You have several choices. Project does not "know" anything other than basic math. So if you say 1 painter can paint 100sf of wall in 1 day. You can:

1. Enter the task "Paint 400 sf of wall" and set the duration to 4 days.
2. Now assign painter 1 - Project will automatically calculate 32 hours of work based upon a 4 day duration task * 100% assignment units.

Now you realize you have two painters. The task "Paint 400sf of wall" task could be done by two painters -- the space is large enough and you have enough paint brushes. So, you ensure the task is effort driven -- this is in the Task Form at the bottom once you have split the screen.

3. You add Painter 2 to the task. Because the task is effort driven and you are adding another resource -- the total amount of work on the task (32 hours calculated from the first assignment) stays stable. Project drops the task duration to 2 days because now each resource can do 16 hours of work at the same time as the other resource does 16 hours of work.

There is no automatic way to tell Project that a painter can paint 100 sf of wall in 1 day, so 400sf should take 4 days.



I hope this helps.

based on your answer,

1-so by telling me to enter duration as 4 days, you are in a way telling me that, the first time I enter duration for an activity, I should first think of it as if it will be done by just one (1) full time resource? such as i should calculate "how long would it take for just one full time resource to finish this", and then i do the math myself outside, and come up with 4 days and enter that?

2-as soon as I assign 4 days, and then enter just one (1) full time resource, the project automatically multiplies 4 days by 8 hours (because lets say we had set a full day to 8 hours) to come up with 32 hours?

You may certainly work by either specifying duration or entering the work for each resource and letting Project calculate duration.

You expressed that you knew how long a task took one person. How long is duration. If you said you knew a painter could paint 100 sq feet of wall in 8 hours of work -- that is work.

Use what ever method makes the most sense for you. You are correct -- if you set duration, Project will calculate work based upon:

Duration * assignment units = Work

You can specify work and Project will recalculate duration based upon:
Work/Assignment units = Work.