How to calculate total weight of order?

Hello,

I have added a customised field for the weight of an item under "Products".
I have also figured out how to make this weight show in templates.
Now the question is, how can I add a calculation at the bottom of quotes/invoices/sales orders/purchase orders to show the total weight of the order (for shipping purposes)?

I'd be grateful for any tips.

Thanks in advance and cheers,
Cheryl