Paid Workspaces can calculate numeric fields in List view and Table view to find their sum, average, or range!
What you'll need
Workspaces on the Unlimited Plan and above can calculate columns.
Everyone, including guests, can calculate columns.
Calculate columns
To calculate a column:
In any List or Table view, click the + icon to show existing columns or add additional columns.
Hover over any numeric column to reveal the Calculate button at the bottom of it.
Select Sum, Average, or Range.
Click Calculate.

This will work for both standard fields as well as numeric Custom Fields that you create.
Once you add a calculation, you'll see results for each group in your List or Table view, as well as a calculation for the entire column.

Calculate an entire column
Eliminate task grouping to see one calculation for your entire column:
Click on a column header in List & Table view to sort.
Select Sort entire column.
Select Sum, Average, or Range.
Click Calculate.
Want to learn more?
Calculate data using Dashboards and Portfolios!
Learn more about List view.
Discover Board view and its features.
Check out all the different types of Custom Fields!