The OTR GroupAlmost every project team meets a Gantt chart sooner or later. It can look like a wall of coloured bars, but the idea behind it is simple. This guide explains what a Gantt chart is, where it came from and how to read one with confidence. It draws on the Association for Project Management (APM) article "A brief history of Gantt charts" by Tim Lyons.
What a Gantt chart shows
The APM describes the common modern format as one that plots a list of deliverables on one axis against time on the other. In most project software the tasks run down the left and the calendar runs across the top, so each bar tells you roughly when a piece of work starts, how long it lasts and when it ends.
Where the idea came from
The APM article credits the American Henry L. Gantt with developing the concept. It says Gantt worked with a colleague, Frederick Taylor, on a method of describing production planning and resource loading for factories and workshops. The first charts were more like tables than the bar charts we know today, but they were among the first uses of the deliverable-against-time idea. (The article gives the date as "around the turn of the 19th Century", which is probably intended to read 20th; we have avoided quoting a date for that reason.)
From hand-drawn bars to live schedules
Before computers, the APM explains, Gantt charts were drawn by hand as bar charts, and each change meant manual revision. Computers made it possible to calculate how linked tasks affect delivery dates and to show changes in the critical path and float on screen and in reports.
Five things to look for
- The bars. Each bar is a piece of work. Longer bars are longer tasks, and the position along the calendar tells you when it is planned.
- The links. Where one task depends on another, the chart normally shows the second starting after the first finishes. The APM notes that this logic can be hard to follow on a complex programme, and that a PERT chart is often a better way to show it.
- The critical path. The APM says critical paths can be highlighted on a chart. Look for the chain of linked tasks that determines the end date: a delay there moves the finish.
- The "time now" marker. Modern systems show a Time Now or Data Date line, with progress drawn along the bars. Anything that should be finished to the left of the line but is not deserves a question.
- The baseline. Many tools can show baseline bars alongside current forecast bars, so you can see how far the plan has moved.
A warning about charts without logic
The APM warns that charts drawn manually in Excel or PowerPoint have no logic holding them together. It adds that such charts are often cited in contracts and have been held up in court as evidence. If a chart matters commercially, ask whether it comes from a scheduling tool with linked tasks, or whether it is simply a drawing.
Questions to ask your project manager
- Which tasks are on the critical path?
- Is this the baseline or the latest forecast?
- Are the links between tasks built into the tool, or is this a picture?
Used well, a Gantt chart is a shared picture of the plan. Used carelessly, it is a picture that looks certain when it is not. Knowing what to look for helps you tell the difference.