Bond repayment calculator

Twenty years is a long time to pay interest without once checking the arithmetic. This runs the full amortisation schedule on a South African home loan, and it runs it twice, so you can put one plan against another: a bigger deposit, a shorter term, an extra R2,000 a month, a bonus paid in as a lump sum in month 14.

The mechanics are standard SA bond mechanics. Repayments are worked out on the rate and term you set, held level across the loan. Almost every home loan in the country tracks prime rather than a fixed contract rate, so the rate you enter is whatever your bank quoted you against prime, and prime moves when the Reserve Bank's Monetary Policy Committee moves the repo rate. Testing a rise is a separate job, and the rate shock tool does it.

Have these ready:

• the purchase price and your deposit, or the outstanding balance if the bond already exists
• the rate your bank quoted, and the term in years
• any extra monthly payment, plus any lump sum and the month it lands
• your take-home pay and other monthly debt, for the debt-load gauge at the bottom

Results come in three layers. The stat cards compare the two scenarios on monthly payment, payoff time and total interest, and give the interest saved going from one to the other. The true-price strip shows everything that leaves your account over the term, deposit and principal and every rand of interest, set against the purchase price. Then the charts: balance falling, the principal-versus-interest split of each payment, cumulative interest, and a schedule you can open year by year down to the month.

Early payments on an SA bond are mostly interest. That is why money aimed at the principal moves the payoff date so hard, and the schedule shows how hard at your own numbers.

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Money Cat is an information tool, not financial advice. Confirm any figure that matters with your bank, lender or a registered professional.