What is a Gantt chart? A plain-English guide with an example
What is a Gantt chart? Learn what it shows, its main parts, when to use one and how to build your first chart with a simple worked example.
If you have ever been asked "what is a Gantt chart?" in a project meeting and nodded along, this guide is for you. A Gantt chart is a bar chart that lays a project out over time. Each task is a horizontal bar, its position shows when it starts and ends, and its length shows how long it takes. In one picture you see what has to happen, in which order, and when the work should be done.
The chart is named after Henry Gantt, an American engineer who popularised this kind of schedule chart in the early twentieth century. The idea has barely changed since: time runs left to right, tasks run top to bottom.
What a Gantt chart shows
A good Gantt chart answers four questions at a glance:
- What work is in the project (the list of tasks on the left).
- When each piece of work starts and finishes (the position of the bars).
- How long each piece takes (the length of the bars).
- In what order the work must happen (the links between tasks).
A spreadsheet can list the same tasks and dates, but it cannot show overlap, gaps or a chain of tasks at a glance. That is the real value of the chart: it turns a list into a shape you can reason about. If you want to compare the two formats in detail, see Gantt chart vs spreadsheet.
The main parts of a Gantt chart
The timeline
The top of the chart is a calendar scale, usually days, weeks or months. Everything below it is positioned against this scale. A vertical line often marks today, so you can see what should already be finished.
Tasks and durations
Each task has a name, a start date and a duration. The bar starts on the start date and stretches for the number of days the task takes. Keep tasks small enough that you can tell whether they are done: "Write invitation text" is a task, "Marketing" is not. For help choosing realistic numbers, read how to estimate task duration.
Dependencies
A dependency says that one task cannot start, or finish, until another one does. It is usually drawn as an arrow between two bars. You cannot print flyers before the design is approved, so the printing task depends on the design task. Dependencies are what make a Gantt chart a schedule rather than a drawing: when one task moves, everything that depends on it should move too. The details are covered in task dependencies in a Gantt chart.
Task groups and subtasks
Large projects need structure. A task group, sometimes called a summary task or parent task, collects related subtasks under one heading such as "Planning" or "Launch". The group bar spans from the earliest subtask to the latest, so you can collapse a phase to one line or open it to see the detail. See subtasks in a Gantt chart for how deep to nest.
Milestones
A milestone is a point in time with no duration, such as "Client approves design" or "Event day". Milestones mark the moments that matter to other people. Our guide to the milestone in a Gantt chart shows how to choose them.
Progress
Many charts fill part of each bar to show how much of the task is complete. Progress turns the plan into a status report: you can see what is on track and what is late without asking anyone.
A worked example: planning a small event
Imagine you are organising a one-day workshop with a start date four weeks away. A simple Gantt chart for it could look like this:
| Task | Start | Duration | Depends on |
|---|---|---|---|
| Choose venue | Day 1 | 3 days | none |
| Book venue | Day 4 | 1 day | Choose venue |
| Write programme | Day 1 | 5 days | none |
| Design invitation | Day 6 | 2 days | Write programme |
| Send invitations | Day 8 | 1 day | Design invitation, Book venue |
| Collect replies | Day 9 | 10 days | Send invitations |
| Workshop day | Day 20 | milestone | Collect replies |
Even this tiny chart shows useful things. "Choose venue" and "Write programme" run side by side, because nothing forces one to wait for the other. "Send invitations" has two predecessors, so it can only start once both the venue is booked and the invitation is designed. If the programme takes two extra days, the chart makes it obvious that the invitations, the replies and the workshop date are all affected. For a fuller version of this idea, follow the Gantt chart example for a small project.
When a Gantt chart is useful
A Gantt chart earns its place when:
- The project has a deadline and several tasks that overlap.
- Tasks depend on each other, so a delay in one has knock-on effects.
- More than one person works on the plan, and everyone needs the same picture of the schedule.
- You need to report progress to a client, manager or team without writing a long update.
It is usually overkill for a short list of independent chores, or for work that arrives continuously with no fixed end, where a simple list or a board may serve you better. Do you need a Gantt chart? gives an honest checklist, and Gantt chart vs Kanban compares the two most common alternatives.
How to build your first Gantt chart
You do not need special training. Follow these steps:
- List the work. Write down every task needed to finish the project. If a task feels vague, split it.
- Estimate durations. Give each task a number of days. Use ranges when unsure, then pick a sensible value.
- Set start dates. Place the first tasks at the project start and let the rest follow.
- Link dependencies. Connect tasks that must happen in order. Leave independent tasks unlinked so they can run in parallel.
- Add milestones. Mark the dates others care about.
- Group into phases. Collect related tasks under headings so the chart stays readable.
- Review the whole thing. Look for gaps, overloaded weeks and a final date that is later than your deadline.
The full process, with advice at each step, is in how to create a project schedule step by step.
You can do all of this in a free online editor such as ProGantt Flow: add tasks with durations, link them with dependencies, add milestones and track progress as the project moves. Because the plan is a real chart rather than a picture, changing one date updates the schedule instead of leaving you to redraw it.
Common mistakes to avoid
- Too much detail. A chart with three hundred tasks is unreadable. Plan at the level at which you can actually check progress.
- No dependencies. Without links, the chart is just coloured bars and will not warn you when something slips.
- A plan that is never updated. A Gantt chart is only useful while it matches reality. A short weekly routine fixes this; see how to keep a project plan up to date.
- Ignoring the critical path. Some tasks can slip without consequence and some cannot. Learn to spot the difference with the critical path in a Gantt chart.
Key takeaways
A Gantt chart is a timeline of your project: tasks as bars, dependencies as links, milestones as key dates, and progress as filled bars. It helps you plan realistically, see the effect of changes early, and share one schedule with everyone involved. Start small, keep tasks checkable, link what truly depends on something else, and keep the chart current. Once you can read and build one, you can use it for anything from a workshop to a product launch.