Critical Path Calculator
| Activity | Duration | Predecessors | |
|---|---|---|---|
Enter predecessor activity names separated by commas. Leave blank for activities with no dependencies.
Enter Your Project Activities
Add your activities with durations and predecessor relationships, then click Calculate to see the critical path, Gantt chart, and full schedule table.
How to Use the Critical Path Calculator
Enter Your Activities
Type each project activity into the table. Give every activity a unique name and a numeric duration in your chosen time unit (days, weeks, hours, or months). Start with activities that have no predecessors, then add dependent activities. Use the Add Activity button to insert new rows, or click Load Example to see a pre-filled 7-activity software project.
Define Predecessor Relationships
In the Predecessors column, type the names of all activities that must be completed before each activity can begin. Separate multiple predecessors with commas, for example: Design, Test. Activity names must match exactly (the tool is case-sensitive). Activities with no predecessors should have the Predecessors field left blank — these are the project start activities.
Choose CPM or PERT Mode
Select CPM mode if you have a single deterministic duration for each activity. Switch to PERT mode if you want to model schedule uncertainty using three-point estimates: Optimistic (O), Most Likely (M), and Pessimistic (P). In PERT mode the calculator computes the expected time te = (O + 4M + P) / 6, standard deviation σ = (P − O) / 6, and variance V = σ² for each activity, then sums variance along the critical path for project-level risk analysis.
Read the Results
The Critical Path is shown at the top as a chain of activity names (e.g., Requirements → Design → Backend Dev → Integration → Testing → Deployment). The Gantt chart displays all activities on a shared timeline — critical activities appear in the primary color, non-critical in grey, and float is shown as a dashed extension. The schedule table provides ES, EF, LS, LF, Total Float, and Free Float for every activity. Enable Independent Float via the checkbox in the controls panel for the most conservative float estimate.
Frequently Asked Questions
What does Total Float (TF) mean and how is it different from Free Float (FF)?
Total Float is the maximum amount of time an activity can be delayed without extending the overall project duration. It is calculated as LS − ES (or equivalently LF − EF). Free Float is more restrictive: it is the maximum delay an activity can absorb without delaying any of its immediate successors. An activity can have large total float but zero free float if it shares that float with downstream activities. For scheduling purposes, Free Float is safer to consume because it guarantees no successor is affected, while consuming Total Float requires coordination across the entire path. Critical activities always have TF = 0 and FF = 0.
Can a project have more than one critical path?
Yes, and it is quite common on complex projects. Multiple critical paths exist when two or more parallel chains of activities each have zero total float and the same total duration from project start to finish. When a project has multiple critical paths, every activity on all of those paths must be managed with equal rigor — a delay on any one of them delays the project finish date. Multiple critical paths also increase schedule risk because there are now several chains that could independently cause overruns. Projects with near-critical paths (very small positive float) require close monitoring even if they are not technically critical.
What is the difference between CPM and PERT?
CPM (Critical Path Method) uses a single deterministic duration for each activity and focuses on identifying the critical path and float values. It was developed for construction and engineering projects where durations are well-known from historical data. PERT (Program Evaluation and Review Technique) was developed for research and development projects with high uncertainty, using three time estimates — Optimistic, Most Likely, and Pessimistic — to compute an expected duration and variance. PERT adds a probabilistic dimension: by summing variances on the critical path you can estimate the standard deviation of the project finish date and calculate the probability of meeting a deadline. Both methods use the same CPM scheduling algorithm; PERT simply adds the three-point estimation layer.
What does Independent Float mean and when should I use it?
Independent Float (IFF) is the most conservative float measure. It is the amount of time an activity can slip assuming all predecessors finish as late as possible (at their LF) and all successors start as early as possible (at their ES). Because it uses the worst-case scenario for both ends, IFF is often zero or negative (clamped to zero). It represents the slack that is truly 'owned' by the activity with no dependency on how surrounding activities are scheduled. Independent Float is most useful when activities are managed by different teams or contractors, and you need to guarantee that an activity can be delayed without any coordination with adjacent teams. Enable it via the 'Show Independent Float' checkbox.
What happens if I enter a circular dependency?
A circular dependency occurs when activity A depends on B, B depends on C, and C depends on A — forming a loop. This is logically impossible in a real project because it would mean no activity can ever start. The calculator detects circular dependencies using a topological sort algorithm (Kahn's algorithm). If the topological sort cannot order all activities — because one or more form a cycle — the calculator displays a circular dependency error and does not produce results. To fix this, review your predecessor assignments and break any cycles. Common causes include accidentally typing the wrong activity name as a predecessor or misunderstanding a dependency direction.
How do I use this calculator to speed up (crash) a project?
Project crashing is the process of reducing a project's duration by adding resources to critical activities at increased cost. To crash a project manually using this calculator: first note which activities are on the critical path (TF = 0). For each critical activity, determine how much it can be shortened (crash duration) and at what cost (crash cost minus normal cost divided by days saved = cost slope). Then reduce the duration of the critical activity with the lowest cost slope by one period, re-run the calculator, and check whether the critical path has changed. Repeat until you reach your target duration or all critical activities are fully crashed. The calculator auto-recalculates on every change, so you can iterate quickly by directly editing the duration of each activity.