What is a mortgage top-up?

A top-up is borrowing more on your existing mortgage, often to fund a renovation, a car or other large cost. It increases your loan balance, so your repayments rise and you pay more interest over the term.

How much will a top-up add to my repayments?

It depends on the top-up amount, your rate and the remaining term. This calculator works out the extra repayment per fortnight or month and the additional interest over the life of the loan, so you can see the true cost.

Is topping up the mortgage a good idea?

It is usually cheaper than other borrowing because the rate is lower, but spreading a short-term cost over a long mortgage term can mean paying interest on it for decades. Consider repaying a top-up faster than the rest of the loan.

Mortgage Top-Up Calculator

This calculator shows what happens to your mortgage repayments when you borrow more against your home, whether for a renovation, a car or another large cost. A top-up is often cheaper than other borrowing because home loan rates sit below personal loan and credit card rates, but stretching a short-term cost over a long remaining term can quietly add far more interest than you expect. You enter the top-up amount, your interest rate and the remaining term in years, and the calculator treats the top-up as its own loan repaid on a standard table mortgage basis. It returns the extra repayment added to your mortgage each month, the equivalent extra per fortnight, the extra interest you pay over the remaining term, and the total you repay on the top-up once that interest is included. Use these figures to judge whether a top-up suits you, and to see how much you could save by repaying it faster than the rest of your mortgage, for example through extra payments or a shorter term on that portion of the loan. It is a useful check before agreeing to a top-up with your bank or broker, so you understand the real long-term cost rather than just the headline monthly increase. Figures are estimates based on the rate and term you enter and are not a loan offer; confirm exact numbers with your lender.

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Top-up amount

Interest rate

Remaining term (years)

extra repayment per month from the top-up

Extra per fortnight$0

Extra interest over the term$0

Total repaid on the top-up$0

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.

How it works

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.

Worked example

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.

Related calculators

Mortgage Repayment Calculator: full repayments.
Mortgage Extra Repayment Calculator: paying it down faster.
Home Renovation Budget Calculator: budget a reno.
Home Equity Calculator: equity to draw on.

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