CPM Scheduling: Activities, Relationships, Float, and the Critical Path That Defines Project Duration
Critical path method (CPM) scheduling is how commercial construction projects get planned and tracked. Activities with durations and predecessor relationships produce a network that calculates earliest and latest start and finish dates, activity float, and the critical path — the longest path through the network, which determines project duration. CPM drives project coordination, claim analysis, and schedule-related decisions.
Understanding CPM basics is essential for anyone working on scheduled construction projects. This post covers the fundamentals — activities, relationships, forward and backward passes, float, critical path — and the common mistakes that produce misleading schedules. The mechanics are specific; getting them right matters.
Activities are schedule building blocks:
Activity characteristics
- Specific scope of work
- Defined duration
- Assignable responsibility (subcontractor or self-perform)
- Measurable completion
- Bounded start and finish
- Not too large — typically 5-20 working days
- Not too small — typically not under 1 day
Activity sizing matters. Too-small activities ("install drywall on wall 123A") produce unmanageable schedule detail. Too-large activities ("finish interiors") lose schedule precision. Right-sized activities support coordination without overwhelming schedule management.
Relationships define logic:
Relationship types
- Finish-to-Start (FS) — most common; successor starts when predecessor finishes
- Start-to-Start (SS) — both start together
- Finish-to-Finish (FF) — both finish together
- Start-to-Finish (SF) — rarely used
- Leads and lags — modify relationship timing
- Multiple relationships per activity possible
FS relationships dominate. Concrete foundations must finish before framing starts. Roof sheathing must finish before roofing starts. Most construction logic is sequential FS.
Forward pass calculates early dates:
Forward pass calculation
- Start at project start date
- Earliest Start (ES) of first activity = project start
- Earliest Finish (EF) = ES + Duration
- Next activity's ES = predecessor's EF (for FS)
- When multiple predecessors, take latest of predecessors
- Continue through network
- Last activity's EF = earliest project completion
Forward pass determines the earliest each activity can happen given schedule logic. Every activity has ES and EF — these are the earliest possible dates without violating any relationships.
Backward pass calculates late dates:
Backward pass calculation
- Start at project end date
- Latest Finish (LF) of last activity = project end
- Latest Start (LS) = LF - Duration
- Predecessor's LF = successor's LS
- Multiple successors — take earliest of successors
- Continue backward through network
- First activity's LS = latest possible project start
Backward pass determines the latest each activity can happen without extending project completion. LS and LF represent the latest possible dates.
Float measures schedule flexibility:
Float calculations
- Total Float (TF) = LS - ES (or equivalently LF - EF)
- Free Float (FF) = earliest ES of successor - EF
- Critical activities have zero float
- Activities with float can shift within their float without delaying project
- Negative float means project is behind schedule
- Float ownership — typically project, not individual activities
Float represents flexibility. An activity with 10 days of float can start up to 10 days late without delaying project completion. Zero-float activities (critical) drive completion directly.
Critical path is the defining concept:
Critical path characteristics
- Longest path through network
- Activities with zero total float
- Sum of durations equals project duration
- Multiple critical paths possible
- Delay to any critical activity delays project
- Critical path can shift as project progresses
- Near-critical paths should be monitored
Critical path is where project duration lives. Adding duration to any critical activity extends project. Shortening critical activities shortens project (until a different path becomes critical). Project management focuses heavily on critical path.
The critical path isn't fixed for project duration — it can shift. An activity not currently critical can become critical if it experiences delay. Near-critical activities (low float) are watch items because they can become critical with modest disruption.
Get AP insights in your inbox
A short monthly roundup of construction AP + accounting posts. No spam, ever.
No spam. Unsubscribe anytime.
Schedule Updates
Schedules update as work progresses:
Schedule update practices
- Regular updates (weekly or biweekly typical)
- Actual start and actual finish dates entered
- Remaining duration updated for in-progress activities
- Logic changes documented
- Schedule recalculated with updates
- Critical path may shift with updates
- Baseline schedule preserved for comparison
Updated schedules reflect current reality. Baseline schedule is preserved for comparing actual vs plan. Variance from baseline surfaces problems early.
CPM scheduling has common errors:
Common CPM errors
- Missing logic — activities without predecessors or successors
- Open-ended activities — no logic, float unrealistically high
- Logic loops — activity precedes itself through chain
- Fragmented network — disconnected sub-networks
- Constrained dates overriding logic — manual dates masking true critical path
- Durations not matching actual work
- Float manipulation — forcing specific activities to appear critical or not
Each error produces misleading schedule. Missing logic makes activities appear flexible when they're not. Fragmented networks hide dependencies. Manual date constraints create fake float. Scheduler review catches these; software also flags them.
CPM uses specialized software:
Scheduling software options
- Primavera P6 — industry standard for large projects
- Microsoft Project — common on smaller projects
- Asta Powerproject — used in UK and internationally
- Procore Schedule — integrated with construction management
- Specific features vary — constraints, resource loading, baselines
- File interchange (XER, XML) between systems
Primavera P6 dominates large commercial and infrastructure. MS Project is common for smaller projects. Software choice affects features available and integration with other systems.
Schedules have evidentiary role:
Schedule evidentiary uses
- Baseline schedule — agreed project plan
- Schedule updates — actual progress
- Time impact analysis — delay claim support
- As-built schedule — what actually happened
- Schedule comparison in disputes
- Expert schedule analysis in litigation
Schedules are contract documents on most commercial projects. Baseline schedule gets approved; updates reflect progress; deviations support claims. Schedule documentation discipline matters for disputes.
CPM scheduling uses activities, relationships, forward and backward passes, float calculations, and critical path identification to produce project schedules. Activities are right-sized with specific scope and duration. Relationships (predominantly FS) define sequence. Float represents flexibility; critical path activities have zero float. Schedule updates reflect progress. Common errors — missing logic, loops, constrained dates, fragmented networks — produce misleading schedules. Specialized software (Primavera P6, MS Project) supports CPM. Schedules serve as evidence in claims and disputes. Understanding CPM basics is essential for anyone working on scheduled construction projects. The mechanics are specific and unforgiving — correct application produces schedules that support coordination and claim analysis; incorrect application produces plans that mislead teams and fail in disputes.
Written by
Marcus Reyes
Construction Industry Lead
Spent twelve years running AP at a $120M general contractor before joining Covinly. Lives in the world of AIA G702/G703, retainage schedules, and lien waiver deadlines. Writes about the construction-specific workflows that generic AP tools get wrong.
View all posts