The four steps of CPM
- List the activities and their durations.
- Link them with their logic: which activity must finish (or start) before another can start (or finish).
- Forward pass: from the project start, work out each activity's earliest start (ES) and earliest finish (EF = ES + duration).
- Backward pass: from the project finish, work out the latest finish (LF) and latest start (LS = LF − duration). Total float = LS − ES. Activities with zero float are critical.
Worked example
| Activity | Duration (days) | Predecessor | ES | EF | LS | LF | Float |
|---|---|---|---|---|---|---|---|
| A Foundations | 10 | none | 0 | 10 | 0 | 10 | 0 |
| B Steel erection | 15 | A | 10 | 25 | 10 | 25 | 0 |
| C Equipment delivery | 20 | none | 0 | 20 | 5 | 25 | 5 |
| D Equipment setting | 5 | B, C | 25 | 30 | 25 | 30 | 0 |
| E Piping | 20 | D | 30 | 50 | 30 | 50 | 0 |
The critical path is A → B → D → E, 50 days. Equipment delivery has 5 days of float: it can slip by up to 5 days without moving the finish date.
Link types and lag
| Link | Meaning | Example |
|---|---|---|
| FS (finish-to-start) | B starts after A finishes | Pour concrete after rebar is fixed |
| SS (start-to-start) | B starts after A starts | Insulation starts 5 days after hydrotest starts (SS+5) |
| FF (finish-to-finish) | B finishes after A finishes | Painting finishes 3 days after steel erection finishes (FF+3) |
| SF (start-to-finish) | B finishes after A starts | Temporary power runs until permanent power starts |
Lag delays the successor by a number of days; a negative lag (lead) lets it overlap. In Planline you type links as 4, 4SS+5 or 4FF-2.
Calculate your own critical path
Enter your activities and links in the free scheduler. It runs the forward and backward pass on a working-day calendar with holidays, shows total float for every activity, highlights the critical path in the Gantt chart and lists it in the Excel report.
Frequently asked questions
What is total float?
The number of days an activity can be delayed without delaying the project finish date. Activities with zero total float are on the critical path.
Can there be more than one critical path?
Yes. Two or more chains can have the same longest duration, and all of them are critical.
What is the difference between CPM and PERT?
CPM uses one duration per activity. PERT uses optimistic, most likely and pessimistic durations to estimate the probability of finishing on time.
Try it on your own schedule
Planline is free, needs no sign-up and keeps your data in your browser. Start from the sample project or import your own file.