According to the PMBOK effort is defined as the number of labor units required to complete an activity or other project element. Usually expressed in staff hours, staff days, or staff weeks. Whereas, duration is defined as the number of work periods (not including holidays or nonworking periods) required to complete an activity or other project element. Usually expressed as workddays or workweeks. Sometimes incorrectly equated with elapsed time.

Effort is usually given in terms of hours, and duration is usualy given in terms of days with an end date. You need to calculate effort first and then determine the duration by dividing the number of effort hours by the number of resources, divided by the number of productive hours they spend on an activity each day.

Here is a summarized process for calculating effort and duration in a project taken from an article written by Tom Mochal.

Estimating Effort

1.Create estimate of effort hours using an estimating technique.

2.Determine effort hours based on skill level of assigned resource.

3.Consider rework by either adding it to the original estimate, or as separate activities, or as a separate block of time.

4.Add project management time by calculating 15% of total effort hours.

5.Add contingency hours to take risk into account.

6.Calculate total effort.

7.Document assumptions.

Estimating Duration

1.Estimate the productive hours per day using a productivity factor of 6.0 to take into account breaks, sick time, meetings and administrative activities.

2.Determine number of resources assigned to each activity.

3.Determine available workdays to take into account vacation, holidays and training.

4.Identify part-time resources.

5.Estimate multitasking productivity loss.

6.Determine any activities that have lag time or delays.

7.Determine activities that can be done in parallel.

8.Document assumptions.