Gantt Charts

Gantt charts are used to schedule project tasks. Invented by Henry Gantt, they take the form of a horizontal bar chart with time on the horizontal axis and tasks on the vertical axis. The ends of the bars represent the start and finish times of the tasks.

The above example shows a Gantt chart for making a cup of coffee. Although you wouldn’t normally use a Gantt chart for a project as short as ten minutes, the familiarity with coffee making will help you understand the principles, which you can then apply to larger projects.

Constructing a Gantt chart

1. Identify the tasks that have to be undertaken to complete the project.
2. Give each task an identity code (optional).
3. Identify how long each task will take (duration).
4. Identify the start time for each task.

5. Identify the finish time of each task by adding the duration to the start time.

   ID	Task	Duration	Start Time	End Time
   1	Get kettle	0:00:20	0:00:00	0:00:20
   2	Fill kettle	0:00:30	0:00:20	0:00:50
   3	Boil water	0:03:00	0:00:50	0:03:50
   4	Get coffee jug	0:00:20	0:00:55	0:01:15
   5	Get coffee	0:00:10	0:01:15	0:01:25
   6	Put coffee in jug	0:00:30	0:01:25	0:01:55
   7	Get cups	0:00:20	0:01:55	0:02:15
   8	Get spoons	0:00:10	0:02:15	0:02:25
   9	Get sugar	0:00:10	0:02:25	0:02:35
   10	Put sugar in cups	0:00:05	0:02:35	0:02:40
   11	Pour water on coffee	0:00:20	0:03:50	0:04:10
   12	Brew coffee	0:04:00	0:04:10	0:08:10
   13	Get milk jug	0:00:20	0:06:00	0:06:20
   14	Get milk	0:00:10	0:06:20	0:06:30
   15	Fill milk jug	0:00:10	0:06:30	0:06:40
   16	Warm milk	0:00:50	0:06:40	0:07:30
   17	Depress plunger	0:00:05	0:08:10	0:08:15
   18	Pour coffee	0:00:30	0:08:15	0:08:45
   19	Add milk	0:00:05	0:08:45	0:08:50
   20	Stir coffee	0:00:10	0:08:50	0:09:00

6. Draw a horizontal bar chart with time on the horizontal axis and the tasks on the vertical axis, with the ends of the bars representing the task start and finish times.

Some start times depend on the completion of other tasks and cannot start until these pre-requisite tasks are completed. For example, the water cannot be boiled until after the kettle has been filled.

The start of some tasks needs to be delayed until sufficient resources become available. For example, getting the coffee jug was delayed until the kettle had been switched on.

Tasks which, if delayed, lengthen the duration of the project are called critical tasks. In this example, the following are the critical tasks:

• Get kettle
• Fill kettle
• Boil water
• Pour water on the coffee
• Brew coffee
• Depress plunger
• Pour coffee
• Add milk
• Stir coffee

Other uses for Gantt Charts

Gantt charts can be used for more than just scheduling important project tasks. They are often used to display the results of a critical path analysis, with the critical tasks drawn along the top and dotted lines used to show the window through which the non-critical activities have to be carried out if the project is not to be delayed.

Gantt charts can also be used to track progress by shading the completed tasks. In the example shown on the next page, we are seven minutes into the project. Brewing the coffee is ahead of schedule, but warming the milk has been delayed and has not yet started. However, we still have time to warm the milk without delaying the project.