Activity duration

Hello, I have a question about activity duration. I'm studying in Rita's book. Here the activity duration is assumed EF-ES or LF-EF. In some Internet exercises I find a different calculation: EF-LF+1. Which is the right one? Why there is this difference? Thanks

My guess is, you are getting confused between formula for calculating float and fomula for calculating activity duration. I am saying so because, LF-EF (mentioned in your question) is a formula to calculate Float of an activity and not the duration of it.

Fomula for calculating activity duration is EF-ES+1 or LF-LS+1.

For example:-

  • EF=5th Day
  • ES=2nd Day

So, effectively your team will be working for 4 days (2nd, 3rd, 4th and 5th) and not 3 days.

  • EF-ES= 5-2 = 3 days
  • EF-ES+1= 5-2+1 = 4 days


Hemant, PMP

Thank you for explanation.Your approach is different from Rita's chart: this is the reason of my confusion. I think that the difference is because Rita start from 0, not from 1, in the critical path calculation. ex activita A (with duration 4) start at 0 and end at 4. So the  duration is EF-ES=4-0.

Other exercises in other books use the formula EF-ES+1. My doubt is: wich is the correct approach for PMP exam?


 refer blog dated  26th of Dec 12