Mozo guides

How are mortgage repayments calculated?

Person counts cash for their mortgage repayment

Every month or fortnight, your home loan lender will bill you a specific amount to chip away at the total cost of your mortgage until you discharge it. These mortgage repayments vary based on the home loan type, your interest rate, the size of your principal, and any administrative fees you incur. 

So how do lenders calculate your mortgage repayments? Let’s break down what you need to know.

The costs of a home loan repayment

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Home loan repayments chip away at the total debt you took out at the start of your loan. A standard mortgage repayment typically includes these costs: 

  • Principal repayments
  • Interest charges
  • Fees.

Your principal repayment is a chunk of the original amount you borrowed. If you borrowed $500,000, your principal is $500,000. This money is divided by the number of months or fortnights in your home loan term. For example, a 25-year home loan will spread the principal out over 300 months, making 300 principal repayments. 

Some home loans just charge interest-only repayments for the first few years. These loans don’t pay down any of your principal, so you don’t own any more of the property, but they can reduce the size of your repayments for a while.

Interest is calculated daily by your lender and added to your principal repayments. At the end of every business day, your lender multiplies your loan’s interest rate by how much of your principal you have left to repay, then divides this amount by 365 days, or 366 if it’s a leap year. A monthly mortgage repayment will usually have 30 to 31 days of interest included in it.

The type of interest rate can also affect your mortgage repayments. Variable interest rates can ‘vary’ over time, which changes the amount of interest you pay, while fixed interest rates stay the same for a short term, typically 1 to 5 years.

Lastly, your lender may charge administrative fees. These cover the maintenance and account-keeping required for your mortgage. Fees can be ongoing or once-offs. There is no such thing as a truly fee-free home loan, since third-party charges may apply. Always read the product disclosure statement and target market determination before applying for a home loan. 

How to calculate your mortgage repayments

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When comparing home loans, it is useful to do your own calculations to see how much a home loan could charge you every month in repayments. 

Mozo’s mortgage repayment calculator is designed to take your principal, loan term, and interest rate into account and give you an idea of how much you would pay monthly and how much interest you could pay over time.

Our other home loan calculators can run your mortgage through different repayment scenarios to give you an idea of how much a home loan could cost in total.

  • Our rate change calculator shows you what happens to your monthly repayments if your interest rate goes up. 
  • Our home loan comparison calculator compares two different home loans to give you an idea of how much each loan might cost.
  • Our borrowing calculator looks at your income and expenses and comes up with an idea of how much you can borrow and afford to repay monthly.

Aside from your mortgage repayments, however, there are some other home-buying costs you may need to consider. 

For instance, our stamp duty calculator determines how much you could have to pay in stamp duty. Stamp duty is a government tax on transferring a property title from the seller to the buyer and could cost tens of thousands of dollars. While stamp duty is not part of your home loan, it is still an important bill to budget for when buying a home.

How much of your income should mortgage repayments be?

A good rule of thumb is that your home loan costs should not comprise more than 30% of your paycheck. The Australian government considers 30% to be the threshold for mortgage stress. 

To find your mortgage stress limit, multiply your after-tax income by 0.3: this is the maximum amount you should be paying for your mortgage repayments.

Do home loan repayments decrease over time?

Yes, home loan repayments decrease over time. This is because the amount of interest you pay decreases over time. 

Because you pay down more and more of your loan, your interest rate is calculated with a smaller outstanding loan balance. So while the size of your principal repayment will remain consistent for all your mortgage repayments, the amount of interest charged will fall.

How to reduce home loan repayments

Person winks holding a piggy bank

There are several strategies you can take to lower your mortgage repayments. 

The main way to reduce the size of your mortgage repayments is to reduce the size of your principal, which means looking for cheaper properties. This way, you don’t have to borrow nearly as much – or save as much for a deposit

Speaking of which, the size of your home loan deposit can also impact your mortgage repayments. Deposits (and later your home equity) establish your loan-to-value ratio (LVR). The lower your LVR, the lower your interest rate. Aim for a 20% deposit or an 80% LVR to access a lender’s most competitive rates.

If you’re already servicing your mortgage, lowering your principal may not be possible. Instead, you can focus on reducing your interest. You can do this by:

Note if you want to refinance, you’ll need to establish your home equity by doing a property valuation. If your LVR has lowered either because your home value has risen or you’ve repaid more of your home loan, you may be eligible for a lower LVR tier and thus, a lower interest rate. 

Additionally, when comparing home loans, pay attention to the fees charged with a mortgage account. While they don’t make up most of your home loan, a $20 monthly fee still adds up to $240 yearly. 

FAQs about mortgage repayments

Are mortgage repayments tax deductible?

If you are a property investor, you can claim the interest from your mortgage repayments on your tax return as a deduction. You cannot claim the principal. Other rental property expenses, such as agent fees, may be tax deductible.

Always consult a tax advisor when lodging your tax return.

Compare home loans in the table below.

Compare home loans - last updated 9 December 2023

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* WARNING: This comparison rate applies only to the example or examples given. Different amounts and terms will result in different comparison rates. Costs such as redraw fees or early repayment fees, and cost savings such as fee waivers, are not included in the comparison rate but may influence the cost of the loan. The comparison rate displayed is for a secured loan with monthly principal and interest repayments for $150,000 over 25 years.

** Initial monthly repayment figures are estimates only, based on the advertised rate. You can change the loan amount and term in the input boxes at the top of this table. Rates, fees and charges and therefore the total cost of the loan may vary depending on your loan amount, loan term, and credit history. Actual repayments will depend on your individual circumstances and interest rate changes.

^See information about the Mozo Experts Choice Home Loan Awards

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Evlin DuBose
Evlin DuBose
Money writer

Coming from a diverse background in filmmaking, music production, and creative writing, Evlin is passionate about putting money-matters in relatable, personal contexts. Budget what? Finance who? She’s keen to find out!