Gantt chart
The Gantt chart presents a project schedule on a time axis. Each activity takes one row. Its bar starts at the start date and its length gives the duration. Diamonds mark the milestones; arrows link dependent activities. During the project, a baseline bar shows the approved dates and a shaded part shows progress. The chart displays dates that the critical path method calculated on the network diagram; it calculates none. The name comes from the title of a book by Wallace Clark published in 1923. Clark had worked with the engineer Henry Gantt, whose charts compared the work promised with the work done, day by day.
Goal
The PMBOK Guide glossary defines the Gantt chart as a bar chart: activities on the vertical axis, dates on the horizontal axis, each duration shown as a bar placed between its start and finish dates.
The chart makes a schedule readable for people who did not build it. The sponsor, the steering committee, the teams and the suppliers read on one page who works on what, in which order and until when. Reading down from one date shows what runs in parallel on that date. At each report, the project manager reads the variance between the current dates and the baseline, then spots the activities that are slipping.
The deliverable is the chart, drawn from the project's schedule model: the scheduling software file that holds the activities, their durations, their dependencies and the calendar. The chart exists in two states: the baseline, then the version updated at each status date, the date on which progress is recorded. The PMBOK Guide defines the baseline as the approved version of the schedule model, used as the basis for comparison with actual results. To change it, the project manager submits a change request to the committee, following the formal change control procedure.
The precedence diagram sets the dependencies between activities, and the critical path method derives the dates and floats from them. A Gantt chart drawn without this calculation puts hopes on a time axis. Nothing in it says whether the dates hold or which dates a slip pushes back.
Usage
When to use it
- A schedule to present to a committee or a sponsor: show the phases and milestones on one page.
- Sequential work with a fixed deadline: IT migration, office move, legal compliance; read the sequence up to the deadline.
- Several teams or suppliers working in parallel: see who works when and which activities overlap.
- Periodic progress report: compare the current dates with the baseline and spot the late activities.
- A change request awaiting a decision: show the committee the recalculated effect on the milestones before it decides.
- Weekend cutover: sequence the go-live steps and their owners hour by hour.
When not to use it
- No calculated schedule: the chart would put hopes on a time axis; set the dependencies in a precedence diagram, then calculate the dates with the critical path method.
- Work reprioritised at every iteration: the bars cannot keep up; order a backlog and track the flow on a Kanban board.
- Milestones drifting from one report to the next: a Gantt chart shows a single state; run a milestone trend analysis.
Description
The elements of the chart
Each element displays a value taken from the schedule model or from the progress report.
| Element | Representation | Source of the value |
|---|---|---|
| Time axis | Horizontal, graduated in days, weeks or months | Project calendar: working days, public holidays, leave |
| Activity row | One per activity, with its code and name | Activity list from the project's work breakdown structure (WBS) |
| Bar | Placed from the start to the finish of the activity; its length gives the duration | Dates calculated from the durations and dependencies |
| Summary bar | Covers the activities of a phase or a deliverable | Work breakdown structure |
| Milestone | Diamond, zero duration | Milestone list: decisions, deliveries, deadlines |
| Dependency link | Arrow between the ends of two bars | Precedence diagram |
| Critical path | Bars with a distinct colour and pattern | Critical path method |
| Total float | Line extending the bar to the late finish | Critical path method |
| Baseline bar | Thin bar under the current bar | Approved schedule baseline |
| Progress | Shaded part of the bar | Reported by the activity owner |
| Status date | Vertical line | Date on which progress is recorded |
| Resource | Name to the right of the bar | Resource assignments |
The project calendar sets the length of the bars. An activity of ten working days covers two calendar weeks when no public holiday falls within it. Public holidays vary from canton to canton: Federal Fast Monday is a public holiday in the canton of Vaud but not in the canton of Zurich. A project on two sites needs one calendar per site or per resource.
A milestone marks an event without duration: a committee decision, a delivery, a legal deadline. The links use the four dependency types of the precedence diagram: finish-to-start, start-to-start, finish-to-finish and start-to-finish. For a finish-to-start dependency, the most common type, the arrow leaves the end of the predecessor and reaches the start of the successor. Software colours the critical path. A colour-blind reader misses colour alone; a pattern or a thick border adds a second cue.
Building the chart
The APM describes how the schedule model is built: list the activities, estimate their durations, sequence them, set the dependencies, assign the resources. In Microsoft Project, the planner enters the activities in the Gantt view, so the chart seems to produce the dates: the scheduling engine calculates them and the view draws them.
- Choose the audience. The committee receives one summary bar per phase, with the milestones. A team reads its activities for the next four to six weeks, with their links. Rows the reader does not act on hide the rows they do act on.
- Choose the time scale. Weeks suit a project of a few months, months a multi-year programme, hours a weekend cutover. The scale sets the shortest readable duration: on a monthly axis, a three-day activity becomes a line.
- Order the rows. Follow the work breakdown structure, phase by phase or deliverable by deliverable, then the start date within each group. The reader finds an activity by its deliverable and sees the work of a phase unfold in sequence.
- Show what the calculation produced. The critical path, the floats and the links explain why a bar sits where it does. Without them, the reader cannot tell which bar can slip without consequence.
- Freeze the baseline. Once the schedule is approved, the project manager saves the baseline: the software copies each activity's start and finish dates into its baseline fields.
- Update at each status date. The project manager enters the actual start and finish dates, then the remaining duration of the activities in progress. The software recalculates the dates of the following activities, and the chart shows the current dates beside the baseline.
Reading progress
The shaded part of a bar gives a percentage complete. The activity owner estimates this percentage, and the reader compares it with the time elapsed. A bar shaded halfway when three quarters of its duration have elapsed signals a delay, if the work progresses at a steady rate. The remaining duration says more than the percentage. The owner states how many days of work remain, and the software derives the new finish date from that figure.
The status date line divides the chart. To its left, all work should be done: an unshaded part there is late work. The distance between the baseline finish and the current finish measures how far the activity has slipped.
The project manager saves a new baseline only after an approved change: an extended scope, a finish date that the sponsor has pushed back. Replacing the baseline to erase a delay removes the variance from the chart and with it the record of the delay. Software keeps several baselines. The project manager keeps the old one to show the committee where the new date comes from.
What the chart hides
Two parallel bars assigned to the same analyst, each full-time, load her at 200% while they overlap. Nothing in the bars signals it. The resource histogram, which most software places under the Gantt chart, shows this overload. Levelling resolves it by moving activities, often at the cost of the finish date.
Software accepts constraints such as "must start on": the activity stays on its date when its predecessor slips, and the chart shows an overlap that the logic forbids. The planner reserves date constraints for an external deadline, such as a supplier delivery or a legal date. The planner justifies each constraint in writing.
Beyond a few dozen rows, the reader can no longer follow a bar to its successor. A schedule of four hundred activities is read in extracts: filtered on one team, on the critical path or on the weeks ahead. Printed in full, it covers a wall and nobody reads it.
The Gantt chart and neighbouring techniques
The network and the Gantt chart present the same data. The network shows the logic: who waits for whom and which branch the critical path runs through. The Gantt chart shows the same activities on the time axis.
Network
Gantt chart
The PMBOK Guide lists the Gantt chart among the scheduling approaches, with the critical path method, critical chain and Kanban. It also names a bias to raise with the team: the belief that a schedule is valid only if it is displayed as a software-generated Gantt chart.
Milestone trend analysis tracks the forecast date of each milestone, recorded at every report and plotted on a chart. The Gantt chart shows the forecast of one report. The trend shows how that forecast has moved from one report to the next. A milestone that slips by a week at every report signals an unrealistic schedule.
The product roadmap places themes on a broad time scale, in quarters or horizons. It carries no dated activities.
Where the name comes from
Henry Gantt (1861-1919), an American mechanical engineer, worked with Frederick Taylor early in his career, then practised as a consulting engineer in scientific management. His books Work, Wages, and Profits (1916) and Organizing for Work (1919) published his charts: machine records, bonus charts, production sheets. According to the scheduling historian Patrick Weaver, most of them were filled in after the working day, to compare the work promised with the work done.
In 1917 and 1918, the Frankford Arsenal and the Ordnance Department in Washington, under General William Crozier, tracked munitions production with these charts. The name "Gantt chart" comes from the title of a book by Wallace Clark, a colleague of Gantt: The Gantt Chart: A Working Tool of Management, published in 1923. Gantt named his charts after their use, as with the load chart.
The bar chart on a time axis is older. Weaver traces it back to Joseph Priestley's Chart of Biography (1765) and William Playfair's commercial atlas (1786). In 1896, in his harmonogram, the Polish engineer Karol Adamiecki placed activities on paper strips along a date axis, each predecessor to the left of its successors. Weaver concludes that Gantt's charts, which compared planned with actual every day, are closer to earned value than to the bar schedule that bears his name.
AI considerations
The scheduling engine calculates the dates. A language model calculates them badly: it miscounts working days and does not know the project's public holidays. A Gantt chart that it draws on its own dates carries errors that nothing flags.
From a work breakdown structure or the notes of a planning session, the language model proposes, in the software's import format, an activity list that the team corrects. It writes a chart in a text syntax, Mermaid or PlantUML, for a wiki page. On an exported schedule, it spots structural defects: activities with no predecessor or successor, date constraints, progress unchanged from one export to the next. It drafts the commentary on the baseline variances for the committee.
The team owns the durations and the dependencies. A language model proposes plausible values for work it does not know, and the chart presents them as crisply as calculated dates. A schedule also carries supplier names, amounts and contractual dates. The team does not send it to an online service without the approval of the data protection advisor.
Examples
The concept: the variance between the current dates and the baseline, read at a status date. A municipality in the canton of Vaud is replacing its resident registration software. The project manager draws up the report at the end of week 6.
The configuration bar (B) is shaded up to the status date: no late work appears to the left of the line. Its baseline bar, however, ends in week 7. The owner reports two weeks of remaining work: the current bar ends in week 8. Acceptance testing (D) waits for the end of configuration and of data migration (C). It slips by one week, and go-live with it. Data migration, finished on time, makes up for nothing.
Visualisations
The steering committee reads the milestone chart, which keeps only the diamonds, each with its baseline date and its current date. The summary Gantt chart keeps one bar per phase. A team's Gantt chart covers the weeks ahead, with links and floats displayed.
The tracking view overlays each bar with its baseline bar. Some software also draws a progress line. It starts from the status date and joins the end of the shaded part of each activity in progress; it bends to the left where work is late.
Cost
| Dimension | Level | Rationale |
|---|---|---|
| Preparation | Low | The schedule model supplies the data; the audience, the level of detail and the scale remain to be chosen. Without a model, the network comes first. |
| Execution | Medium | At each status date, the project manager collects the actual dates and the remaining durations from the activity owners. |
| Documentation | Medium | Each baseline stays archived with the approved change that replaced it. |
Tooling
A whiteboard or a sheet of squared paper is enough for a first schedule in a workshop: the teams move the bars around. Nothing there propagates a delay.
A spreadsheet draws bars through conditional formatting. It suits a schedule of a few dozen rows without dependencies. The planner updates every later date by hand after each slip.
Microsoft Project, Oracle Primavera P6 and the open-source ProjectLibre calculate the dates and display the critical path, the baseline, progress and the resource histogram. In Microsoft Project, a tracking view overlays the baseline on the current bar. GanttProject, also open source, is enough for a small project.
Work management tools such as Jira or Smartsheet offer a timeline view with bars. Their dependencies do not always recalculate the dates. The planner checks this before keeping a schedule with a contractual deadline in one of them.
Mermaid and PlantUML describe a Gantt chart in text, versioned with the project documentation.
Sources
- Gantt, Henry L. (1916), Work, Wages, and Profits, The Engineering Magazine Co., 1916 edition (link to the 1919 printing): Gantt's production and bonus charts.
- Gantt, Henry L. (1919), Organizing for Work, Harcourt, Brace and Howe: the second book in which Gantt published his production charts.
- Weaver, Patrick (2012), "Henry L Gantt, 1861–1919: Debunking the myths", PM World Journal, vol. I, no. 5: Gantt's practice, the use of his charts in 1917 and 1918, the origin of the name with Wallace Clark and the earlier bar charts, from Priestley to Adamiecki.
- Project Management Institute (2025), A Guide to the Project Management Body of Knowledge (PMBOK Guide), 8th edition: §2.3.3.6 Scheduling Approaches and Methods for the chart's place among the scheduling approaches; §2.6.3 Tailoring Considerations for the bias of the schedule displayed as a Gantt chart; Section 4, entries "Project schedule" and "Schedule baseline", for the definition of the baseline; glossary, entry "Gantt chart", for the definition.
- Association for Project Management, "What is a Gantt chart?": the practical definition, the data each bar carries and the order in which the model is built.