Slack / Float
Slack (also called Float — the two words mean the same thing) tells you how much an activity can be delayed without pushing back the project's finish date. It is one of the most practical numbers in scheduling because it shows you exactly where you have breathing room.
You can calculate it two equivalent ways, and both give the same answer. Either subtract the Early Start (the earliest an activity can begin) from the Late Start (the latest it can begin): Float = LS − ES. Or subtract the Early Finish (earliest it can end) from the Late Finish (latest it can end): Float = LF − EF. Because the early numbers come from the forward pass and the late numbers come from the backward pass, float is simply the gap between "as soon as possible" and "as late as allowed."
Reading the result is simple. A float of zero means the activity has no room to slip at all — any delay pushes the whole project late. These zero-float activities form the "critical path," the sequence that controls the project's end date. A positive float, say 4 days, means that activity could be delayed up to 4 days before it starts to matter.
Project managers lean on float constantly. It tells them where they can afford a hiccup and where they absolutely cannot. When resources are stretched thin, a manager can borrow time from activities with plenty of float to protect the critical, zero-float ones.
Think of it like the buffer time you leave before a flight. If your flight is at 3 p.m. and you could safely arrive at the airport anytime between 1 p.m. and 2 p.m., you have one hour of float — you can afford to be an hour late leaving home. But if you must leave at exactly the earliest time to make it, your float is zero and any delay means you miss the plane.
Say Activity C can start as early as day 5 (Early Start = 5) but as late as day 8 (Late Start = 8). Its float is: 8 − 5 = 3 days. Let's double-check with the finish figures: if its Early Finish is day 7 and its Late Finish is day 10, then 10 − 7 = 3 days — the same answer. This tells the project manager that Activity C could slip up to 3 days without delaying the project. If a different activity had ES of day 5 and LS of day 5, its float would be 0, meaning it's on the critical path and cannot be delayed at all.
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