Decimal outDec;
if (Decimal.TryParse("<some string>", out outDec))
{
// Sets Decimal as a Percentage out of 100
PercentageColumn.Text = Convert.ToInt32(outDec * 100).ToString();
}
Edit
To do it specifically using String
formatting :
PercentageColumn.Text = "#PERCENT{P0}";
From: Social.MSDN - Shows Percentages on Pie Chart ( Aspnet) and MSDN - Standard Numeric Format Strings