Auto-Calculating PDF Work Order

I’ve built a fillable work-order PDF for my team and now I want it to do the math for us. The form already has fields for Quantity, Unit Price, Subtotal, Tax, Total, and Discount, but they’re all static... (Budget: $250 - $750 USD, Jobs: Adobe Acrobat, Adobe Creative Cloud, Electronic Forms, JavaScript, PDF, PHP, Scripting, Software Development)