Critical Path Method Example: A Step-by-Step Walkthrough

A Quick Refresher: The Five Numbers Behind Every CPM Schedule

New to these terms? The history and terminology guide covers the basics. Here’s the short version of what we’re about to calculate for every activity:

Early Start (ES) and Early Finish (EF)

The soonest an activity can start and finish, found by working forward through the network from day zero.

Late Start (LS) and Late Finish (LF)

The latest an activity can start and finish without pushing back the project finish date, found by working backward from the end.

Total Float (TF)

The cushion an activity has: LS minus ES (or LF minus EF). An activity with zero float is critical — it can’t slip at all without delaying the project.

Worked Example: A Small Sitework-to-Framing Sequence

Here’s a simplified activity sequence for the early phase of a project — mobilization through framing — with logic (which activities depend on which) and durations in days.

Step 1: Forward Pass (Early Start / Early Finish)

Starting at day 0, each activity’s Early Start is the latest Early Finish among everything it depends on. Where two activities feed the same successor — like C and D both feeding E and F — the later of the two governs.

Step 2: Backward Pass (Late Start / Late Finish)

Starting from the project’s finish (day 40, driven by the forward pass), work backward: an activity’s Late Finish is the earliest Late Start among everything that depends on it.

Step 3: Total Float and the Critical Path

Subtracting Early Start from Late Start gives Total Float for each activity. Here’s the complete table:

Critical path: A → C → E → G → H → I → J, for a 40-day project duration. Every activity on that chain has zero total float. B and D form a parallel branch with 4 days of float each — they could each start up to 4 days late (combined) without affecting the finish date. F has 3 days of float for the same reason: the parallel footings-and-walls path (E-G) simply takes 3 days longer.

Here’s every activity in the worked example redrawn as that same box, all at once — this is called an activity-on-node (AON) diagram, one of the terms from the history and terminology guide. The arrows carry the same dependencies as the network diagram earlier; the boxes now carry the dates too.

Try It Yourself: A Small Renovation Sequence

Here’s a smaller network to practice on. Work out the Early Start/Finish, Late Start/Finish, and Total Float for each activity, then identify the critical path — the answer is below when you’re ready to check your work.

Frequently Asked Questions

What is the critical path in a CPM schedule?

The critical path is the longest chain of dependent activities from the start of a project to its finish. Every activity on it has zero total float, meaning any delay to one of them delays the whole project's finish date.

How do you calculate total float?

Total float equals an activity's Late Start minus its Early Start (or equivalently, Late Finish minus Early Finish). Early dates come from a forward pass through the network; late dates come from a backward pass.

What is the difference between total float and free float?

Total float is how much an activity can slip without delaying the whole project's finish. Free float is the smaller amount it can slip without delaying the early start of its very next successor activity.

Can a schedule have more than one critical path?

Yes. Whenever two or more paths through the network both work out to zero total float, they are all critical simultaneously, and a delay to any one of them delays the project.

What does negative total float mean?

Negative total float means the schedule's logic and durations already show the project finishing later than a required or contractual date. It's a red flag that shows up immediately in a schedule health check.