward pass and backward pass
Forward pass: start at day 0 and work left to right. Each activity's early start is the latest early finish of its predecessors. Pull feeders can't start until both the gear (day 3) and the conduit (day 6) are done, so its early start is day 6.
Backward pass: start at the finish (day 11) and work right to left. Each activity's late finish is the earliest late start of its successors. That gives you the latest each activity can finish without moving the end date.
Total float vs. free float
Total float is how many days an activity can slip before it pushes the project finish: late finish minus early finish. Free float is how many days it can slip before it bumps the very next activity: the successor's early start minus this activity's early finish.
In the example, the pad (A) has 3 days of total float but zero free float. If the pad slips a day, the gear setting slips a day too, but the job still finishes on day 11. That's why float isn't free time: whoever burns it first takes it from everyone downstream.
What breaks the critical path
- Open ends. An activity with no successor can't push anything, so it can't show up as critical.
- Hard constraints. Forced dates override the logic and hide the real path.
- Calling everything critical. If everything's critical, nothing is. See stop calling everything critical.
In Primavera P6 you can define critical activities either by total float at or below a threshold or by the longest path. Know which setting your project uses. The DCMA 14-point assessment includes a critical path test that checks the path is actually connected.
Common questions
What is the critical path in construction?
The longest chain of logically connected work from start to finish. Its length sets the earliest possible finish date. Any delay to an activity on it delays the project unless you recover the time somewhere else.
Can the critical path change?
Yes, often. When work on another path slips past its float, that path becomes the new longest path. That's why schedulers review the critical path every update.
What is the difference between total float and free float?
Total float is how long an activity can slip before it delays the project finish. Free float is how long it can slip before it delays the next activity. Free float can never be more than total float.
Keep going
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- What is the DCMA 14-point assessment?
- What does a construction scheduler (P6 scheduler) do?
- How do you become a construction scheduler from the trades?
- All guides
- BlueCollarPMP FAQ