If you know the unit cost and the quantity of units you are buying, you can use Excel to automatically calculate the total cost for the order. By performing the calculation in Excel, you can update and change either the quantity purchased or the unit cost to update the total cost of the purchase. This is useful for an invoice spreadsheet so you do not need to recreate an invoice for each purchase.
Type "Order Quantity" in cell A1.
Type "Unit Price" in cell A2.
Type "Total Cost" in cell A3.
Enter the quantity purchased in cell B1.
Enter the unit price in cell B2.
Type "=B1*B2" in cell B3.