Understanding the Critical Path Method
The Critical Path Method (CPM) turns a list of activities, durations, and dependencies into a schedule. The forward pass walks the network from the start, computing each activity's Early Start (the latest Early Finish among its predecessors) and Early Finish (ES + duration). The largest Early Finish in the network is the project duration. The backward pass then walks from the finish, computing Late Finish (the earliest Late Start among successors) and Late Start (LF − duration) — the latest each activity can run without delaying the project.
Total float vs. free float: Total float (LS − ES) is how long an activity can slip before the project end date moves. Free float is how long it can slip before the next activity's early start moves — it never exceeds total float. The distinction matters in practice: an activity can have days of total float yet zero free float, meaning any slip immediately disrupts its successor's plan even though the project end survives. This calculator reports both.
The critical path is the chain of activities with zero total float — the longest path through the network, and the sequence that sets the project duration. A day lost on the critical path is a day lost on the project. Networks can carry multiple critical paths at once, which increases schedule risk: recovery effort can't focus on a single chain. Near-critical activities (small float relative to project duration) deserve the same attention, since minor slips promote them onto a new critical path.
Zero-duration activities work here too — enter milestones with a duration of 0 and they'll participate in the passes normally. Dates are expressed in elapsed periods from a project start of 0, the standard classroom convention; map period 0 to your real start date to convert.
Related tools: Build your duration inputs with the PERT Calculator, then track execution against this baseline with the Earned Value Management (EVM) Calculator.