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surcharge-calculator
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# surcharge-calculator
![](https://img.shields.io/badge/written%20in-Javascript-blue)
Determine what surcharge is worth paying in order to offset a loan.
## Download
- [⬇️ surcharge-calculator.html](dist-archive/surcharge-calculator.html) *(1.88 KiB)*

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<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
</head>
<body>
<div id="config-area">
Surcharge: <input type="number" min="0" value="2" id="surcharge-percent">%
<br>
Debt rate: <input type="number" min="0" value="5" id="debt-percent">% p.a.
<br>
Reward rate: <input type="number" min="0" value="0" id="reward-percent">%
<br>
Debt compounding period: <input type="number" min="1" value="1" id="compounding-period"> Day(s)
<br>
Interest free days: <input type="number" min="1" value="30" id="interest-free-days"> Day(s)
</div>
<hr>
<div id="output" style="white-space:pre;">
</div>
<script type="text/javascript">
function $(query) {
return document.querySelector(query);
}
function output(sz) {
$("#output").textContent = sz;
}
function recalculate() {
var S = + $("#surcharge-percent").value;
var D = + $("#debt-percent").value;
var R = + $("#reward-percent").value;
var T = + $("#compounding-period").value;
var P = + $("#interest-free-days").value;
// @ref http://money.stackexchange.com/questions/72654/what-credit-card-surcharge-rate-should-i-accept
var lhs = (1 + (S - R)/100); // net expense of the surcharge minus the rewards
var rhs = (1 + ((T/365)*D/100)) ** P; // interest that you would pay otherwise
var ret = "DECISION: ";
if (lhs < rhs) {
ret += "Better to use CREDIT.";
} else if (lhs > rhs) {
ret += "Better to use DEBIT.";
} else {
ret += "No difference.";
}
ret += "\n\n";
ret += "Net expense of (surcharge minus rewards): " + lhs + "\n";
ret += "Interest that you would pay otherwise: " + rhs + "\n";
output(ret);
}
function setup() {
var items = document.querySelectorAll("#config-area input");
for (var i = 0; i < items.length; ++i) {
items[i].addEventListener('change', recalculate);
items[i].addEventListener('keyup', recalculate);
}
recalculate();
}
setup();
</script>
</body>
</html>