If everyone worked eight hours per day on your project, and
was absolutely 100% productive for all eight hours, you could easily calculate
duration by taking the number of effort hours, divided by the number of
resources, divided by the number of hours they work per day. For instance, if
an activity was estimated at 80 hours, and you have one person assigned, and that
person works eight hours per day, the duration would be (80 / 1 / 8) = 10 days.
Likewise, if four people were assigned full time, the duration would be (80 / 4
/ 8) = 2.5 days.

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However, as we all know, those perfect circumstances are not
indicative of how work is actually performed. There are many reasons why the
duration of the work is different than the effort hours divided by eight. You
need to use the following process and techniques if you want to make your
schedule estimate as realistic as possible.

  1. Estimate the productive hours per day.
    The first step is to determine how many productive hours of work you can
    count on each person working per day over time. The rule of thumb is to
    use a factor of 6.0 to 6.5 productive hours per day. This takes into
    account socializing, ramping up in the morning, going to the bathroom etc.
  2. Factor in multi-tasking productivity
    loss for part-time resources
    . If one person is working on multiple
    projects, or perhaps a combination of projects and support, you need to
    take into account a further reduction in productivity. This reflects the
    fact that if a person is shared on two or more unrelated efforts, it takes
    time to stop one and start up another. For instance, if a person is on two
    projects for 20 hours each week, this might result in a 10% loss of
    productivity on both projects.
  3. Determine how many resources will be
    applied to each activity
    . In general, the more resources you can apply
    to activities, the quicker they can be completed. Obviously two resources
    may be able to complete an activity faster than one person (but it may not
    be twice as fast).
  4. Factor in available workdays. Take
    into account holidays, vacations and training. This was not included in
    the productivity factor in the first item, since this non-project time can
    be scheduled and accounted for in advance
  5. Take into account any resources that
    are not full time
    . If you have a resource 50% of the time, it will
    take that resource at least twice as long to do any individual activity.
  6. Calculate delays and lag-times.
    Some activities have a small number of effort hours, but a long duration.
    For instance, if you’re counting on vendor resources, you may need to wait
    until the vendor is ready before you can begin. Another example is the
    duration required to get a deliverable approved. You may estimate the
    effort at only a few hours, yet it may take a number of days or weeks to
    gain the actual approval.
  7. Identify resource constraints.
    When you build your initial workplan, you
    identify the activities that can be done sequentially and those that can
    be done in parallel. If you have enough resources, all of the parallel
    activities can, in fact, be done in parallel. However, if you don’t have
    enough resources (you rarely do), you’ll find that some of the parallel
    activities need to be done sequentially, since the same resource needs to
    be assigned. This results in extending the duration further than what you
    might initially expect.
  8. Document all assumptions. You will
    never know all the details of a project, so it’s important to document all
    the assumptions you’re making along with the estimate.