How Can I do Sum and Multiply columns in DBGrid

My database is connect to a data cotrol that it diplay into a DBGrid1. So I wan to sum and multiply some colums and and totalise a new column.

I do not know in which event can I do this

Dim QtySales, TaxSales, NewTot

QtySales = DBGrid1.Columns(3) .Value * DBGrid1.Columns(4) .Value

TaxSales =DBGrid1.Columns(5) .Value + QtySales

NewTot = TaxSales


I Need a new tota column to be createdl

I tried this in Form Load Event and it do not work when the user enter values in the rows in DBGrid1. How can I do that


Please help me