Critical path method calculator — free, with float on every task

The critical path is the longest chain of dependent tasks through your project. Its length is your project duration, and every task on it has zero float: slip one day, slip the whole project. This calculator does the forward and backward pass for you and hands back early and late dates, total float and free float on every task, and the critical path marked.

Open the critical path calculator

Free, no signup. Upload an Excel file or start from the sample.

How the calculation works

There is no trick to it, just two passes and a subtraction:

• Forward pass — walk the network from the start. A task's early start is the latest early finish of everything it depends on; its early finish is that plus its duration. The largest early finish in the network is the project duration.
• Backward pass — walk back from the end. A task's late finish is the earliest late start of everything that depends on it; its late start is that minus its duration.
• Total float = late start − early start. How long that task can slip without moving the project end date.
• Free float = how long it can slip without delaying its own successors. Always less than or equal to total float, and it is the number you actually want when deciding whether to move one crew.
• The critical path is the chain where total float is zero.

The part most CPM calculators skip

Classic CPM assumes infinite resources. It will happily tell you that four tasks all start Monday with zero float, when all four need the same two electricians and only two of them can actually start. The moment you apply real crew limits, a task can become critical because of people, not because of logic — and its float against the logical network is meaningless.

This tool computes criticality with crew constraints applied, so the tasks it marks critical are the ones that will actually move your finish date. That is the difference between a CPM homework answer and a schedule you can hand to a foreman.

What it does beyond CPM

• Resource leveling — rearranges the work so you stop needing six people on a three-person crew, and tells you what that costs in days.
• Crashing — if the date is fixed, it picks which tasks to shorten for the least money.
• MS Project XML — import and export, so a plan can round-trip through Project.
• Pin a task — drag a bar to fix its date and everything else recalculates around it.

Common questions

What is the difference between total float and free float? Total float is slack against the project end date. Free float is slack against the very next task. A task can have ten days of total float and zero free float, which means you can delay it only if you delay its successor too.

Can a project have more than one critical path? Yes, and it is common on real plans. Two parallel chains of exactly the same length are both critical, and both have to be shortened together to pull the date in.

Does the critical path change? Constantly. Every time a task finishes early or late, or a crew is short, a different chain can become the longest one. A critical path calculated once at the start of a project is a historical document by week three.

Is CPM the same as PERT? No. CPM uses one duration per task. PERT uses three — optimistic, likely, pessimistic — and gives a probability distribution. This tool is CPM: single durations, exact answers.

Do I need software for a small project? Under about fifteen tasks, a forward and backward pass on paper is genuinely fine and you will understand your plan better for having done it. Past that, the bookkeeping is where mistakes live — and if crews are constrained, paper stops working entirely.