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Gantt Charts & Bar Charts
Quantitative Analysis · Chapter 7: Project Planning with PERT and GanttSection 7.2 · lesson 32 of 5511 min · free with a free account
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In this lesson
The legacy of Henry Gantt and the power of bar charts.
- Read a Gantt chart: bar position, bar length and stacking order
- Add dependency arrows and milestone markers to a bar chart
- Use baseline overlays and resource histograms for project control
- Build a Gantt chart and test a delay against an activity's slack
Transcript
Everything said in the video, section by section (1,637 words). Read along, or search it.
Gantt Charts & Bar Charts · Section 7.2
A Visual Timeline for Every Project
Open almost any project management tool on the market today. The first view you get is a row of horizontal bars, stretched across a calendar. That view has a name, and it is more than a century old. It is the Gantt chart. This section takes it apart, piece by piece. We start with the bars, and what their length and position actually encode. Then we add the two features that turn a picture into an analysis. Dependency arrows, which say why the order is what it is. And milestone markers, which flag the moments management cares about. We finish with a worked exercise, and a delay that is harmless only up to a point.
The Structure
Time Rendered as Space
At its core, a Gantt chart is a horizontal bar chart. Each bar represents one project activity. Nothing more than that. The bar's position along the time axis tells you when the activity starts and finishes. The bar's length is directly proportional to its duration. That proportionality is the whole trick. Because length maps onto time, you read duration with your eyes, not with arithmetic. Activities are stacked vertically down the left hand axis. They are usually listed in the order they are scheduled to occur. From that one arrangement, four things fall out at a single glance. The total project duration, the sequence of work, and any periods of concurrency where bars overlap. And the gaps, where no work at all is planned.
Four Layers
What a Gantt Chart Can Carry
It helps to think of a Gantt chart as four layers, laid one on top of another. The base layer is the bars themselves, on a time axis. That layer answers exactly one question. When does each thing happen? The second layer adds dependency arrows between the bars. Those arrows answer a different question. Why is the order what it is? The third layer overlays the original plan on top of what actually happened. There the chart stops describing intent and starts measuring performance. The fourth layer sits underneath, sharing the same time axis. It is a resource histogram, showing how much of a chosen resource each period demands. Stop at layer one, and the chart will tell you far less than it could.
The Legacy
Henry Gantt, and Why the Chart Survived
The chart carries the name of Henry Laurence Gantt. He lived from eighteen sixty-one to nineteen nineteen. An American mechanical engineer and management consultant, he developed the technique in the nineteen tens. Gantt worked alongside Frederick Taylor, during the era of scientific management. He wanted a way to see the progress of manufacturing operations and shipyard production. His charts were used extensively during the First World War. They managed United States munitions production. Later they were adopted for the Hoover Dam and the Interstate Highway System. What is remarkable is how little the idea has needed to change since. We now have network diagrams, agile boards, and scheduling engines driven by artificial intelligence. And still the Gantt chart is the primary view in nearly every major platform. The reason is not inertia. Humans process timelines visually, and a bar renders time as space.
Dependencies
Arrows Answer the Question 'Why This Order?'
A basic Gantt chart shows you when activities occur. It does not show you why they occur in that order. That is a real gap, and it bites the moment something slips. Modern charts close it by drawing arrows between the bars. Each arrow is a dependency relationship. The most common by far is finish to start. One activity must finish before the next can begin. The arrow runs from the end of the predecessor bar, to the start of the successor bar. Start to start means two activities must begin together. Finish to finish means two activities must end together. And start to finish means one activity must start before another can finish. That last one is relatively rare, and the least intuitive of the four.
Milestones
Zero Duration, So No Bar
A milestone is a significant point in time. A key deliverable. An approval gate. The end of a phase, or a major decision point. Now here is the property that governs how it gets drawn. A milestone has no duration. So it cannot be a bar, because a bar of zero length would simply be invisible. Instead it is drawn as a diamond shaped marker, placed at the appropriate date. The diamond is not decoration. It is a reading aid. Milestones give stakeholders quick reference points, without forcing them to read every activity bar. Common examples are design approval, regulatory sign off, prototype delivery, and project handover. Notice what those four have in common. Each is a moment of managerial significance, not a stretch of work.
Baseline vs Actual
From a Plan to a Control Instrument
So far the chart is a plan. Now we make it an instrument of control. The method is disarmingly simple. Draw every activity twice. The first bar is the baseline, holding the originally planned start and finish. The second is the actual bar, updated continuously as the work proceeds. The visual gap between those two bars is the schedule variance. It is a direct, intuitive measure of whether you are ahead, on, or behind schedule. By convention the baseline is drawn hatched or outlined. The actual is drawn solid, in the project's primary colour. Here is why that beats a status percentage in a spreadsheet. A number tells you the project is slipping. The overlay tells you which activities are slipping. That is the difference between an alarm and a diagnosis.
Resource Loading
The Histogram Beneath the Chart
A Gantt chart shows the timing of work. It says nothing, by itself, about how much resource that work demands at any moment. The fix is a resource histogram, attached directly beneath the chart. Crucially, it shares the same horizontal time axis. For each day or week, it shows the total quantity of one chosen resource. Most often that resource is labour hours, but it can equally be equipment, budget, or a specialist skill. The total is taken across all concurrent activities, and that is the whole point. Its chief diagnostic value is finding overallocation peaks. Those are the periods where demand exceeds the capacity you actually have. Once you can see a peak, you can flatten it. Resource levelling and smoothing shift non critical activities within their available slack. You can also split tasks, or stagger start dates, while holding the project end date fixed.
Limitations
Where the Bar Chart Runs Out
For all its strengths, the Gantt chart is not a universal solution. Understand the limits before you rely on it as your only planning instrument. First, dependency clarity. Arrows can be added, but on a large project they become visually tangled. A network diagram, PERT or activity on node, shows the logic far more clearly. Second, scale. Hundreds or thousands of activities produce a chart that fits on no screen. Summary roll ups help, but they lose detail. Third, critical path visibility. It is not obvious unless the software colour codes it. Fourth, uncertainty. A standard Gantt chart assumes deterministic durations. It has no natural home for a probabilistic estimate, which is what PERT and Monte Carlo are for. Fifth, iterative work. Agile and Scrum have short iterations and backlogs that change constantly. They fit awkwardly into a fixed bar paradigm, and Kanban boards are often preferred.
Worked Exercise
Exercise 7.2 - The Activity List
Let us build one, using the activity list from the previous section. The brief has three parts. Convert the activity list into a Gantt chart. Identify and mark the critical path activities. Then introduce a two week delay in one non critical activity, and check whether the completion date moves. There are eight activities, labelled A through H. Define scope takes three days, and stakeholder meetings take two. Neither has a predecessor. Procurement takes five days, and training setup takes four. Both follow define scope. Implementation takes six days and follows procurement. Pilot testing takes three days, documentation takes two, and both follow implementation. Project closure takes two days, and waits on both pilot testing and documentation. The durations sum to twenty-seven days. The project will not take twenty-seven, and overlap is why.
Critical Path
Plotting the Bars, Reading the Slack
Plot each bar at its earliest start, respecting every predecessor relationship. The schedule falls out immediately. Define scope runs days one to three. Procurement then runs days four to eight. Implementation runs days nine to fourteen. Pilot testing runs days fifteen to seventeen. And project closure runs days eighteen to nineteen. That chain is the critical path. It runs A, then C, then E, then F, then H, and totals nineteen days. The critical path is the longest sequence of dependent activities. It determines the minimum possible project duration, and its bars are conventionally shown in red. The other three activities are not on it. Stakeholder meetings, training setup and documentation each possess slack, also called float. Slack is how far an activity can be delayed before it starts delaying something else.
The Delay Test
A Delay Is Harmless Only Inside Its Slack
Now the test. Take training setup, activity D. It is not on the critical path, it finishes on day seven, and nothing downstream needs it soon. Work its slack out from the activity list, and you get twelve days. That one number decides everything that follows. Delay training setup by two days, and it simply eats part of its own float. It shifts to days six through nine. Ten days of slack still remain. No successor on the critical path moves, and the project still ends on day nineteen. But be careful with the full two week version. Fourteen days is more than twelve, so on activity D that delay would exceed its slack. Push an activity past its float, and the float stops protecting you. The end moves to day twenty-one. Apply the same fourteen day delay to stakeholder meetings instead. That activity carries seventeen days of slack, so the delay is fully absorbed, with three days to spare. So the principle carries a condition, and the condition is the whole lesson. Delays on non critical activities leave the completion date alone, provided they stay inside their slack.
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