How to calculate the critical path: a worked renovation example

To calculate the critical path, list every task with its duration and what it waits for. Work out each task's earliest start and finish (forward pass), then its latest start and finish (backward pass). Tasks whose earliest and latest dates match have zero float. Together they form the critical path, the chain that decides the finish date.

What do you need before calculating a critical path?

You need 4 inputs per task, and the result is only as good as the weakest of them:

1. A task list. Each task should be one piece of work with a clear end, such as "Install the electrics", not "Kitchen".
2. A duration in working days. Use realistic estimates, not best-case ones.
3. What each task waits for. Write down which tasks must finish before it can start. A task can wait for more than one.
4. A working calendar. Decide which days count. Here it is Monday to Friday, so weekends are skipped.

The GAO Schedule Assessment Guide treats a valid critical path and reasonable float as best practices for project schedules. Both depend on these inputs being complete.

What does the flat renovation example look like?

The example is a bathroom and kitchen renovation in a flat, starting on Monday 2 November 2026. The dates were calculated by tokidu's date engine. The calendar is Monday to Friday, "1 day" means 6 hours of work, and each person does one task at a time.

The project finishes on Friday 20 November 2026. Plastering cannot start until both the plumbing and the electrics are done, and the final clean waits for both painting and the fixtures. Those 2 merge points are where the calculation needs care.

How do you calculate the critical path by hand in 4 steps?

Count in working days from the start. Day 0 is the start of Monday 2 November, so a task finishing at "day 2" ends on the evening of the second working day.

1. Run the forward pass. Give each task an earliest start equal to the latest earliest finish among the tasks it waits for. Earliest finish is earliest start plus duration. At a merge point, take the biggest number.
2. Run the backward pass. Set the project's finish (day 15) as the latest finish of the last task. Work backwards: latest start is latest finish minus duration. A task's latest finish is the smallest latest start among the tasks waiting for it.
3. Work out the float. Float is latest start minus earliest start. It is how long a task can slip without moving the finish date.
4. Join the zero-float tasks. Follow the tasks with zero float from start to finish. That chain is the critical path.

Which tasks are on the critical path of the flat renovation?

The calculation gives this result for the example. Numbers are working days counted from the start.

The critical path runs through demolition, electrics, plastering, tiling, then painting and fixtures in parallel, and finally the clean. Its durations add up to 15 working days, which is exactly 3 working weeks from Monday 2 November to Friday 20 November.

Only the plumbing is off the critical path. It is shorter than the electrics and both lead into plastering, so plastering waits for the electrics either way.

Why can a renovation have two critical paths at the same time?

Two tasks with the same duration that wait for the same task and feed the same task are both critical. Painting and fitting the fixtures each take 2 days after tiling, and the final clean needs both. If either slips by a day, the clean moves, and so does the end of the project.

This matters on site: watching only one of the parallel trades gives a false sense of safety. Both Lucía and Marta must finish on time.

What does one day of float on the plumbing mean in practice?

One day of float means the plumbing can finish one working day later than planned and plastering still starts on Monday 9 November. Use that day as a buffer for a delivery delay or a surprise behind the old tiles.

Float is not free time to hand out twice. If the plumbing uses its spare day, it becomes critical too, and any further delay pushes the finish date.

Which 4 mistakes give a wrong critical path?

1. Missing a dependency. If plastering is linked only to the plumbing, the electrics seem to have float and the finish date looks a day earlier than it really is.
2. Mixing calendar and working days. Counting weekends as working time shortens the plan and moves every date.
3. Ignoring who does the work. The classic method assumes unlimited people. Here Pablo's 3 tasks already follow each other through dependencies, but if one person had 2 parallel tasks, one would have to wait.
4. Taking the wrong value at merge points. The forward pass takes the largest earliest finish; the backward pass takes the smallest latest start. Swapping them hides critical tasks.

For how dependencies are named and chained, see the predecessor and successor tasks guide and the 4 types of task dependency.

How do you check a hand-calculated critical path in software?

Microsoft documents how to show the critical path in Project, and Planner's timeline view covers dependencies and the critical path. Enter the same tasks, durations and dependencies, and compare the result with your hand calculation.

In tokidu, the project manager by Sinergia Barcelona, the ‘Map’ shows the waits between tasks and marks with a thick border the tasks that decide when the project finishes. Tapping a planned date opens ‘Why that date?’, a sentence saying what pushes it: which task it waits for, what that person has first, or which days they do not work. Both are described with the Map and date features. To draw the plan as bars, follow the free online Gantt chart steps; to work out the end date itself, see when a project will finish.