We often find it beneficial to estimate how long a given task will take. This comes in handy when trying to quote billable time, set deadlines, or map out a project timeline on a Gantt chart. In scientific management, we often use the Program Evaluation and Review Technique, or PERT. Under PERT, we let O become the best-case, or optimistic, time; P becomes the pessimistic time, and M is the mean or average time. We then use the following formula:

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T = (O * 4M * P) / 6 

As you can see, it’s pretty simple as it’s really just a weighted average. For example, for a task with an optimistic time of 4 hours, a pessimistic time of 7 hours, and an average time of 5 hours, you could reasonably plan on this task taking about 5.17 hours. So the next time you need estimate how long a task will take, whether it’s delivering a pizza or building parts for a submarine, think about using this PERT formula.