Home loan repayment calculator

This calculator simulates the entire loan term by processing every single calendar day — one by one. For a 30-year loan, that’s over 10,950 days calculated in just a split second. It’s the only accurate way to calculate interest, and this is the first (and only) Australian home loan calculator that does it.

It’s the only calculator that:

• Calculates interest daily, while still charging monthly — just like your bank does.
• Accurately shows the true interest savings from weekly or fortnightly repayments.
• Generates a complete transaction schedule across the life of the loan.
• Supports extra repayments at any time and frequency.
• Supports withdrawals.
• Supports offset account transactions at any interval.
• Tracks how far ahead you are — including advance payments.
• Models existing loans by letting you enter the actual repayment amount and duration in months.
• Accounts for leap years in daily interest calculations.
• Accurately handles rounding methods for interest and charges.
• Provides a full breakdown of interest calculations, day by day.
• Supports different methods of calculating interest — depending on the lender.
• Supports different repayment calculation methods for weekly and fortnightly repayments. This is one of the biggest reasons other calculators get it wrong. Many Australian lenders calculate this differently, and the difference can add up to over $50,000 in interest on a typical loan.

I’ve verified the accuracy of these calculations against real transaction schedules from several major Australian banks — though not all.

Over the years, this calculator has been used and tested by mortgage brokers and financial planners across Australia. I’ve received a lot of positive feedback about its accuracy, and I’m continually working to improve it.

While it may never reach 100% precision — due to the many subtle differences between Australian lenders — my goal is to get as close as possible.

That said, you should treat the results as a guide only. They’re not a substitute for personalised financial advice.

This calculator was built specifically for the Australian home loan market, and I don’t have enough knowledge about lending practices in other countries to say for sure if it’s accurate elsewhere.

If you’re from New Zealand (my home country 🇳🇿), the lending practices seem pretty similar — so it might work well for you. But again, no guarantees!

I spend a lot of time on Reddit’s r/AusFinance community, and I’ve noticed just how confusing home loans can be for many Australians. It’s easy to see why — the information out there is often complicated, inconsistent, or just plain wrong. And sometimes, even well-meaning advice is based on misunderstandings.

I wanted to build something that could help people better understand how home loans actually work and give them a tool to make better financial decisions. I started working on this calculator after seeing a post where someone was trying to figure out how much they could save by switching to weekly repayments. They were using a popular calculator, but the results didn’t add up.

When I dug deeper, I realised most calculators were seriously flawed. All of them charged interest at the same time as repayments, which isn’t how real loans operate – because weekly and fortnightly repayments are still charged interest monthly. The repayment calculation methods also widely varied between banks, leading to differences of tens of thousands of dollars. I found many other errors too, leading to even more inaccurate results and lost my faith in them completely.

For many people, a home loan is the biggest financial decision they’ll ever make. It seemed crazy to me that in such a huge Australian home loan market, there wasn’t a truly accurate calculator available. I wanted to change that by building a tool that offered transparency where lenders hadn’t — one that helps people understand their loans, make better decisions, and pay them off faster. That’s not something most banks are exactly motivated to help with in their own calculators.

You might’ve used another calculator and seen huge interest savings by switching to weekly or fortnightly payments. But in reality, those savings are often exaggerated — the difference is usually quite small.

That’s because Australian lenders use different methods to calculate weekly and fortnightly repayments. Most split your monthly repayment into 52 weeks or 26 fortnights. But some use a lazy, simplified method that assumes there are 48 weeks per year — effectively 4 extra weekly payments (or 2 fortnightly).

This is why you might see wildly different results across calculators. It depends on which calculation method that bank is using.

Most lenders aren’t very transparent about this, and most calculators don’t even mention it. This is the only calculator that lets you choose which repayment calculation method to use. You’ll find it under the Repayments section — when Frequency is set to Weekly or Fortnightly, you can change the Calculation method to Split monthly to simulate the less accurate method with 48 weeks.

So yes, weekly or fortnightly payments do save some interest because you’re paying more often. But the much bigger difference comes from how your lender calculates those payments in the first place.

Up next:

1. Interest rate fluctuation

   See how rate changes could affect the loan.

2. Save results 💾

   Managed access to your previous calculations. This will be easier than dealing with share links, which can be difficult to keep track of.

3. Split loans 🖖

Sometimes your loan may be paid off earlier than the original loan term — even if you haven’t made any extra repayments.

Here are some possible reasons why:

• You’re making fortnightly or weekly repayments, which reduces your loan balance more frequently and saves interest over time.
• Some lenders calculate weekly or fortnightly repayments in a way that leads to roughly 13 monthly repayments each year — effectively adding extra payments that reduce your loan faster.
• You’ve set weekly or fortnightly payments and entered a very high interest rate, which can amplify slight mismatches between repayment calculations and interest calculations. (They are surprisingly disconnected algorithms.)

After your last repayment, there may be a closing payment added to balance the loan. This is expected.

The formula lenders use to calculate your minimum repayments is not completely accurate. It doesn’t account for the odd numbers of days per month or extra days in leap years (when using the Actual/365 interest calculation method).

Is your chart trending upwards? 📈 It’s probably because you’re overriding the Minimum repayment with an amount too low to cover interest charges.

This could also happen when combining an extremely high interest rate with the Actual/365 interest calculation method. This effectively charges an additional day of interest every leap year, which the repayment formula was not built to factor in.

Surprisingly most Australian lenders use the Actual/365 interest method, but some use Actual/Actual which evenly distributes interest across every day, including leap years.

• Built a time-based algorithm using date-fns for precise calendar date handling.
• Written in JavaScript. While that may seem obvious, many legacy calculators still run Java or C++ on the server, which makes them so much slower to load and interact with.
• All calculations run client-side, avoiding the latency and overhead of calling out to an external API.
• The calculation logic is offloaded to a separate thread using a web worker, keeping the main thread free and the UI responsive.
• The transaction schedule is powered by react-virtualized, allowing thousands of rows to render efficiently as you scroll with minimal memory use.
• Built with React and TypeScript.
• The site is server-side rendered and statically generated using Next.js.
• Components are all handcrafted, leading to minimal external dependencies. I plan to open-source the design system soon.