Need to replace null value by zero to calculate

I have a datagridview (dgv) that is bound to database:

QtyOnHand     QtyOnHold       QtySold         QtyAvailable
10              2               1               null
12              null            null            null
null            null            null            null
7               5               null            null
25              null            5               null

I want to iterate through the datagridview and calculate the last column with this formula:

QtyAvailable = QtyOnHand - QtyOnHold - QtySold

so the datagridview should become:

QtyOnHand     QtyOnHold       QtySold         QtyAvailable
10              2               1               7
12              null            null            12
null            null            null            null
7               5               null            2
25              null            5               20

I use this code:

foreach (DataGridViewRow row in dgv.Rows)
{
     if (string.IsNullOrEmpty(row.Cells["QtyOnHand"].Value.ToString()))
     {
          row.Cells["QtyAvailable"].Value = Convert.ToInt32(row.Cells["QtyOnHand"].Value) - Convert.ToInt32(row.Cells["QtyOnHold"].Value)
                                   - Convert.ToInt32(row.Cells["QtySold"].Value) - Convert.ToInt32(row.Cells["QtyAvailable"].Value) 
     }
}

The problem is when any of the value on the right side of the equation is null, the whole equation is invalid. I need to replace the null values by 0 on the fly so that it can be calculated. Note that I do not want to actually replace the null values in the second and third columns of the table, just replace it in the equation whenever it applies to make the calculation work.