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Critical path method explained

Every plan has a handful of tasks where a single day of delay moves the end date, and a long tail where it does not. The critical path method tells you which is which, before the sponsor does.

Step 1: List the tasks, durations and dependencies

The critical path method needs three things for every task: how long it takes, and which tasks must finish before it can start. Everything else follows from those.

Here is a small example with made-up numbers, as an illustration. Task A takes 3 days. Task B takes 4 days and follows A. Task C takes 2 days and follows A. Task D takes 3 days and needs both B and C to finish.

• Use working days and take out bank holidays and known leave.
• Record real dependencies, not preferences about order.
• Add a single start and a single finish so every path joins up.

Step 2: Trace every path

A path is a chain of tasks from start to finish. In the example there are two: A, B, D, and A, C, D.

Add up the durations on each. A plus B plus D is 3 + 4 + 3 = 10 days. A plus C plus D is 3 + 2 + 3 = 8 days. The longest path is the critical path, so the project takes 10 days.

• Critical path: A, B, D at 10 days.
• Non-critical path: A, C, D at 8 days.
• Any delay to A, B or D moves the finish date day for day.

Step 3: Forward and backward pass

The forward pass finds the earliest each task can start and finish. A runs from day 0 to day 3. B runs 3 to 7 and C runs 3 to 5. D cannot start until both B and C are done, so it starts at day 7 and finishes at day 10.

The backward pass works from the finish date to find the latest each task can finish without delaying the project. D must finish by day 10 and start by day 7. That means C must finish by day 7, even though it could finish at day 5.

• Earliest start and earliest finish come from the forward pass.
• Latest start and latest finish come from the backward pass.
• Where the earliest and latest dates are equal, the task is critical.

Step 4: Read the slack

Slack (also called float) is how long a task can slip without moving the end date. Subtract the earliest finish from the latest finish. For C that is 7 - 5 = 2 days. A, B and D have zero.

Zero slack is the signal. Those tasks get your attention, your best people and your earliest warning.

• Zero slack: critical, protect it.
• Some slack: monitor, but you can use it to level resources.
• Negative slack: you are already late against a fixed date.

Step 5: Use it, then keep it current

The critical path moves. If C overruns by three days, it becomes the longest path and the project finishes on day 11. Recheck after every material change.

In Senshin the plan highlights the critical path on the Gantt chart, and what-if simulations let you test the effect of a slipped task before you tell the sponsor.

• Recalculate whenever a duration or dependency changes.
• Report the critical tasks, not the whole plan, to the steering committee.
• Shorten the path by overlapping tasks or adding capacity, and check the cost.

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Last updated: 2026-10-22