Backward Pass — Late Finish (LF)
Every project is made up of activities (tasks) that connect to one another, often in a chain. Some activities can't start until others finish. To figure out our scheduling flexibility, project managers do two "passes" through the schedule. The forward pass works from the start of the project to the end and finds the earliest times each activity could happen. The backward pass works in reverse, from the project's end date back to the start, and finds the latest times each activity could happen without pushing the whole project late.
Late Finish (LF) is one result of that backward pass. It answers a simple question: "What is the very latest this activity can finish and still not delay the finish of the entire project?" To calculate it, you look at the activity that comes right after (called the "successor") and take its Late Start (LS) — the latest that next activity can begin. Then you subtract 1. We subtract 1 because in this counting method each day is a whole unit, so the current activity must wrap up one day before the next one is allowed to begin.
How do you read it? A larger LF (further out in time) means the activity has room to breathe — it can finish later without harm. If the LF is the same as the activity's Early Finish (the earliest it could finish), then there is no wiggle room at all and the activity is time-critical.
A project manager uses LF to spot which activities are tight and which have some cushion, so they know where a delay would hurt the project and where it wouldn't.
Think of it like a relay race where the next runner is allowed to start no later than a certain moment. The runner before them must hand off the baton just before that deadline — the latest they can finish their leg is one step before the next runner's absolute latest start.
Imagine Activity B must start no later than day 10 — that is, its Late Start (LS) is day 10. Activity A comes right before Activity B (A is B's predecessor, B is A's successor). To find the Late Finish of Activity A, we take B's Late Start and subtract 1: LF = 10 − 1 = 9. So Activity A can finish as late as day 9 without delaying Activity B, and therefore without delaying the whole project. If A finishes on day 10 or later, it would force B to start late, and the project would slip.
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