Javascript runtime error: 'calcAmt' not defined

function calcAmnt() 
var text1 = document.getElementById('<%= txt_qty_OS.ClientID %>');
var text2 = document.getElementById('<%= txt_rate_OS.ClientID %>');
var txt_amnt = document.getElementById('<%= txt_amt_OS.ClientID %>');

                                           if (text1.value.length == 0) {
                                               text1.value = 0;
                                           }

                                           if (text2.value.length == 0) {
                                               text2.value = 0;
                                           }
                                           var x = parseFloat(text1.value);
                                           var y = parseFloat(text2.value);

                                           txt_amnt.value = x * y;
                                           if (txt_amnt.value == "NaN")
                                               txt_amnt.value = 0;
                                       }

<table class="sTable">
<tr>
<td class="sTD_R">
Quantity:
   </td>
 <td>
<asp:TextBox ID="txt_Qty_OS" runat="server" Height="20px" onChange ="calcAmt()" width="167px">0</asp:TextBox>
</td>
</tr>
<tr>
<td class="sTD_R">
Rate:
</td>
<td>
<asp:TextBox ID="TXT_RATE_OS" runat="server" Height="20px" OnChange ="calcAmt()" width="167px">0</asp:TextBox>
</td>
</tr>
<tr>
<td class="sTD_R">
Amount:
</td>
<td>
<asp:TextBox ID="TXT_AMT_OS" runat="server"Class="textbox" Height="22px" Width="165px">0</asp:TextBox>
</td>
</tr>
</table>