What are the key differences between forward pass and backward pass calculations in project networks

Question

I’m brushing up on network diagram techniques—can someone explain the practical difference between forward pass and backward pass calculations when scheduling tasks?

Answer 1

Forward pass and backward pass are two critical computations in the Critical Path Method (CPM) for determining early and late start/finish dates for jobs. Understanding the distinctions aids in identifying work flexibility and timetable pressure.

• Forward Pass – Calculates Earliest Start (ES) and Earliest Finish (EF)

  • Begins at the project start date and moves forward through the network

  • ES = maximum EF of all immediate predecessors

  • EF = ES + task duration

  • Determines how early activities can start and finish without any delay

  • Helps establish the project's earliest possible completion date

• Backward Pass – Calculates Latest Start (LS) and Latest Finish (LF)

  • Begins at the project end date and moves backward

  • LF = minimum LS of all immediate successors

  • LS = LF - task duration

  • Identifies how late a task can be completed without delaying the project

  • Used to calculate total float (slack) and identify critical path tasks (zero float)

The forward pass looks at "how early" things can happen, whereas the backward pass looks at "how late" they can happen.