Home / Tutorials / Critical Path Method in AutoCAD, Explained
Video 5 of 7 · 6:02 · Watch in order

Video 5: Critical Path Method in AutoCAD, Explained

Before you trust a schedule, it helps to understand the math behind it. This tutorial walks through the Critical Path Method itself, the forward pass, the backward pass, and how float and the critical path come out of that calculation inside CPMCAD.

How to understand the critical path method behind CPMCAD

  1. Model the project as a network. Every activity has a duration and is connected to other activities through logical relationships. From that network, CPM calculates the minimum project duration and identifies which activities control the timeline.
  2. Run the forward pass. Starting from the project start date, CPMCAD calculates the earliest possible start and finish for every activity, working forward through the network. Where an activity has multiple predecessors, the latest of their finish dates governs its start.
  3. Run the backward pass. Starting from the project end date, CPMCAD works backward to calculate the latest allowable start and finish for every activity without extending the project. Constraints such as Finish No Later Than override the date that would otherwise come from successor logic.
  4. Calculate Total Float and Free Float. Total Float is Late Finish minus Early Finish, how much an activity can slip without delaying the project. Free Float measures how much it can slip without affecting its immediate successor. An activity can have a lot of Total Float and zero Free Float at the same time.
  5. Identify the critical path. The critical path is the longest chain of activities through the network, every one of them with zero Total Float. A one day delay on any critical activity moves the entire project completion date by one day.
  6. Know what CPMCAD does and does not do. CPMCAD implements the core CPM algorithm correctly, forward pass, backward pass, both float values, four relationship types with lags, and six constraint types. It is not a full enterprise resource leveling or portfolio management platform, that is a job for tools like Primavera P6 or Microsoft Project.

What you will see in the video

CPMCAD network diagram showing the critical path
The critical path traced through the network, every activity on it carrying zero Total Float.
Gantt chart distinguishing critical and non critical activities
Critical and non critical activities are visually distinct on the Gantt chart.
Video overview (from the production script)
The Critical Path Method is a scheduling technique developed in the late 1950s and remains the foundation of project scheduling across construction, engineering, and infrastructure. At its core, CPM is a network analysis technique. You model a project as activities connected by logical relationships, each with a duration, and from that network you calculate the minimum project duration and identify which activities control the timeline. The forward pass calculates the earliest possible start and finish for every activity, working from the project start date forward through the network. Each activity's Early Start is determined by the latest Early Finish of all its predecessors, since you cannot start until every predecessor allows it. The backward pass works in reverse, starting from the project end date and calculating the latest allowable start and finish for every activity without extending the project. Constraints modify this pass directly, a Finish No Later Than date becomes the Late Finish for that activity rather than a date derived purely from successor logic. With both passes complete, Total Float is Late Finish minus Early Finish, and Free Float measures how much an activity can slip before its immediate successor is affected. The two numbers answer different questions, and confusing them leads to bad scheduling decisions. The critical path is the set of activities where Total Float equals zero, the longest path through the network, and any delay there delays the whole project. CPMCAD implements the full CPM algorithm, forward pass, backward pass, separated Total and Free Float, all four relationship types with lags, and six constraint types, giving you a correctly calculated schedule with native AutoCAD output. Note: this summary is adapted from the production script for this video.

Try it on your own schedule

CPMCAD is a free trial with up to 10 activities, or $250/year for unlimited activities and clean, watermark-free output.

Download Trial Version Purchase Professional Version