The forward pass identifies the earliest dates on which project activities can start and finish. The backward pass answers a different question: how late can each activity occur without delaying the required project completion date?

This analysis calculates two additional values: Late Finish (LF) and Late Start (LS). Together with the early dates from the forward pass, they reveal how much float each activity has and help identify the project’s critical path.

Work Backward from the Completion Date

The backward pass begins with the final activity in the network. Suppose Level 5 Integrated Systems Testing is the final activity in a data-center schedule, and the forward pass has established Day 320 as the earliest project completion date. In that case, the activity’s Late Finish is anchored to Day 320.

The analysis then moves from right to left through the network. The basic formula is:

Late Start = Late Finish − Duration + 1

The addition of one reflects the same inclusive-day convention used in the forward pass. If testing has a duration of 10 working days and must finish by Day 320, its latest allowable start is Day 311: Days 311 through 320 represent 10 working days.

That Late Start becomes the Late Finish for the activity’s predecessor. The calculation continues backward through the schedule until every activity has a latest allowable start and finish date.

When Several Paths Converge

The backward pass also requires careful attention when one activity has multiple successors. Consider the installation of main electrical switchgear, which feeds both the UPS system and the mechanical chiller plant.

If the UPS cable pull has a Late Start of Day 200 and the chiller cable pull has a Late Start of Day 215, the switchgear’s Late Finish is Day 200—the smallest of the successor Late Start dates.

Why? Because the switchgear must be complete in time to support both successor activities. The tighter requirement controls. Allowing the switchgear to finish on Day 215 would leave the UPS cable pull five days late, potentially delaying downstream commissioning and the project completion milestone.

This is the backward-pass counterpart to the forward-pass merge rule. When several predecessor paths converge during the forward pass, the highest Early Finish controls. When an activity has several successors during the backward pass, the lowest Late Start controls.

From Dates to Float and Critical Path

Once the forward and backward passes are complete, float can be calculated by comparing the early and late dates. An activity with no difference between its early and late start dates has zero total float. It has no scheduling flexibility without affecting the controlling completion date and is therefore part of the critical path.

The backward pass transforms the schedule from a collection of possible dates into a map of allowable risk. It shows which activities have room to move, which activities require close protection, and where a delay could affect the project’s contractual completion date.

The calculations are simple, but the discipline behind them matters. A schedule is not an abstract spreadsheet; it is a model of the physical sequence required to deliver the project. Every late date must be supported by sound logic, realistic durations, and a clear understanding of the work.

The forward pass tells us how early the project can finish. The backward pass tells us how much flexibility exists along the way. Together, they reveal the schedule’s controlling path—and provide the foundation for managing it.