How do I get the task level calculated field information in a project level calculated field?

Short answer = via Fusion.

Long answer:

Calculated fields only update when the object they are part of is triggered to update.  Meaning in the example above, the calculated field on the project will only update when the project object has an update - not when the duration on the specific task is updated.

If that's acceptable, or if you go the Fusion route - I'd suggest creating a custom field that works as a metadata tag to find the "right" task.  For example, in our world we use a custom field called "Task - Event"; it has a drop down of about 20+ options that can be tagged to tasks.  We use that to find the "correct" task in a number of reports and automations.

I accidentally clicked "Me Too" on this, but this is not a problem for me. 

Jason's approach is 100% valid. If you don't have access to Fusion or similar tool, then you don't have the option to do this in a calculated field. Project-level calculated fields cannot pull collection-level (task/issue/etc) data. Only the inverse is true - those collections can reference project-level data. 

The closest you can get to this is by building a view in a project list. A project list view can look into collections like tasks, use an if/then expression to only display data from specific tasks, and display whatever task attributes are needed. If needed, this expression can be customized to display custom strings if certain attributes are true for task ABC. E.g., "if task ABC is less than one week, display 'on target' else if task ABC is less than two weeks, display 'at risk' else display 'in trouble'." This will help if you only need to display a specific task duration in a list of projects. It cannot help if you are using the calculated field to enable conditional formatting or grouping. 

If you need the calculated field to build a filter, like "show me all projects where the duration of task ABC is more than 1 week" then you can build an EXISTS filter for your projects list to accomplish that, and use it in conjunction with the view approach described above. 

There are several posts throughout the community on how to do all the above that can be tailored to your specific criteria. Good luck!

Hi Pravin - 

You can accomplish this in a project report IF the name of the task will remain unchanged, or if you happen to have it tagged as a milestone. For example:

column.17.displayname=
column.17.listdelimiter=<hr>
column.17.listmethod=nested(tasks).lists
column.17.sharecol=true
column.17.textmode=true
column.17.type=iterate
column.17.valueexpression=IF(CONTAINS("TASK ABC",UPPER({name})),{duration})
column.17.valueformat=HTML

Example using a milestone id:

IF({milestoneID}="5b27d511002cd8db965c9de0a5223fde",CONCAT(" ",{plannedCompletionDate}, ", ",{actualCompletionDate}))

Using this method to show the task duration in a project report alleviates the need to recalculate a custom expression every time the duration changes. 

Hope this helps,

Teale