function calculate() {

var loanAmount = document.pay_points.loan_amount.value;
var  interestRate = document.pay_points.interest_rate.value  / 100 / "12";
var numberPayments = document.pay_points.number_payments.value;

var  pt_interestRate = document.pay_points.pt_interest_rate.value  / 100 / "12";

var yearsOwn = document.pay_points.years_own.value;

var discountPercent = document.pay_points.discount_percent.value  / 100;

// Now compute the monthly payment without points
    var x = Math.pow(1 + interestRate, numberPayments);
    var  monthlyPayment = (loanAmount * x * interestRate)/(x-1);
   
// Now compute the monthly payment with points
    var x = Math.pow(1 + pt_interestRate, numberPayments);
    var pt_monthlyPayment = (loanAmount * x * pt_interestRate)/(x-1);

document.pay_points.monthly_payment.value = decimal(monthlyPayment);

document.pay_points.pt_monthly_payment.value = decimal(pt_monthlyPayment);

var discountAmount = parseFloat(loanAmount) * parseFloat(discountPercent);
document.pay_points.discount_amount.value = decimal(discountAmount);

var paymentDifference = parseFloat(monthlyPayment ) - parseFloat(pt_monthlyPayment);
document.pay_points.payment_difference.value = decimal(paymentDifference);

var pointsMonths = parseFloat(discountAmount) / parseFloat(paymentDifference);
document.pay_points.points_months.value = decimal(pointsMonths);

var pointsYears = parseFloat(pointsMonths) / 12;
document.pay_points.points_years.value = decimal(pointsYears);

var pointsSavings = (parseFloat(paymentDifference)* parseInt(yearsOwn * 12)) - parseFloat(discountAmount);
document.pay_points.points_savings.value = decimal(pointsSavings);

}