This is a smart little chunk of code, used to calculate vat in sterling.

<html>
<head>
<title>vat calculator</title>
</head>
<body>

<p>This will calculate the VAT contained within a total amount, or the VAT charged on a
net amount.</p>
<script language="javascript">

// starts calculation
function calculate() {
var vat, total, vatTotal, finalTotal;

var frm = document.calculator;

total = evalnum( frm.total.value );
if (total == "NaN") total=0;
if (total == 0) {
alert("Please enter an ammount first.");
frm.total.focus()
return
}
else
frm.total.value = evalpound( total );
if (frm.chkvat.checked) {
vatTotal = total * (7/47);
finalTotal = total - vatTotal;
frm.calcvat.value = evalpound( vatTotal );
frm.calctotal.value = evalpound( finalTotal );
}
else {
vatTotal = total * 0.175;
frm.calcvat.value = evalpound( vatTotal );
frm.calctotal.value = evalpound( total + vatTotal );
}
}

</script>
<script language="JavaScript">
//----------------------------------------------------------------------------
// Scripts for VAT calculator
//----------------------------------------------------------------------------

var frm = document.calculator;

// ----------------------------------------------------------------------------
function selectBox(frmE, selectText) {

var i;
var selectIndex = 0;

for(var i=0;i<document.calculator[frmE].length;i++) {
if(document.calculator[frmE].options[i].value == selectText) { selectIndex = i;}
}

return selectIndex;
}

// ----------------------------------------------------------------------------
function evalnum(numstr) {

var i,c,neg;
var ret="";
var p=false;

neg = 1;
if (numstr.indexOf("-") >=0) { neg = -1; }

for (i=numstr.length-1; i>=0; i--) {
c=numstr.charAt(i);

if (c == ".") {
if (p == false) p=true;
else c="";
}

if ((c < "0" || c > "9") && c != ".") c="";

ret = c + ret;
}

if (ret=="" || ret=="NaN") ret="0";
return parseFloat(ret*neg);

}

// ----------------------------------------------------------------------------
function evalpound(num) {

var i,l,d;
var nums;
var ret;

nums = String(Math.round(num*100));

while (nums.length <3) nums = "0" + nums;

l = nums.length-3;
ret = "." + nums.charAt(l+1) + nums.charAt(l+2);

d=0;
for (i=l; i>=0; i--) {
ret = nums.charAt(i) + ret;
d++;
if (d==3 && i>0) {
ret="," + ret;
d=0;
}
}

ret = "" + ret;
return ret;

}

</script>


<form name="calculator">
<table align="center" width="440" border="0" cellspacing="0" cellpadding="0">
<tr>
<td width="440" height="50"><table width="440" height="50" border="0" cellspacing="0"
cellpadding="0">
<tr>
<td width="26" height="20"></td>
<td width="393" height="20" bgcolor="#BDC4D5"><table width="390" height="100" border="0"
cellspacing="0" cellpadding="2">
<tr>
<td bgcolor="#BDC4D5"><table width="390" border="0" cellspacing="0" cellpadding="3">
<tr>
<td colspan="2"></td>
<td></td>
</tr>
<tr>
<td class="calcbody" width="220" valign="top">Input amount:</td>
<td class="calcbody" width="150"><input type="text" class="calcinput" name="total" title
min="0" max format value size="9" style="width: 100px; text-align: right;"
onchange="calculate(this)"></td>
<td class="calcbody" width="20"></td>
</tr>
<tr>
<td class="calcbody" width="220" valign="top">Inclusive of VAT?</td>
<td class="calcbody" width="150"><input type="checkbox" class="calcinput" name="chkvat"
title onchange="calculate(this)" value="ON"><script language="javascript">document.calculator.chkvat.checked = false</script>Yes</td>
<td class="calcbody" width="20"></td>
</tr>
<tr>
<td class="calcbody"></td>
<td class="calcbody"><table width="100" height="22" border="0" cellpadding="0"
cellspacing="0">
<tr>
<td width="100" height="22" class="calcbody" align="center"><a class="calcbutton"
href="javascript:calculate(this)">Calculate</a></td>
</tr>
</table>
</td>
</tr>
<tr>
<td colspan="2" class="calcsubtitle"><b>Results</b></td>
<td></td>
</tr>
<tr>
<td class="calcbody" width="220" valign="top">VAT:</td>
<td class="calcbody" width="150"><input type="text" class="calcinput" name="calcvat" title
min max format value size="9" style="width: 100px; text-align: right;" readonly="true"></td>
<td class="calcbody" width="20"></td>
</tr>
<tr>
<td class="calcbody" width="220" valign="top">Amount:</td>
<td class="calcbody" width="150"><input type="text" class="calcinput" name="calctotal"
title min max format value size="9" style="width: 100px; text-align: right;"
readonly="true"></td>
<td class="calcbody" width="20"></td>
</tr>
</table>
</td>
</tr>
</table>
</td>
<td width="21" height="20"></td>
</tr>
</table>
</td>
</tr>
</table>
</form>

<p>&nbsp;</p>
</body>
</html>