Understanding Estimating Activity Duration in Project Management

Understanding Estimating Activity Duration in Project Management


Last updated June 19, 2018


Aspiring project managers must learn how to perform a variety of tasks in order to get a project started and estimating activity duration is one of them. It’s not always an easy task to estimate the project duration, because doing so needs a Project Manager’s knowledge of some theory, along with application of some best practices and methodologies.

To estimate activity duration is a lengthy process and it takes time for a long-term project. Ideally, you should involve your core project team in estimating the duration. You should involve the project team, who will work on the project to achieve the project objectives, project management team, and all the identified key stakeholders to make a consensus on the milestone dates, and delivery schedules. Depending on this duration, you can develop a schedule, with milestones for your project. 

Estimating activity duration can be for a project phase or product phase and is iterative in nature. It can be changed as work progresses and as new activities identified in your project. Initially, you can estimate your project based on the network diagram (sequencing the activities). It can be further modified based on the changes done in the project during the execution, controlling and monitoring process.

The PMI® recommends few project management tools, methodologies, best practices for estimating the duration. They are as follows:

Analogous Estimating

This is a very important methodology that gives you the entire duration estimation. Based on the information of past projects that are similar, you can get a ballpark idea of the entire duration estimation. You may be able to get this information from the Project Management Office or another project manager; you can use this template for your current project. 

Analogous estimating the activity duration is not so accurate in nature; however, it is less costly and less time consuming than other methods. You can frequently use this method when you don’t have sufficient time to do a proper estimate, and you have a deadline. Analogous estimating can be of a part of project as well, since you don’t have to estimate the entire project. You can estimate activity duration for a single phase of the project, if the requirement /nature of the phase are very similar to the previous processed project.

Parametric Estimating

This method is more accurate in nature. It is based on a statistical calculation of historical data and other variables. For example, in the design phase in case of a construction project, you may already have the activity duration: cutting 1,000 stones will take 8 hours and so on. If your project is of a similar nature to one performed in the past, it is worth your while to find existing activity duration estimates and historical data from the past project.

Three-Point Estimating

This kind of estimating is based on considering various options like the assignment of the resources, uncertainty, and the risk associated—such as best scenario-based and worst-scenario based. The three-point estimating concept is originated from PERT (Program Evaluation and Review Technique). Through PERT, we generally estimate three types of duration:

  • Most Likely (M)
  • Optimistic (O)
  • Pessimistic (P)

Of these, Most Likely (M) is based on the resources assigned and Optimistic (O) is based on the best scenario-based analysis. And the last one—Pessimistic (P)—is based on the worst possible scenario-based analysis. Once you have these, you can use some very useful formulas to derive the expected duration. They are as follows:

TE = (O + 4M + P) ÷ 6

Here, TE is expected time, and O, M, P are Optimistic, Most Likely, and Pessimistic duration respectively. In this way, you can understand the importance of PERT in estimating the duration for your project.

Heuristic Estimating

This is also known as Rule of Thumb. You can roughly guess that entire design phase can take 45% of your entire project time and so on…this is a very useful technique in case of known phases where you can put an imaginary figure based on the experience.

Reserve Analysis

A reserve analysis is a type of contingency reserve. After fixing a schedule for each activity level depending on the activity attributes, you can create contingency reserve timing by adding some percentage in the derived schedule of each activity. It can be changed as work progresses; reducing or increasing depending on the situation.


Estimating the duration of a project—or its various phases—can help you successfully manage a project that comes in on time and on budget. Learn from the lessons your organization has already digested to save yourself time when doing an activity duration estimate. To learn more about successfully managing projects of all types, look to Simplilearn’s online project management training courses. 

Find our PMP® Certification Online Classroom training classes in top cities:

Name Date Place
PMP® Certification 22 Dec -26 Jan 2019, Weekend batch Your City View Details
PMP® Certification 6 Jan -21 Jan 2019, Weekdays batch New York City View Details
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