Currently there is no field that we can expose on the Item Record that will show the total onhand for all Locations. An open enhancement asking for this exists. Its details are as follows:
Enhancement 148130: Lists > Accounting > Items > Inventory Tab: Show Total Quantity on Hand (Total for all Locations).
As an Alternate Solution you can create a Sublist under the Item Record using an Item Search. Below are the steps to take to accomplish this:
1. Lists > Accounting > Items > Search
---Click Create Saved Search
---Set Search Title = Item Sublist Search for Total Locations (or what ever you like)
---Under Criteria tab - leave as is
---Under Results tab > Add Inventory Location and Location on Hand (and possibly other location fields such as Location Available and Location On Order -- basically exposing same fields as under the Location tab of the Item
----------Set Show Totals = Yes
---Under Available Filters add Internal ID mark Show in Filter Region checkbox
2. Customization > Forms> Sublists > Item tab
---Under Search column click the drop down to lookfor above search and select it
---Under Label column set Total On Hand - All Locations (or what ever you like)
---Under Tab column select Locations (or Inventory or the tab you would prefer to see this under)
---Check off all of the Item Types you want this Sublist to show under
3. Now proceed to any item record and under the tab you set this sublist to show; you will see the results.