Amortization Calculator

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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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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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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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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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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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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");

		oAmortizationTable.style.visibility="visible";

		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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