Hi there, @Firstcarpentry.

Currently, calculating the volume of lumber in a complete aspect is unavailable. Although, you can add the unit of measure in board foot so that it will be easy for you to identify the item. Then, perform the rest of the calculations when you create a transaction. I'll show you how to do that. Also, I added some screenshots so that you'll see how it looks like.

First, we have to activate the Units of Measure so that you can add board foot. Here's how:

1. Go to the Edit menu and select Preferences.
2. Choose the Company Preferences tab.
3. In the Unit of Measure section, click Enable or choose Multiple U/M Per Item.
4. Once done, select OK. 
   
   
   
   

Here's how to add an inventory item in QuickBooks.

1. Go to the Lists menu and select the Item drop-down button.
2. Select New item and choose Inventory Part in the Type section.
3. In the Unit Of Measure section, select the drop-down icon and select Add new.
4. Choose Length then, click Next. Then, tick Other and click Next.
5. Enter the Name such as, "Board foot" and then the Abbreviation as "bdft."
6. Tick the box under the  Add column and enter the Name such as "2'' x 4'' x 12ft."
7. Under the number of BDFT, enter 8 and click Next.
8. Confirm the transaction after choosing the item name of the sales, purchase,and shipping name of the measurement.

Once done setting up the unit of measure, enter the Item name such as, "Wood." Verify the details of your item and then select Ok.

Now, let's apply the scenario you've given by creating an invoice transaction.

1. Go to the Customers menu and then select Create Invoice.
2. Enter the necessary details.
3. Under the Item code column, choose Wood item with the description of "2'' x 4'' x 12ft. or 8 Board foot."
4. Verify the details and then, select Save and close.
   
   

I can see that this article can be handy in the future with managing your customer's invoices: Set up and send progress invoices in QuickBooks Desktop.

Please let me know if you need anything else in QuickBooks Online. I'll be here, ready to help. Stay safe.