function calculateBond() {
var priceInput = document.getElementById("propertyValue").value;
var depositInput = document.getElementById("depositAmount").value;
var termInput = document.getElementById("bondTerm").value;
var rateInput = document.getElementById("bondRate").value;
// Validation
if (priceInput === "" || termInput === "" || rateInput === "") {
alert("Please fill in all required fields (Property Price, Term, and Bond Rate).");
return;
}
var price = parseFloat(priceInput);
var deposit = parseFloat(depositInput) || 0;
var termYears = parseFloat(termInput);
var annualRate = parseFloat(rateInput);
if (isNaN(price) || isNaN(termYears) || isNaN(annualRate)) {
alert("Please enter valid numbers.");
return;
}
var principal = price – deposit;
// Handle full cash purchase
if (principal <= 0) {
document.getElementById("monthlyRepayment").innerHTML = "R 0.00";
document.getElementById("totalPrincipal").innerHTML = "R 0.00";
document.getElementById("totalInterest").innerHTML = "R 0.00";
document.getElementById("totalCost").innerHTML = "R " + price.toLocaleString('en-ZA', {minimumFractionDigits: 2});
document.getElementById("results-area").style.display = "block";
return;
}
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = termYears * 12;
// Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
var monthlyPayment = 0;
if (monthlyRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
var x = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPayment = (principal * x * monthlyRate) / (x – 1);
}
var totalPayment = monthlyPayment * numberOfPayments;
var totalInterest = totalPayment – principal;
// Display Results
var formatter = new Intl.NumberFormat('en-ZA', {
style: 'currency',
currency: 'ZAR',
minimumFractionDigits: 2
});
document.getElementById("monthlyRepayment").innerHTML = formatter.format(monthlyPayment);
document.getElementById("totalPrincipal").innerHTML = formatter.format(principal);
document.getElementById("totalInterest").innerHTML = formatter.format(totalInterest);
document.getElementById("totalCost").innerHTML = formatter.format(totalPayment);
document.getElementById("results-area").style.display = "block";
}
Understanding Bond Rates in South Africa
Entering the South African property market requires a clear understanding of how bond rates affect your long-term financial commitments. A bond, commonly known internationally as a mortgage, is likely the largest debt instrument a consumer will undertake. The cost of this debt is primarily driven by the South African Reserve Bank's (SARB) repurchase rate (repo rate), which directly influences the prime lending rate offered by commercial banks.
Current Market Context: In South Africa, banks typically quote bond rates linked to the "Prime" rate (e.g., Prime + 1% or Prime – 0.5%). Your personal rate is determined by your credit score, the size of your deposit, and the property value.
How the Calculation Works
This calculator determines your monthly repayment obligation based on standard amortization principles used by major SA banks (Standard Bank, FNB, Absa, Nedbank, Capitec). The formula considers three critical variables:
Principal Bond Amount: This is the purchase price of the property minus any cash deposit you put down. A higher deposit reduces the principal, thereby reducing the total interest accrued over the lifespan of the loan.
Bond Term: The standard term in South Africa is 20 years (240 months), though 30-year terms are becoming increasingly common to lower monthly installments, albeit at a higher total interest cost.
Annual Bond Rate: This is the percentage interest charged on the diminishing balance of your loan.
The Impact of a Deposit
Providing a cash deposit does more than just reduce the amount you need to borrow. It signals lower risk to the bank, which can often help you negotiate a more favorable interest rate (e.g., Prime minus). Even a 10% deposit can save hundreds of thousands of Rands in interest over a 20-year term.
Affordability and Additional Costs
When calculating your bond repayments, remember that banks in South Africa typically require that your monthly repayment does not exceed 30% of your gross monthly income. Furthermore, this calculator focuses on the capital and interest repayment. Prospective buyers should also budget for:
Bond Registration Costs: Fees paid to the bond attorney to register the bond at the Deeds Office.
Transfer Duties: A tax levied by SARS on properties valued above a certain threshold (currently R1,100,000).
Monthly Levies and Rates: Municipal charges that are separate from your bond repayment.
Using accurate tools to estimate these costs ensures you can make an informed offer to purchase without overextending your financial capacity.