The plain-English guide
The critical path method
The longest chain of dependent tasks sets the earliest project finish. Learn how to find it and use it to assess the effect of a delay.
From the studio behind ganttchart.ai · a 6-minute read
The critical path is the longest chain of dependent tasks in a project — the run of work where each task can't start until the one before it finishes. Add up the durations along that chain and you have the shortest time the whole project can possibly take. Every other task is running alongside it with room to spare.
The critical path method (CPM) is the procedure for finding that chain. A day of delay on it pushes the finish back by a day. Other tasks have slack that can absorb some delay. A Gantt chart can show both the critical tasks and those with slack.
Trace the chain
Find it by eye
One real plan — the six-week bakery-website launch. Two tasks run with slack; the highlighted run is the critical path. Follow it and the end date explains itself.
Copy and Photos each run alongside the chain — either could slip a week without moving the launch. That freedom is their slack.
The method, in five moves
How to find the critical path
No software required for a plan this size. The whole method is a list, some durations, and the discipline to add up the longest chain.
- 01
List the tasks
Break the job into pieces small enough to estimate. This is your work breakdown .
- 02
Give each one a duration
Write down how long each task actually takes — its duration, in days or weeks. Use estimates based on the work and time available.
- 03
Map what waits on what
Draw the dependencies: which tasks can only start once another finishes. These arrows are what turn a list into a network of chains.
- 04
Add up the longest chain
Follow each unbroken chain of arrows from start to finish and total its durations. The longest total is the critical path — the run that sets the earliest the project can possibly end.
- 05
Mark the zero-slack tasks
Every task on that longest chain has zero slack: slip one by a day and the whole project slips a day. Check these tasks closely when reviewing the schedule.
Want to watch the method run on five different plans? See the worked examples →
How it helps a schedule review
Where to spend your attention
The critical path shows where a delay would affect the finish date. Shortening a task off that path may leave the finish unchanged, while shortening a critical task can bring it forward.
Use it to decide where to allow a buffer before the finish, which task updates need close attention, and how much delay other tasks can absorb within their slack allowance.
One warning: the critical path moves. Shorten the longest chain and a different one becomes longest; let a slack task slip far enough and it joins the path. Recalculate it after schedule changes so the highlighted tasks reflect the current plan.
Common questions
The critical path, briefly
What is the critical path method, in one sentence?
The critical path method (CPM) finds the longest chain of dependent tasks in a project, which determines its earliest possible finish date.
How do you calculate the critical path?
By hand: list every task with its duration, draw the dependencies (which task waits on which), then trace each unbroken chain of tasks from the first to the last and total the durations along it. The chain with the largest total is the critical path, and its length is the shortest time the project can take. Formally, software does a "forward pass" to find each task's earliest start and a "backward pass" to find its latest start; any task where those two are equal has zero slack and sits on the critical path.
What's the difference between the critical path and the critical chain?
The critical path is the longest chain of dependent tasks by duration. Critical chain project management (CCPM), from Eliyahu Goldratt, is a later refinement that also accounts for shared resources — two critical tasks that need the same person can't truly run in parallel — and gathers each task's private padding into one shared buffer at the end. For most small plans the plain critical path is all you need; critical chain earns its complexity on big, resource-constrained programs.
Can a project have more than one critical path?
Yes. If two separate chains happen to have the same longest total, both are critical — every task on either one has zero slack. It also happens the moment you shorten the longest chain: a different chain becomes the new longest, so the critical path moves. That is why it is worth recomputing after any real change, rather than treating the first critical path you found as permanent.
Do I have to calculate the critical path by hand?
No — and past a handful of tasks you shouldn't. On a small chart you can trace the longest chain by eye; it is the longest unbroken run of arrows from the first bar to the last. Beyond that, let software compute it: describe the project to ganttchart.ai and it draws the plan with the critical path highlighted, and recomputes it the moment a date changes.
Every term above is defined, jargon-free, in the plain-English glossary.
Let the chart do the arithmetic
Describe your project in a sentence and ganttchart.ai draws the plan with the critical path already highlighted — and recomputes it the moment a date moves, so the longest chain is always the one you're looking at.
Try ganttchart.ai →