3.1Critical Path & Float
The essentials in 30 seconds
- The critical path is the sequence of zero-float tasks that sets the finish date: any delay on one of them pushes back delivery.
- Total float = delay possible without moving the finish; free float = without moving the successor; Project calculates both automatically.
- You shorten a project by acting on the critical path: crashing (more resources) or fast-tracking (parallelizing), then rechecking the critical path.
Running mini-case — Meriem secures the delivery of a site
Meriem is a project manager at an engineering & construction (BTP) company in Algiers. She is overseeing the handover of an office building and finds that the delivery date does not move whatever she does on certain tasks. She displays the critical path in the Gantt chart: it is the structural works tasks, then the technical packages, that set the finish — all with zero float. Around them, several tasks (finishes, landscaping) have comfortable float. Meriem decides to focus her vigilance on the critical path and to use the float of the finishes to absorb a materials-delivery contingency, without touching the finish date. She notes the reflex: watch the critical tasks, use the float of the others, and recheck the critical path after each optimization.
Standalone case for this section: it comes back in the lesson, the diagram, the script and the 48h challenge of section 3.1.
1. What is the critical path
The critical path is the longest sequence of tasks in the schedule: it sets the finish date. These tasks have zero float — the slightest day of delay carries straight through to delivery. Meriem spots the red bars in the Gantt chart: they are the tasks that 'command' the end of the project.
2. Total and free float
Outside the critical path, tasks have float. Total float is the delay possible without moving the project finish date; free float is the delay possible without moving the start of the next task. Project calculates them automatically. Meriem uses this float to absorb small contingencies without touching the critical path.
3. Shortening without breaking
To shorten a project, you act only on the critical path: crashing (adding resources, often more costly) or fast-tracking (parallelizing tasks planned in sequence, riskier). Beware: after a compression, a new critical path may appear. Meriem always rechecks the critical path after each optimization.
🔎 Concrete example — 🔎 Concrete example — Shortening without breaking
Meriem wants to gain a week on delivery. Two levers on the critical path:
Crashing : adding resources to critical tasks (costly).
Fast-tracking : parallelizing sequential tasks (increased risk).
-> Afterwards, RECHECK the critical path (a new one may appear).Reading: you act only on the zero-float tasks, the ones that move the finish date.
Lesson: shortening a task with float is useless; and every compression is rechecked.
Section vocabulary
- Critical path
- Sequence of zero-float tasks that determines the project finish date.
Simple example: Meriem spots the red bars of the structural works in the Gantt chart.
What it is not: a finishing task that has several weeks of float.
Reflex: what sets the finish → critical path.
- Total float
- Delay a task can take without moving the project finish date.
Simple example: Meriem shifts a finishing task by three days with no consequence.
What it is not: the delay possible without moving the next task.
Reflex: without moving the finish → total float.
- Free float
- Delay a task can take without moving the start of its successor task.
Simple example: Meriem slides an upstream task without pushing back the next one.
What it is not: the delay possible without moving the project finish date.
Reflex: without moving the successor → free float.
- Compression (crashing)
- Shortening a critical task by adding resources, often at the cost of an overrun.
Simple example: Meriem reinforces a team on a critical task to gain days.
What it is not: shifting a task that already has plenty of float.
Reflex: saving time by paying → crashing.
- Fast-tracking
- Shortening by parallelizing normally sequential tasks, at the cost of increased risk.
Simple example: Meriem launches two technical packages in parallel instead of chaining them.
What it is not: waiting for a task to finish before starting the next one.
Reflex: saving time by parallelizing → fast-tracking.
In practice — In practice — Analysing and using the critical path
- Display the critical path in the Gantt chart (Format > Critical Tasks) and spot the red zero-float bars, like Meriem.
- Add the Total Float column to visualise the available slack on non-critical tasks.
- Identify the critical tasks to watch in priority and the usable float to absorb contingencies.
- To shorten the project, act on the critical path (crashing or fast-tracking) then recheck the critical path.
Step-by-step script — reading the critical path like Meriem (4 min)
1) Display the Gantt chart and enable Critical Tasks. 2) Spot the red bars: they are the zero-float tasks. 3) Add the Total Float column to see the slack of non-critical tasks. 4) Sort: critical tasks to watch, float to use. 5) To save time, compress the critical path (crashing/fast-tracking) then recheck the critical path.
Keep this reflex: what slips on a task with float does not always reach delivery, but what slips on the critical path pushes it back.
Copy-paste template — my critical path sheet
Critical path : zero-float tasks -> set the finish date
Total float : delay without moving the project FINISH
Free float : delay without moving the SUCCESSOR
Compression : crashing (resources) or fast-tracking (parallelize)
Reflex : recheck the critical path after each optimizationA contingency on a task with float
Meriem absorbs a materials-delivery delay in the float of the finishes, without touching the finish date.
A contingency on a critical task
The structural works run a week late: Meriem compresses the critical path or accepts the delay.
An unexpected compression
After parallelizing two tasks, a new critical path appears: Meriem rechecks and monitors it.
A float used
Meriem moves a non-critical task to keep a team busy during a wait, with no consequence for the finish.
✓ Best practices
- Watch critical tasks in priority.
- Use float to absorb contingencies.
- Compress only the critical path, and recheck it.
✗ Common pitfalls
- Shortening a task with float (no effect on the finish).
- Forgetting that a new critical path may appear.
- Thinking that adding resources always divides the duration.
Key takeaways
- The critical path = zero-float tasks that set the finish date.
- Total float (without moving the finish) and free float (without moving the successor).
- You watch the critical tasks and use the float of the others.
- Shortening a project = crashing or fast-tracking on the critical path, then rechecking.
48h challenge — apply it tomorrow
Like Meriem, open a Gantt chart, display the critical tasks and list the ones that set the finish date. Add the Total Float column and note a task with float that you could shift to absorb a contingency. Finally, write in one sentence whether you would choose crashing (more resources) or fast-tracking (parallelizing) to save time.
- I can explain what the critical path is and why it sets the finish.
- I distinguish total float from free float.
- I know how to shorten a project by acting on the critical path (crashing / fast-tracking).
Frequently asked questions
Why focus on the critical path rather than on all tasks?
Because only critical tasks determine the finish date: a delay on a task with float can shift that task without pushing back delivery, whereas a critical delay pushes everything back. Your attention is limited: focusing it on the critical path means watching what can really derail the project.
How do you shorten a late project without breaking everything?
Act only on the critical path. Crashing adds resources (effective but costly, without automatically dividing the time); fast-tracking parallelizes sequential tasks (free but risky). You often combine the two, and you recheck the critical path: compressing can make a new one appear.
What is the critical path?