Amortization Calculator

Reply Thu 21 Aug, 2003 03:36 pm
I've been toying with online tools such as an Amortization Calculator and have a question for the beanies here.

Well, how accurate is it?

Amortization Calculator
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Reply Thu 21 Aug, 2003 03:44 pm
Hmm... I compared it to Yahoo's calculator and it matches. Then I ran it against this one: http://ray.met.fsu.edu/cgi-bin/amortize and it comes up off by $.02 in total. I used a $6000.00 loan at 10% APR over 24 months for all 3.

This FSU site makes a note about the final payment being adjusted to account for rounding through the life of the loan.
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Reply Thu 21 Aug, 2003 04:07 pm
Very timely. The Ginnie Mae Calculator attempts to factor in such factors as insurance, pmi, and so forth. Phoenix furnished that one on Abuzz a year ago.
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Craven de Kere
Reply Thu 21 Aug, 2003 04:08 pm
Thanks, I had no idea (since I'm a nOOb to financial matters) what could be a factor in making it innacurate.
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Reply Thu 21 Aug, 2003 04:17 pm
Yours is good because Ginnie Mae is making assumptions that might not prove out. Did you get involved with the Present Value formula in setting up the calculator? I used it once for a bond amortization schedule and it is a beast even for a master typist.
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Craven de Kere
Reply Thu 21 Aug, 2003 04:39 pm
Actually I'm just using a javascript that has been around the net for a while. It just uses basic math.

Code: function getAmortization(a,n,p) {
var i=0;
var sATline="";
var oAmortizationTable=document.getElementById("amortizationtable");
var sCR=String.fromCharCode(13);
var sTab=String.fromCharCode(27);

/* Calculate amortization and write table to text area **/
var payment = getPayment(a,n,p);
oAmortizationTable.value = sCR + "Monthly Payment = " + (Math.round(payment*100)/100) + sCR + sCR;
oAmortizationTable.value += "Month Balance Principal Interest Payment Total Interest" + sCR;
var balance=a;
var interest = 0.0;
var principal=0.0;
var totalinterest=0.0;
for (i=1;i<=n;i++) {
interest = balance*p/1200;
totalinterest += interest;
principal = payment-interest;
balance -= principal;
sATline = i.toString()+getSpaces(7-i.toString().length);
sATline += Math.round(balance*100)/100;
sATline += getSpaces(18-sATline.length);
sATline += (Math.round(principal*100)/100);
sATline += getSpaces(29-sATline.length);
sATline += (Math.round(interest*100)/100);
sATline += getSpaces(39-sATline.length);
sATline += (Math.round(i*payment*100)/100);
sATline += getSpaces(50-sATline.length);
sATline += (Math.round(totalinterest*100)/100) + sCR;
oAmortizationTable.value += sATline;

function getSpaces(n) {
var i=0;
var sSpaces="";
for (i=0;i<n;i++) {sSpaces += " ";}
return sSpaces;

function getPayment(a,n,p) {
/* Calculates the monthly payment from annual percentage
rate, term of loan in months and loan amount. **/
var acc=0;
var base = 1 + p/1200;
for (i=1;i<=n;i++)
{ acc += Math.pow(base,-i); }
return a/acc;
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