Calculate grand total from multiple rows of quantity and price/unit

Hello everyone,

I've been working on an automated ordering tool for my workplace, while trying to learn as much as I can for VBA programming (only schooled in the very basics). Something I still have difficulties with though, is successfully implementing loops.

For this ordering tool, I would like to calculate and present the grand total, starting from a fixed range of two columns (each 10 rows), where one has quantities, and the other has the price/unit. Since I'm not allowed to change the layout of the ordering sheet (we still fax orders...), I can not add a column for the subtotals. Also, not all rows are filled necessarily.

I had some success with this as a worksheet function: =IF(ISERROR(H30*I30);0;H30*I30)+..., but I would really like to have it as a part of a big VBA function, since colleages might alter/delete the worksheet formula.

I looked and messed around with For loops, but I can't make it work.

Can anyone help me with this?