Critical Path Calculator
Enter activities, durations and predecessors. The calculator runs the forward and backward pass, finds every critical path and shows how much each activity can slip.
| ID | Activity | Duration (days) | Predecessors |
|---|
How the critical path is calculated
Forward pass: each activity's Early Start (ES) is the latest Early Finish (EF) of its predecessors; EF = ES + duration. The largest EF is the project duration.
Backward pass: starting from the project end, each activity's Late Finish (LF) is the earliest Late Start (LS) of its successors; LS = LF − duration.
Float: Total float = LS − ES. Activities with zero total float form the critical path — the longest chain of dependent work and the shortest possible project duration.
How to shorten a project
- Crashing: add resources to critical activities (costs money; check diminishing returns).
- Fast-tracking: overlap critical activities that were planned in sequence (adds risk and rework).
- Re-scoping: remove or defer work on the critical path.
Shortening a non-critical activity changes nothing. Always re-run the calculation after a change — a new critical path often appears. Learn more in our Critical Path Method guide.
Frequently asked questions
What is total float?
Total float (or slack) is how long an activity can be delayed without delaying the project finish date. It equals Late Start minus Early Start (LS − ES). Critical activities have zero total float.
What is free float?
Free float is how long an activity can be delayed without delaying the early start of any successor. It is always less than or equal to total float.
Can a project have more than one critical path?
Yes. Several chains can tie for the longest duration. Projects with multiple critical paths carry more schedule risk because a slip on any of them delays the finish.