Forward Pass — Early Start (ES)
Early Start, or ES, is the earliest day an activity can possibly begin. An activity usually can't start until the task before it — called its "predecessor" — is finished. ES captures exactly that: the moment the previous work is done, this one is free to begin.
You calculate it by taking the Early Finish (EF) of the predecessor — the earliest day that prior task can be completed — and adding 1. The formula is ES = EF of predecessor + 1. You add 1 because the new activity starts on the very next day after the previous one finishes; they don't share the same day. If a task has no predecessor (it's the first activity in the project), its ES is simply day 1.
To read the result, remember a smaller ES means the task can begin earlier, and a larger ES means it must wait longer because more work stacks up ahead of it. By itself ES isn't "good" or "bad" — it's a stepping stone you use, along with the activity's duration, to calculate its Early Finish, and then the Early Start of whatever comes next.
A project manager uses ES as part of the "forward pass" — working through the schedule from start to end — to build the whole timeline. It reveals the soonest each piece of work can realistically kick off, which is essential for planning resources, setting expectations, and spotting the earliest date the project could finish.
Think of it like a relay race. The next runner can't start sprinting until the runner before them crosses the handoff line. The instant the baton is passed — the very next step — the new runner takes off. Adding 1 is simply that clean handoff: one task ends, and the next begins the following day.
Imagine the activity "Install flooring" must happen before "Paint the walls." You've already calculated that flooring has an Early Finish (EF) of day 4 — the earliest it can be completed. Apply the formula: ES of painting = EF of flooring + 1 = 4 + 1 = 5. So the earliest the painting can start is day 5, the day right after the flooring wraps up. You'd then use that ES of 5 to work out the painting's own Early Finish and keep the chain going.
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