Slack Time Calculator

Float, or slack, falls out of two passes over an activity network. A forward pass propagates earliest start and earliest finish dates from the project start; a backward pass propagates latest finish and latest start back from the completion date. Total float is then Latest Finish minus Earliest Finish, which must equal Latest Start minus Earliest Start; if the two disagree, the network logic or the durations are wrong. Activities with zero total float form the critical path, the longest chain through the network and therefore the shortest achievable project duration. Free float is stricter: the earliest start of the nearest successor minus this activity's earliest finish, the delay that disturbs nothing downstream at all. Practitioners read the numbers in bands. Zero float is critical; near-critical is commonly drawn at around 10 working days, because such paths become critical after modest slippage; negative float appears when an imposed constraint or contract date sits earlier than the calculated finish, signalling an infeasible plan that needs crashing, fast-tracking or re-scoping. Two caveats outweigh the rest. Total float belongs to a chain rather than to a single activity, so spending it on one task silently removes it from every other task on the same path. And ordinary CPM float ignores resource limits: once a schedule is resource-levelled, apparent float may be unusable because the crew is committed elsewhere.

The Critical Path Method was developed in 1956 and 1957 by Morgan R. Walker of DuPont and James E. Kelley Jr of Remington Rand Univac, who were looking for a computable way to plan chemical plant shutdowns and maintenance turnarounds. Their formulation, published in 1959, introduced the forward and backward pass and with it the float values this calculation still uses. Almost simultaneously the US Navy Special Projects Office, working with Booz Allen Hamilton and Lockheed, produced PERT in 1958 to schedule the Polaris missile program, differing mainly in its three-point probabilistic duration estimates. Both methods originally drew activities on the arrows of a network. John Fondahl of Stanford published a non-computer approach in 1961 that placed activities on the nodes instead, and this precedence diagramming method became the basis of nearly every modern scheduling package. As the techniques moved into construction contracting, float stopped being a purely technical quantity and became a legal one: whether the owner or the contractor owns the float is a standard contract clause, and delay claims turn on before-and-after float comparisons. AACE International's recommended practices on forensic schedule analysis and successive editions of the PMI body of knowledge codified the vocabulary of total float, free float and near-critical paths that schedulers use today.