What a Gantt chart is
A Gantt chart is a bar chart of tasks against time. Each task is a horizontal bar: the bar's left edge is the task's start date, its right edge the finish date, and its length the duration. Stack every task of a Work Breakdown Structure down the left and lay them against a calendar across the top, and you have turned a list of work into a picture of a plan. Read top to bottom it is the WBS; read left to right it is the schedule.
That is the easy half. What turns a Gantt from a picture into a working schedule is what connects the bars: dependencies and the critical path. Without them a Gantt is just a decorated to-do list. With them it can tell you the order work must run in, which tasks decide the finish date, and what happens to that date the moment one task slips. This guide covers all three — reading a Gantt, building one, and using dependencies and the critical path — with a worked fabrication example.
How to read a Gantt chart
Every Gantt shares the same anatomy. Once you can name the parts, any chart is readable at a glance:
- Task rows — one per WBS task, usually grouped under their phase or parent, so the vertical axis is the work breakdown
- The time axis — days, weeks or months across the top; each bar spans the dates its task occupies
- Bars — position shows start and finish, length shows duration; a shaded portion often shows progress %
- Dependency arrows — links from one bar to another showing that one task drives the next
- Milestones — zero-duration markers (often a diamond) for key dates such as a sign-off or a dispatch
- The critical path — the chain of tasks, often highlighted, that determines the project’s finish date
The single most useful habit when reading a Gantt is to follow the dependency arrows, not just scan the bars. The bars tell you when things are planned; the arrows tell you why they are planned that way — and therefore what moves if reality disagrees with the plan.
Dependencies and the critical path
A dependency records that one task relates to another in time — most commonly that a task cannot start until its predecessor finishes. Formally there are four dependency types, and a good scheduler supports them:
| Type | Meaning | Fabrication example |
|---|---|---|
| Finish-to-Start (FS) | B cannot start until A finishes — the default | Welding starts only after cutting finishes |
| Start-to-Start (SS) | B cannot start until A starts | Quality inspection starts once welding starts, running alongside it |
| Finish-to-Finish (FF) | B cannot finish until A finishes | Documentation cannot be closed until testing is closed |
| Start-to-Finish (SF) | B cannot finish until A starts — rare | The old shift ends only once the new shift begins |
Dependencies can also carry lag or lead. Lag is waiting time added to a link — an FS link with two days' lag means "start two days after the predecessor finishes", such as letting a weld cool or concrete cure before the next task. Lead (negative lag) lets a successor overlap its predecessor, so a task can begin before the predecessor fully completes.
Chain the dependencies together and you get the critical path: the longest sequence of dependent tasks through the project, whose total duration is the project duration. Every task on the critical path has zero float (slack) — delay any one of them by a day and the whole project finishes a day late. Tasks off the critical path have float: they can slip within their window without moving the end date. The practical value is focus. The critical path tells a project manager exactly which tasks to protect: a day saved on a critical task shortens the project; a day saved on a non-critical one usually changes nothing.
How to build a Gantt chart
You do not draw a Gantt bar by bar — you derive it from the WBS. The reliable method is five steps:
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The two Gantts you actually need
A single Gantt answers "when does each task run?" A serious project tool offers a second, resource-wise view that answers a harder question: "when is each machine and crew committed, across every project running at once?" The first keeps the customer's dates honest; the second stops you from promising the same crane to two tasks in the same week. Planning realistically means reading both — the order-planning Gantt for sequence, the resource Gantt for load.
How dependencies decide the delivery date
Take the skid package: design (5 days) → cutting (3) → welding (6) → assembly (4) → testing (2) → dispatch (1), all finish-to-start. Piping (4 days) runs in parallel after design but must finish before assembly (an FS link into assembly). The critical path is design → cutting → welding → assembly → testing → dispatch = 21 days, because piping (design + 4 = day 9, needed by day 14) has five days of float. That float is the insight: if piping slips three days, delivery is unaffected; if welding slips one day, the whole project slips one day. The Gantt makes both facts obvious, and a resource check confirms the single welding bay is not double-booked against another job in those six days.
How Fast Project Software renders the Gantt
Fast Project Software renders the schedule with a built-in Gantt engine on the project view. Every WBS task is laid out as a bar by its start, finish and duration, and predecessor links show the sequencing so the order of work is explicit rather than remembered. Alongside the order-planning Gantt, a resource-wise project Gantt shows how machines and crews are loaded across everything in flight, so clashes surface before work starts.
Because the Gantt is fed by the same tasks that carry the Bill of Resources and roll up to milestone billing, the schedule is not a separate diagram to keep in sync — it is one view of the single project record. Update a task's dates or progress and the plan, the cost position and the billing basis all move together. It is built for EPC, ETO, fabrication and construction scheduling.
Frequently asked questions
What is a Gantt chart?
A Gantt chart is a bar chart of tasks against time. Each task is a horizontal bar whose left edge is its start date, right edge its finish date, and length its duration; tasks are stacked down the left as the Work Breakdown Structure and laid against a calendar across the top. Dependency arrows between bars show the order of work, and the critical path — the longest chain of dependent tasks — determines the project's finish date.
What are the four types of task dependency in a Gantt chart?
The four dependency types are Finish-to-Start (a task cannot start until its predecessor finishes — the default), Start-to-Start (a task cannot start until its predecessor starts), Finish-to-Finish (a task cannot finish until its predecessor finishes), and Start-to-Finish (a task cannot finish until its predecessor starts — the rare one). Links can also carry lag (waiting time) or lead (overlap) to model curing, cooling or parallel work.
What is the critical path on a Gantt chart?
The critical path is the longest sequence of dependent tasks through a project; its total duration equals the project duration. Every task on it has zero float, so delaying any critical task delays the whole project, while tasks off the critical path have float and can slip within their window without moving the finish date. The critical path tells a manager exactly which tasks to protect.
How do you make a Gantt chart?
Derive it from the WBS: list every work package as a task row, estimate each task's duration, link the predecessors with the correct dependency type and any lag, let the tool calculate the schedule and critical path, then level resources against a resource-wise view and baseline the plan. You do not draw bars by hand — the dependency network positions them and reveals the finish date.
What is the difference between a Gantt chart and a project timeline?
A timeline simply plots dates or milestones along a line. A Gantt chart adds task durations as bars and, crucially, dependencies between them and a calculated critical path — so it can re-sequence work when a task slips and show the effect on the finish date. A timeline is a picture; a Gantt is a working schedule that reacts to change.