Estimating Activity Duration: An important work of a Project Manager

Estimating Activity Duration: An important work of a Project Manager


Last updated July 9, 2012


It is not an easy task to estimate the project duration – it needs a Project Manager’s knowledge of some theory, application of some best practices and methodologies.

Estimating 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 and depending on this duration you can develop a schedule, a milestone for your project. So, you can understand how important is to derive the duration for your project. The estimating duration can be of a project phase or product phase and it is somehow 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) and further it can be modified based on the changes done in the project while execution and controlling and monitoring process.

In estimating the duration who else can be involved? 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, delivery schedules as you know you can develop schedule only after estimating the duration for the project.

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 to get the entire duration estimation. Based on the information of the past project of similar nature, you can derive an idea of entire duration estimation. Is not it easy? You can get this information from the Project Management Office; you can use this template for your current project. It is not so accurate in nature. But at the same time it is less costly and less time consuming as well. You can frequently use the same when you have no sufficient time and you have to take a decision at the earliest. Analogous estimating can be of a part of project as well, it is not like you have to estimate the entire project. It can be of a single phase of the project, if the requirement /nature of the phase are very similar to the previous processed project.

Parametric Estimating:

It is more accurate in nature. It is based on a statistical calculation of historical data and other variables. Such as in design phase in case of a construction project, cutting of 1000 stones will take 8 hours and so on. So, you can understand how accurate it can be. If project is of similar nature that we had performed in the past, it can be a best process to get estimation on the duration. It is an estimation based on some unitary calculation from the historical data from the past project.

Three point estimating:

This 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. Three point estimating concept is originated from PERT (Program Evaluation and Review Technique). Through PERT, we generally estimate three types of duration viz. Most Likely (M), Optimistic (O), and Pessimistic (P). Out of this, ‘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. Then you can take help of some very useful formulas in deriving 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:

It 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…It is very useful in case of known phases where you can put an imaginary figure based on the experience.

Reserve Analysis:

It is a type of contingency reserve. After fixing a schedule for each activity level depending on the activity attributes, you can put contingency reserve timing by adding some percentage in the derived schedule of each activity. It can be changed as work progresses. It can be reduced or increased depending on the situation in front.

About the Author

Chandana is working as a Senior Content Writer in and handles variety of creative writing jobs. She has done M.A. in English Literature from Gauhati University. A PRINCE2 Foundation certified, she has a unique and refreshing style of writing which can engross the readers to devour each sentence of her write-ups.


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