Calculate Hours total in datagridview cells

Incidentally, if you want to display the result in hours and minutes rather than as decimal hours, then it would be easier to accumulate the hours and minutes components separately:

// now using ints
int totalHours = 0;
int totalMins = 0;

foreach(DataGridViewRow row in dataGridView1.Rows)
{
   if (row.IsNewRow) break;
   object val = row.Cells[10].Value;
   if (!Convert.IsDBNull(val) && val != null && val.ToString().Length > 0)
   {
      string[] items = val.ToString().Split(':');
      totalHours += int.Parse(items[0]);
      totalMins  += int.Parse(items[1]);
   }
}

totalHours += totalMins/60;
totalMins = totalMins % 60;
label1.Text = String.Format("{0:D2}:{1:D2}", totalHours, totalMins);