How to use the critical path method calculator
- List your tasks. Give each a name and a duration in days.
- In Must finish first, type the names of the tasks that have to be done before this one can start, separated by commas. Leave it empty for tasks that can start right away.
- Read the project length, the critical path and the table with earliest and latest start and finish times and the float for every task.
- Copy or print the schedule to share with the people doing the work.
Formula
Earliest start = the largest earliest finish among the tasks it waits for (or 0)
Earliest finish = earliest start + duration
Project length = the largest earliest finish
Latest finish = the smallest latest start among the tasks that follow (or the project length)
Latest start = latest finish - duration
Float = latest start - earliest start
A task with zero float is critical. If any critical task takes longer, the whole project takes longer by the same amount.
Building a prototype
Six tasks: Design (5 days), Order parts (3, after Design), Build frame (7, after Design), Wiring (4, after Order parts and Build frame), Paint (2, after Build frame) and Final test (2, after Wiring and Paint).
The longest chain is Design, Build frame, Wiring and Final test: 5 + 7 + 4 + 2 = 18 days, so the project takes 18 days. Order parts can start on day 5 and must finish by day 12, so it has 4 days of float (earliest finish 8, latest finish 12). Paint can start on day 12 and has to finish by day 16, which gives 2 days of float. Those two tasks can slip without moving the end date; the other four cannot.
What float tells you
Float is the amount a task can slip without delaying the project. It shows where you have room to be flexible and where you do not. A manager can use the critical tasks to decide where to put the best people or to guard against delay, and the tasks with float to level out workload or absorb surprises.
The critical path is the longest chain of tasks that wait for each other. More than one such chain can exist, and the tool marks every task with zero float. To draw the plan on a calendar, send the same tasks to the Gantt Chart Maker, and use the Project Planner Template to track them.
Limits of the method
The method treats each duration as one fixed number and assumes there are enough people and equipment to run parallel tasks together. Real projects share resources, so a plan can be longer than the critical path suggests. Adding a buffer to the end is often wiser than trusting a single estimate for every task.
Dependencies must not form a loop, since a task cannot wait for itself through other tasks. If a loop is found, the page says so and asks you to remove a link. For status reports on a running project see the Project Status Report Template, and the RACI Matrix Template helps assign responsibility.
Assumptions and limits
- Every task starts as early as its dependencies allow, and all dependencies are finish-to-start.
- Durations are whole or decimal numbers of days, and the schedule counts days from day 0 with no calendar.
- Resources are unlimited. Two tasks that can run at once are assumed to run at once.
- Up to 30 tasks are supported, and task names are not stored or put in share links.
Frequently asked questions
What is the critical path?
It is the longest chain of tasks that must happen one after another. Its total length is the shortest time the project can take, and any delay on it delays the finish.
Can there be more than one critical path?
Yes. If two chains have the same total length, both are critical. The table marks every task with zero float, and the headline lists one chain.
What does it mean when a task has float?
The task can start later or take longer by that many days without changing the end date, as long as nothing else changes. Using up the float makes the task critical.
Why do I get an error about a loop?
A loop means task A waits for B, B waits for C and C waits for A, directly or through longer chains. No schedule can satisfy that, so remove one of the links in the Must finish first column.