How do I calculate square footage in QuickBooks online or is here an app for it?

I know how this feature is beneficial to your business, @michelle182. I'm here to share some details about it and will help you resolve the issue.

In QuickBooks Online the option to enter and calculate square footage is currently unavailable. You can only use quantity when you'll create and send invoice or estimates. Unlike QuickBooks Desktop where you can calculate multiple units of measurements for items.

So, if you want to enter items and track by measurements, you can use a third party app from Intuit QuickBooks Apps page. Also, know that we're unable to suggest any app for you.

If you want guide on how to manage your inventory, you can visit this article: Set up and track your inventory in QuickBooks Online.

Furthermore, you can also visit this following article on how to manage invoice and estimates:
 

• Create invoices in QuickBooks Online
• Create and send estimates in QuickBooks Online
   

I'll be here if you need more help or clarifications about how to track units of measure for items. Take care!