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What is the critical path method in construction?

The critical path method (CPM) is a way of scheduling a project by linking every activity to the ones it depends on and finding the longest chain from start to finish. On this page: a table of total and free float, a worked forward and backward pass, our view, and a free schedule template.

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Updated November 2026 · Reviewed by the Ruh construction team

The definition

Critical Path Method, Explained.

The longest chain of linked activities is the critical path: it sets the project duration, and any delay to an activity on it delays the finish.

CPM turns a list of activities into dates. Each activity gets a duration and its predecessors. A forward pass works out the earliest each one can start and finish, and a backward pass from the finish date works out the latest. Where the earliest and latest match, the activity has no float and sits on the critical path. Where they differ, the gap is float, the time it can slip without moving the finish.

The method was developed in 1957 by Morgan Walker of DuPont and James E. Kelley Jr. of Remington Rand, and it is still the basis of construction scheduling software.

To link activities and see the dates move, use the free construction schedule template. History: Kelley and Walker's account of the origins of CPM (PMI).

How it is measured

CPM is measured in working days. Each activity has an early start, early finish, late start and late finish, and total float is late start minus early start. The project duration is the length of the critical path.

Total Float vs Free Float

Total float protects the finish date, free float protects the next activity. In typical practice, delay claims are argued on the critical path, where total float is zero. This is not legal advice.

Float matters to the money too: delay on the critical path stretches the job's general conditions, the costs that run by the week.

Total floatFree float
What it measuresSlip possible without delaying the project finishSlip possible without delaying the next activity
On the critical pathZeroZero
Used forFinish-date risk and delay analysisDay-to-day sequencing

Our View

Watch float, not just dates. An activity whose float shrinks week after week is the one about to become critical, and that is the time to act on it.

Worked example

A Worked Critical Path Example.

Illustrative example. Four activities. A takes 3 days. B (5 days) and C (2 days) both follow A. D (4 days) follows both B and C.

A (3 days) then B (5) and C (2) in parallel, then D (4). The longest chain is A, B, D at 3 + 5 + 4 = 12 days. C finishes at day 5 but is not needed until day 8, so it has 3 days of float.

ActivityEarly startEarly finishLate startLate finishTotal float
A, 3 days03030
B, 5 days38380
C, 2 days35683
D, 4 days8128120

Forward pass: A runs 0 to 3, B 3 to 8, C 3 to 5, and D starts after the later of B and C, so it runs 8 to 12. Backward pass from day 12: D must start by 8, so B and C must finish by 8, C can start as late as 8 - 2 = 6, and A must finish by 3.

C has 6 - 3 = 3 days of float. A, B and D have none, so the critical path is A, B, D at 3 + 5 + 4 = 12 days.

How Ruh handles it

How Ruh Handles Critical Path Method.

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Critical Path Method: Frequently Asked Questions

What is the critical path in construction?+

The sequence of activities with zero total float. In the example above it is A, B and D, which is why a day lost on B costs a day at the end and a day lost on C does not.

What is float in a construction schedule?+

Float is the time an activity can slip without delaying the project finish (total float) or the next activity (free float). Activities on the critical path have zero total float.

Who invented the critical path method?+

Morgan Walker of DuPont and James E. Kelley Jr. of Remington Rand developed it in 1957. Their first paper on it was published in 1959.

Can the critical path change during a project?+

Yes. If a non-critical activity slips by more than its float, it joins the critical path and the old path gains float. Updating the schedule regularly shows when that happens.

Can you do CPM scheduling in Excel?+

Yes, for simple finish-to-start schedules, using the WORKDAY function and predecessor lookups. Lags, overlaps and large networks need scheduling software.

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Related Terms

Look-Ahead ScheduleGeneral conditionsGeneral contractor (GC)SubcontractorChange order

Keep going: A Worked Critical Path Example., the free construction schedule template or explore General Contractor Software.

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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.

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