This New Zealand calculator shows the repayment effect of topping up your mortgage for a renovation or other cost. Borrowing more raises your repayments and the interest you pay over the term, and spreading a short-term cost over a long loan can be expensive. You enter the top-up, your rate and the remaining term, and the tool shows the extra repayment and the additional interest, so you can see what a top-up really costs.
Assumes the top-up is repaid over the remaining term at the rate shown. Spreading a short-term cost over a long term adds a lot of interest; consider repaying a top-up faster. Estimate only, not a loan offer.
The calculator treats the top-up as its own loan over your remaining term and works out the table repayment using the standard amortisation formula at your rate. It shows the extra you would pay each month and fortnight, the total repaid on the top-up over the term, and the additional interest, which is the total repaid less the amount borrowed. That interest figure is the true cost of spreading the cost across the loan.
A $50,000 top-up at 6.5% over 20 years adds about $373 a month to your repayments. Over the 20 years you would repay roughly $89,500 on the top-up, meaning about $39,500 of extra interest, which is why repaying it faster than the main loan can save a lot.
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