Primavera P6 Critical Path explained.

What the critical path actually is

The critical path is the longest sequence of dependent activities that determines the earliest possible completion date of the project. Any delay to any activity on the critical path delays project completion by the same amount — unless recovery action is taken elsewhere. Any delay to a non-critical activity may not affect completion at all, as long as it stays within the activity's total float.

This is the Critical Path Method (CPM). It underpins virtually all serious project scheduling on capital projects. Primavera P6 is the tool the discipline is built around on EPC, oil & gas, infrastructure and power projects.

The forward pass: early dates

P6 runs the forward pass first. Starting from the project start date (or a specific activity's constraint), it walks the network in dependency order, calculating for each activity:

• Early Start (ES) — the earliest date the activity can start, given its predecessors and calendars
• Early Finish (EF) — ES plus the activity's duration, adjusted for the activity's calendar

When an activity has multiple predecessors, its ES is driven by the latest of the predecessors' imposed dates — the “maximum” rule.

The backward pass: late dates

Once the forward pass is complete, P6 walks the network in reverse from the project finish. For each activity it calculates:

• Late Finish (LF) — the latest date the activity can finish without delaying the project
• Late Start (LS) — LF minus the activity's duration, adjusted for calendar

When an activity has multiple successors, its LF is driven by the earliest of the successors' imposed late dates — the “minimum” rule.

Total float and free float

With the forward and backward passes complete, P6 computes float for every activity:

• Total Float (TF) = Late Start − Early Start (equivalently, LF − EF). How much the activity can slip without delaying the project completion.
• Free Float (FF) = Earliest of successors' ES − this activity's EF. How much the activity can slip without delaying any successor's early start.

Total float is the more commonly reported number, but free float is what tells you whether slipping an activity actually affects downstream work in the short term.

The float & critical path calculator walks through a small worked example with both.

What makes an activity critical

By default, P6 flags an activity as critical if its total float is zero or less. You can raise the threshold (e.g. TF ≤ 5 days) in the Scheduling Options if you want to see near-critical work highlighted too — and on any serious project review, you should.

Critical activities are typically shown in red on the Gantt bar. Near-critical activities are the ones with small positive total float; they don't look urgent today but one bad week can push them onto the critical path.

The two scheduling options that change the answer

P6 has two calculation settings that produce materially different critical-path results. Every planner should understand both:

1. Retained Logic vs Progress Override

When an activity has been actualised partially, its remaining work needs to be re-scheduled. Two philosophies:

• Retained Logic — the remaining work waits for its predecessors' original logic. If the predecessor hasn't finished, the remaining work is delayed. Realistic for most EPC work.
• Progress Override — the remaining work is treated as if it can proceed regardless of predecessor status. Optimistic; sometimes useful for reporting an "already started so nothing can stop it" view.

Contract-defensible planning almost always uses Retained Logic.

2. Longest Path vs Total Float

Two ways to define critical:

• Total Float ≤ 0 (default) — any activity with zero or negative TF is critical
• Longest Path — only activities on the single longest sequence to completion are flagged critical, regardless of TF

Longest Path is stricter and often clearer on complex networks with multiple sub-networks each showing zero TF.

Common CPM mistakes on EPC schedules

• Constraints hiding the critical path — a Must Finish On constraint can push activities forward artificially, obscuring which work is actually driving the finish
• Dangling activities — activities with no successor (or no predecessor) that break the logic chain and produce meaningless float
• Wrong calendar assignments — a night-shift activity on a day-shift calendar produces wildly wrong ES/EF dates
• Progress override left on by default — makes the schedule look better than it is
• Ignoring near-critical activities — a review that only looks at TF = 0 misses the activities that will be critical in three weeks

Critical path and the rest of the tool

The critical path drives most of the downstream project-controls work:

• Baseline comparison shows how the critical path has changed since the plan was approved
• S-curves derive their forecast completion from the current critical-path finish
• Delay analysis and EOT claims both depend on demonstrating impact on the critical path — not on any activity

FAQ