What it does
Every project has a chain of tasks that sets its length: the critical path. Delay any task on it and the whole project slips; tasks off it have float, time they can slip without harm. This tool works it out from a simple table of tasks:
- a forward pass (earliest start and finish) and a backward pass (latest start and finish);
- total float and free float for every task, and the critical path;
- links of all four kinds, finish-to-start, start-to-start, finish-to-finish and start-to-finish, each with a lag (or a lead, a negative lag);
- a Gantt chart, with optional calendar dates that skip weekends;
- with three-point estimates, PERT: the expected duration, its standard deviation and the chance of finishing by a date, plus a Monte Carlo simulation that checks it.
How to use it
- Type or paste one row per task, or open a CSV or Excel file. A header row names the columns: ID, Task, Duration and Predecessors. For PERT, use Optimistic, Most likely and Pessimistic instead of Duration.
- Write predecessors as IDs with spaces between them:
B C.A+2waits 2 days after A finishes;A SSstarts when A starts;A FF+1finishes at least 1 day after A finishes;A SFfinishes after A starts. - Choose the time unit, and a start date if you want calendar dates.
- For PERT, type the time you must finish within.
Worked example
The first example is a small building project with three-point times. Each task’s PERT duration is (o + 4m + p) / 6; for “Draw the plans” (4, 6, 10 days) that is 6.333 days, with a standard deviation of (10 − 4) / 6 = 1 day. The schedule takes 30.5 days, along the critical path A → C → E → G → H → I. “Survey the site” (B) and “Prepare the site” (F) have 4.833 days of float, and “Order materials” (D) 3.833.
The variances along the critical path add up to 5.75, a standard deviation of 2.398 days. The chance of finishing within 32 days is therefore Φ((32 − 30.5) / 2.398) = 73.4 %, and you can be 90 % sure of finishing within 33.57 days. The simulation of 5,000 projects gives 73.5 % and 33.81 days: a little later at the 90 % mark, as simulation usually is, because near-critical paths sometimes take over.
The second example uses single durations with a start-to-start lag (E starts 2 days after C starts) and a finish-to-finish lag (H finishes at least 1 day after G); the project then takes 22 days.
Things to know
- Tasks are scheduled as early as possible, from day 0, with unlimited resources. Resource levelling is not done.
- With a start date and working days only, day 0 is the first working day on or after the start date; finish dates are the last working day of each task.
- Up to 2,000 tasks. The Gantt chart shows the first 150 by start.
Frequently asked questions
What if my predecessors form a loop? The tool names the loop, such as B → D → B, so you can remove a link.
Is my plan uploaded? No. Everything is worked out in your browser and nothing is stored.
Where the numbers come from
- Methods: the critical path method, PERT three-point estimates and the beta-PERT distribution, as in project management and operations research textbooks.
- Checked against a longest-path calculation in Python (networkx Bellman-Ford on the start and finish events) for 1,200 random projects, half with SS, FF and SF links and lags, and the PERT figures against SciPy's normal distribution and a NumPy beta-PERT simulation.
- The example project is made up.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.