People that are not used to project management terminology might have some confusion about the difference between durationeffort and elapsed time. Let’s try and clarify how to use this terms correctly.

When it comes to assessing projects or changes to a service, usually one of the first steps is identifying the main tasks and break them down in smaller (sub)tasks. Subsequently, we estimate what the effort to complete each of them should be. To achieve that we (usually) imagine one person working exclusively on the single task, without interruptions, till its completion.


Therefore, imagine you have been asked to estimate, for example, the effort to translate a 100 pages document from English to German. Knowing that every page requires around 30 minutes for a high-quality translation, you would say that it will take approximately 50 hours to translate the whole document.

Our example will always assume that the working time of our resources is 8x5 (8 hours, 5 days per week).

You could express Effort in man-days (MD). Therefore, in our example, we have to divide the estimated effort by the typical length of a working day. If we assume that our translators work 8 hours per day, in our example, the effort would be:

E = 50h \div 8d = 6.25MD


We can now proceed on evaluating the duration, which is the first step to understanding how long it will take to complete the activity “in the real world”.

Let’s keep going with our example: we already clarified that our 50-hour translation must keep into account the length of the working day. Therefore in a scenario in which one single resource works on the translation every business day, the duration in working-day (WD), would be exactly equal to the effort: 6.25 WD.

Allow me to remind the formula to get the duration:

D = MD \times WDH \div TDE

We already know that WD stands for Working Day(s) and MD for Man Day(s), we just added also WDH as Working-Day Hours and TDE as Total Daily Effort. In our example, with one resource working full time on the translation, we can easily understand why effort and duration correspond:

D = 6.25MD \times 8h \div 8h = 6.25WD

As you can notice, the duration of a task is calculated on the estimated effort against the availability of the resources assigned to it. That is why, if the assignee of the task worked only 50% of his time on it (i.e. 4 hours per day), the duration would be twice greater:

D = 6.25MD \times 8h \div 4h = 12.5WD

If we wanted to improve this scenario we could, for example, assign two people to the same task (so that they could translate 50 pages each), working 100% of their time on the translation, the task would take half the time:

D = 6.25MD \times 8h \div 16h = 3.125WD

Do not forget to convert decimal values correctly: 0.5MD is equal to four hours, 0.25 to 2 hours and 0.125 to 1 hour.

Elapsed time

The step to get the elapsed time is at this point quite easy. It is just the number of calendar days needed to complete the task, since the duration does not keep into account any non-working-day such as weekends or holidays.

Wrap Up

Let’s wrap up. Assuming that one resource only is carrying out the job, working on it up to 25% of his time (i.e. 2 hours per day), starting on September 1st, we would have the following information:

  • Effort: 6.25MD
  • Duration: 25WD (6.25 × 8 ÷ 2)
  • Elapsed: 33d (from Sep 1st to Oct 5th)

We know how to estimate effort, duration and elapsed for a single task, but what about a set of tasks belonging to the same project?

It is not a big deal. You just need to estimate effort and duration for each task of the project, and at the end, you will have:

  • the total effort (TE) as the sum of the effort of every single task (E1, E2, …, Ek) included in the project:

TE = sum_{k=1}^nE_{k}

  • the total duration, as the number of working days between the start date of the earliest task and the end-date of the last one
  • the total elapsed as the number of days between the same dates considered for the duration.

For example, given the following tasks with specified effort, start and end dates:

  • First task – Effort 5 MD – from Aug 18th to Sep 2nd
  • Second task – Effort 1 MD – from Aug 20th to Aug 22nd
  • Third task – Effort 4 MD – from Aug 22nd to Sep 9th

The total effort would be 10 MD, total duration 17 WD (for a 5-day working week, from Aug 18th to Sep 9th). The elapsed would be 23 calendar days.

I am sure that many of you already realised that we are just scratching the surface of this topic. There are many variables to keep into account. What if one resource is assigned to more than one task? What if an activity cannot start as long as another task is ongoing? How to adapt the estimated effort to the real skills of the assignee?

Food for thoughts for you, a topic for another article for me… 🙂

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