A Gantt chart is the graphical representation of a project schedule: each task is a horizontal bar laid on a timeline, its position giving the dates and its length the duration. At a glance you see who does what, when, in which order, and where the end date is decided. This guide explains how to read a Gantt chart, how to build one, and when it stops being enough.
Gantt chart: definition and origin
A Gantt chart is a planning tool that sets the list of project tasks against a calendar. Rows carry the tasks, columns carry time (days, weeks or months), and bars translate start and finish dates. Henry Gantt formalised the chart in the 1910s to steer American industrial production. It is now the reference format for schedules in construction, IT, manufacturing and services, precisely because it needs no training to read: one bar, one task, one date.
Anatomy of a Gantt chart
A complete Gantt chart is more than bars. These are the elements you will find in any Gantt chart software and what each one tells you.
| Element | What it shows | What it is for |
|---|---|---|
| Task bar | Start date, finish date, duration | Placing work in time |
| Summary task (phase) | The envelope of its child tasks | Structuring the schedule by phase or work package (WBS) |
| Milestone | A zero-duration event on a fixed date (diamond) | Marking an approval, a delivery, a go/no-go |
| Dependency link | An arrow between two tasks | Enforcing an order: task B waits for task A |
| Lag | A delay added to a dependency | Modelling curing time, a freeze period, a notice period |
| Critical path | The chain of tasks with zero float, often in red | Knowing which delay moves the project end date |
| Today line | A vertical line on the current date | Comparing planned and actual at a glance |
| Progress | Bar fill in % | Seeing what is done, in progress or late |
| Baseline | A ghost bar of the original schedule | Measuring drift against the commitment |
Gantt chart example: a website redesign
Take a 33-working-day project. The table below is exactly what you type into Gantt chart software; the chart then draws itself from durations and predecessors.
| No. | Task | Duration | Predecessors (type) | Note |
|---|---|---|---|---|
| 1 | Scoping and objectives | 5 d | — | Project start |
| 2 | Mock-ups | 8 d | 1 (FS) | Critical path |
| 3 | Mock-ups approved | 0 d | 2 (FS) | Milestone |
| 4 | Front-end development | 15 d | 3 (FS) | Critical path |
| 5 | Back-end development | 12 d | 3 (FS) | 3 days of float |
| 6 | Content writing | 10 d | 2 (SS + 3 d) | Start-to-start link with a 3-day lag |
| 7 | Acceptance testing | 5 d | 4, 5, 6 (FS) | Critical path |
| 8 | Go-live | 0 d | 7 (FS) | Final milestone, day 33 |
How to read it: tasks 1, 2, 3, 4, 7 and 8 follow each other with no float and add up to 33 days; that is the critical path. Back-end development (12 days) has 3 days of float against the front-end (15 days). Content writing starts 3 days after the mock-ups start thanks to a start-to-start link with lag, and finishes well before testing.
The 4 dependency types and lag
A dependency links two tasks: a predecessor and a successor. The link type says which end of one constrains which end of the other.
- Finish-to-start (FS): B starts when A finishes. The default link, used in 90 % of cases. Example: development starts once the mock-ups are approved.
- Start-to-start (SS): B starts when A starts, possibly with an offset. Example: content writing starts 3 days after the mock-ups start.
- Finish-to-finish (FF): B finishes when A finishes. Example: documentation is completed together with development.
- Start-to-finish (SF): B finishes when A starts. Rare. Example: the legacy system stops when the new one starts.
Lag is a delay added to the link: "FS + 5 days" means B starts 5 days after A finishes (concrete curing, a legal delay, a freeze before production). A negative lag, or lead, allows overlap: "FS − 2 days" starts B two days before A finishes. The critical path, in one sentence: it is the longest chain of dependent tasks in the project, so any delay on one of them moves the end date by the same amount, whereas every other task has float. In the example it is scoping, mock-ups, approval, front-end, testing, go-live. Watch it every week: that is where the deadline is won or lost.
How to make a Gantt chart in 7 steps
- List the deliverables, then break them into tasks of 1 to 10 days (WBS breakdown). A 3-month task cannot be steered.
- Estimate each task in working days, together with the person who will do the work.
- Identify predecessors: for each task, which one must be finished (or started) first? Record the link type and any lag.
- Place the milestones: approvals, deliveries, committees, go-live. Zero duration, committed date.
- Assign one owner per task, and only one.
- Let the software compute dates and the critical path, then compare the end date with the target. If it overshoots, cut scope, parallelise or add resources; do not shorten durations by decree.
- Save a baseline and update progress every week. A Gantt chart that is not maintained is lying by the second week.
Gantt chart in Excel or online Gantt chart software?
An Excel template is enough for a 20-task schedule presented once. As soon as you need to recalculate, share or track, an online Gantt chart takes over.
| Criterion | Excel template | Dedicated Gantt software |
|---|---|---|
| Dependencies | Typed by hand, no recalculation | Typed links, dates recalculated automatically |
| Critical path | Computed yourself | Computed and highlighted |
| Milestones and baseline | Manual formatting | Native, drift measured |
| Changes | Move every cell | Drag a bar, successors follow |
| Sharing | File sent by email, multiple versions | Shared view, real time, role-based rights |
| Progress tracking | A % column kept by hand | Progress, actuals, delay alerts |
| Resources and budget | Absent | Linked to the schedule in a PPM tool |
Advantages and limits of the Gantt chart
Its strengths are well known:
- Everyone reads it without training, from the developer to the executive committee.
- Dependencies make constraints visible before they bite.
- The critical path tells you where to focus.
- Progress against the baseline makes drift measurable rather than felt.
Its limits are less known. A Gantt chart shows neither people's workload (a resource assigned to three simultaneous tasks appears on three bars with no warning), nor cost (a schedule that holds can hide a budget that has blown up), nor the trade-offs between projects competing for the same teams. It also becomes unreadable beyond a few hundred tasks. When you run several projects, when your teams are shared or when the budget is a commitment, move to a PPM tool: the Gantt chart remains the planning view, but it is connected to the workload plan and to budget tracking.
Common mistakes
- Planning without dependencies: the chart becomes a drawing, dates never recalculate.
- Tasks too long or too fine: aim for 1 to 10 days, except for external work packages.
- Forgetting milestones: without checkpoints, a delay is seen too late.
- Ignoring working days, leave and public holidays: a schedule in calendar days is wrong from the first long weekend.
- Assigning 100 % of a person to two parallel tasks.
- Not saving a baseline: drift cannot be measured.
- Trimming durations to hit the date instead of acting on scope or means.
- Not updating: a Gantt chart frozen on kick-off day is no longer a steering tool.
In practice: your Gantt chart template
To get started, a seven-column table is enough; it imports as is into most Gantt chart software.
- Identifier (sequence number, stable even if you reorder)
- Task name, action verb plus deliverable ("Write the specification")
- Duration in working days (0 for a milestone)
- Predecessors with type and lag ("3FS", "2SS+3")
- Owner (one person)
- Progress in %
- Parent phase or work package
Housekeeping rule: weekly update, during the team meeting, starting from the critical path.
The Gantt chart in FoxPlan
FoxPlan, a French PPM software, includes an interactive Gantt chart: you create tasks and milestones, draw dependencies of all four types with their lag, and the critical path is displayed automatically. Every bar you move recalculates its successors, and undo takes you back if the result is not right. Above all, the schedule is not isolated: tasks carry resource allocations in person-days or percentage that feed the multi-project workload plan, and budget envelopes track committed and consumed amounts against it. Gantt, workload and budget tell the same story.
See how to do it in the FoxPlan documentation ↗
Frequently asked questions
What is a Gantt chart?
A Gantt chart is a bar chart that represents a project schedule: each task is a horizontal bar placed on a timeline, whose position gives the dates and whose length gives the duration. Arrows between bars show dependencies, and diamonds mark milestones.
What is a Gantt chart used for?
It is used to plan tasks in order, to see which ones depend on others, to identify the critical path that decides the end date, and to track progress against the initial schedule. It is also the simplest format to share a schedule with a team or a steering committee.
How do you make a Gantt chart?
List the deliverables, break them into tasks of 1 to 10 days, estimate durations in working days, record the predecessors of each task, place the milestones, assign an owner, then let the software compute the dates and the critical path. Save a baseline and update progress every week.
What is the difference between a Gantt chart and a PERT chart?
A PERT chart is a network of tasks and links without a time scale, used to compute the critical path and durations. A Gantt chart places the same tasks on a calendar. Modern software computes the network and displays it as a Gantt chart, so you rarely need to draw a PERT chart yourself.
Can you make a Gantt chart in Excel?
Yes, with a stacked bar chart or conditional formatting on a calendar grid. It works for a small schedule presented once, but Excel does not recalculate dates when a predecessor moves, does not compute the critical path and does not share well. Beyond about twenty tasks, dedicated software saves time.
What is the critical path in a Gantt chart?
It is the longest chain of dependent tasks between the start and the end of the project. The tasks on it have no float: any delay on one of them moves the project end date by the same amount. Gantt software usually highlights it in red.
