Critical Path Method (CPM) Calculator
Early/late start and finish, float per activity, and the critical path from a list of durations and dependencies.
Example: A project with 5 activities where Design (5 days) and Procurement (8 days) can run in parallel, but Construction (12 days) can't start until both finish, has a critical path of 20 days total (8 + 12) — Design has 3 days of float since it finishes before Procurement, meaning it can slip by up to 3 days without delaying the whole project.
Critical Path Method, developed by DuPont and Remington Rand in the late 1950s; float = late start − early start for each activity.
How the critical path is calculated
A forward pass through the dependency network computes the earliest each activity could start and finish, given how long its predecessors take. A backward pass from the project's end date computes the latest each activity could start and finish without delaying that end date. Float is the gap between the two — zero float means there's no slack at all, so those activities form the critical path and directly determine the total project duration.
This calculator uses single, fixed durations per activity — the standard CPM approach. If your durations are uncertain and you'd rather work with optimistic/likely/pessimistic estimates instead, see the PERT Estimating calculator. Once you have a schedule and critical path, the Earned Value Management calculator tracks how actual progress compares against that plan as the project runs.
Frequently asked questions
- What does "float" mean?
- How many time units an activity can slip without delaying the overall project. Critical-path activities have zero float — any delay to them delays the whole project. Everything else has slack.
- What happens if my network has more than one critical path?
- If several parallel chains all have zero float, the table below marks every one of them as critical (the isCritical flag), but the single highlighted "critical path" sequence shows just one of those chains, chosen deterministically. Check each activity's float to see the full picture.
- Does this account for resource availability?
- No — this is duration and dependencies only, the standard simple CPM model. It doesn't know if two activities need the same person or equipment at the same time.
- What is the critical path method (CPM)?
- CPM is a project scheduling technique that identifies the longest sequence of dependent activities through a project — the "critical path" — which determines the shortest possible time the whole project can take. It was developed in the 1950s and remains the standard approach for schedule analysis in project management.
- What is float/slack in project scheduling?
- Float (also called slack) is how much an activity can be delayed without pushing back the overall project end date. Total float considers the whole network; free float measures how much an activity can slip without delaying the very next activity specifically. Critical path activities always have zero total float.
- How do I identify the critical path?
- After computing early/late start and finish times for every activity via the forward and backward pass, the critical path is the sequence of activities with zero float, connected end to end from the project start to the project finish.
- What happens if a task on the critical path is delayed?
- Because critical path activities have zero float, any delay to one directly delays the project's overall completion date by the same amount — unless the schedule is actively re-planned (adding resources, changing dependencies, or reducing scope) to absorb the delay.
- How does CPM differ from PERT?
- CPM uses a single, deterministic duration estimate per activity. PERT (Program Evaluation and Review Technique) instead uses three-point estimates (optimistic, most likely, pessimistic) per activity to account for schedule uncertainty — see the PERT Estimating calculator for that probabilistic approach.