Skip to content
Owais Khan Software Reviews

Critical Path Calculator

Critical path, float and PERT estimates for any task list.

Calculate the critical path

Separate predecessors with spaces. Write a duration as 4/5/8 (optimistic/likely/pessimistic) for PERT.

Project duration

23.83 days

Critical path

Plan → Design → Build → Test → Launch

Chance of finishing by the deadline

91.6%

expected 23.83 ± 1.57 days

Schedule for every activity
ActivityDurationESEFLSLFFloatFree floatTimeline
Plan (critical) 3 0 3 0 3 0 0
Design (critical) 5.33 3 8.33 3 8.33 0 0
Content 4 3 7 11.67 15.67 8.67 0
Build (critical) 10.33 8.33 18.67 8.33 18.67 0 0
Copy 3 7 10 15.67 18.67 8.67 8.67
Test (critical) 4.17 18.67 22.83 18.67 22.83 0 0
Launch (critical) 1 22.83 23.83 22.83 23.83 0 0

Everything stays on this device — your project plan never leaves your browser.

Critical path analysis in four steps

The critical path method finds the longest chain of dependent tasks, which decides the earliest a project can finish. Enter each activity with its duration and what it waits on. The calculator runs the forward pass (earliest start and finish), the backward pass (latest start and finish), and marks every activity with zero total float as critical. The example website launch takes 23.83 days, and only five of its seven activities can delay it.

PERT calculator: odds of finishing on time

Estimates are guesses, so a single duration hides risk. Write a duration as optimistic/likely/pessimistic and this PERT calculator weights them (o + 4m + p) ÷ 6, adds the uncertainty along the critical path, and shows the chance of meeting your deadline. With 26 days available the example has a 91.6% chance, and the same plan against 23.83 days is a coin flip.

Reading float

Float is the slack in the plan. Activities with large float, like Content and Copy in the example, can be resourced flexibly or started late; activities with none need protecting. When two paths tie, both are critical, and the calculator lists them all so you know where a delay would hurt.

Choosing software to manage the schedule

A spreadsheet is fine for a one-off plan; once several people share the schedule you need tooling with dependencies and a timeline. Our comparisons cover project management tools for small teams, construction and marketing agencies.

Frequently asked questions

What is the critical path?
The critical path is the longest chain of dependent activities from the start of a project to its end, and it sets the shortest possible project duration. Every activity on it has zero float: if one of them runs late, the whole project finishes late. Activities off the critical path have float, so they can slip by that amount without moving the finish date.
How do I calculate the critical path?
List each activity with its duration and predecessors. Walk forward: an activity starts when its slowest predecessor finishes (earliest start) and ends a duration later (earliest finish). Then walk backward from the project end to get each latest finish and latest start. Activities where latest start equals earliest start are critical. In the example the critical path is Plan → Design → Build → Test → Launch, 23.83 days long.
What is float (slack) and how is it calculated?
Total float is how long an activity can be delayed without delaying the project: latest start minus earliest start. Free float is narrower: how long it can be delayed without delaying the earliest start of any activity that follows it. In the example, Copy has 8.67 days of total float and the same free float, because nothing on the critical path waits on it until Test. Critical activities always have zero float.
What is the difference between CPM and PERT?
The critical path method (CPM) assumes each activity has one fixed duration and finds the longest path. PERT adds uncertainty: each duration is estimated three ways (optimistic, most likely, pessimistic), weighted as (o + 4m + p) ÷ 6, and the spread of the estimates gives the project a probability of finishing by a given date. This calculator does both: write 5 for CPM or 4/5/8 for PERT.
Can a project have more than one critical path?
Yes. When two chains of activities have exactly the same total duration, both have zero float and both are critical, so a delay on either one delays the project. This calculator lists every critical path it finds. Projects with several critical paths, or with paths that are only slightly shorter than the critical one, carry more schedule risk than the single longest path suggests.
How are three-point estimates and the chance of finishing on time calculated?
Each activity with an o/m/p estimate has a variance of ((p − o) ÷ 6)². Adding the variances along the critical path and taking the square root gives the project’s standard deviation (1.57 days in the example). The chance of finishing by a deadline is the normal distribution evaluated at (deadline − expected duration) ÷ that deviation: 91.6% for 26 days here. It assumes independent activities and ignores near-critical paths, so treat it as an optimistic guide, not a guarantee.
Is my project plan uploaded or processed on a server?
No. The schedule is calculated inside this page, everything you enter never leaves your browser, and there is no sign-up. Your list is saved only in this browser so it is still here when you return. That is enforced rather than promised: this site’s test suite scans the shipped HTML for every browser API capable of sending data off the page and fails the build if it finds one.