Adding square feet calculation

how can I add square feet calculation in the invoice?
for example, I am seeling a product of steel window which calculate in sq feet x quantities. (16 sq feet x 2 pcs = Rs.)
how can I do with our present invoice structure?
thanking you
with regards

If you are considering the windows as inventory items, define the Unit name as Sq. Ft.. See If these windows are not already manufactured, but are being made to order for installation and priced in square feet, create a non-inventory item. For the unit price, enter the price per square foot. See

In either case, enter the number of square feet as the quantity.

thank you for your reply…
i want to sell a product which measured as square feet. so here i calculate sq feet manually then add it to the invoice
is there any way how we could enter sq feet then qty and the amount

Number of square feet is the quantity. If you want to have manager do the calculation for you, enter it in the field, such as X*Y.