invoice form

hi guys,
i am making invoice form in vb6, i have to make grid view so that user can add item details in row just by clicking on it and new Grid is open when user want to add new item.
like
productname description qnt rate amount

row1
row2
.....

TOTAL AMOUNT
how can i calculate fields in datagrid colums and get the result in other colums.like when i enter quantity and rate amount and TOTAL AMOUNT will automatically calculated. how can i do this..