Bond Calculator

Understanding your home loan repayment

A bond (the South African term for a home loan or mortgage) is almost certainly the largest debt you will ever take on, and small changes to the rate or term move the numbers by hundreds of thousands of rands. Every monthly instalment is split in two: a portion that pays down the actual amount you borrowed (the capital) and a portion that is pure interest charged by the bank. In the early years most of your payment is interest; only later does the balance tip towards paying off capital. This calculator shows you the monthly instalment, the total interest over the life of the loan, and how sensitive both are to the inputs.

The formula behind the instalment

The monthly repayment on a fixed schedule comes from the standard amortisation formula:

M = P × i × (1 + i)^n ÷ ((1 + i)^n − 1)

P = the amount you borrow (rands)
i = monthly interest rate = annual rate ÷ 12
n = number of months = years × 12

You do not need to work this out by hand - the calculator does it - but seeing it explains why a longer term lowers the monthly amount yet raises the total interest dramatically.

Why the prime rate matters

Most South African home loans carry a variable rate quoted relative to prime, for example "prime minus 0.5%". Prime itself tracks the Reserve Bank's repo rate, which the Monetary Policy Committee adjusts through the year. When the repo rate rises, prime rises, and your instalment goes up at the next adjustment - even though you signed nothing new. When budgeting, stress-test your repayment one or two percentage points above today's rate so a hiking cycle does not catch you out. Do not treat any single rate as fixed; confirm the current prime and your specific margin with your bank.

A worked example

Borrow R1 000 000 over 20 years (240 months) at an annual rate of around 11.75%. The instalment works out to roughly R10 800 a month. Over the full 20 years you repay about R2.6 million in total - meaning roughly R1.6 million of that is interest alone, more than the house price itself. Shorten the term to 15 years and the monthly payment rises, but the total interest falls sharply, because the bank has fewer years to charge it.

Costs the instalment leaves out

• Your deposit, which reduces the amount financed and often earns you a better rate.
• Transfer duty to SARS and conveyancing (transfer) attorney fees.
• Bond registration costs at the Deeds Office.
• Monthly municipal rates, levies (in a complex or estate), and home insurance.

Paying it off faster

Because interest is charged on the outstanding balance, even a modest extra payment each month goes almost entirely towards capital and can cut years off the loan and hundreds of thousands off the total interest. Try adding a few hundred rand to the instalment in the calculator and watch the total cost drop - it is the clearest argument for paying more than the minimum whenever you can.