List Aggregators allow you to quickly view a summary of your data in the footer of a list and can be customised to display SUM, MIN, MAX or AVERAGE functions at the click of a button.
To set up or update your aggregators, select the settings cogwheel in the list header.
Then, toggle on any columns to enable their aggregators and select the function you wish to apply. 
By saving this to your personal list settings, any selected aggregators will appear each time you view the list or export to PDF.