It simulates the entire loan term by processing every calendar date one by one. For a 30 year loan, over 10,950 days are calculated in less than a second. This is the only accurate method of calculating interest, and the first Australian home loan calculator to do so.

This is the only calculator that:

• Calculates interest daily.
• Always charges interest monthly.
• Shows true interest savings of weekly/fortnightly repayments.
• Generates a complete transaction schedule.
• Allows extra repayments to be made at any interval.
• Supports withdrawals.
• Supports offset account transactions.
• Tracks amount paid in advance.
• Supports existing loans by allowing the actual repayment amount to be entered, and duration in months.
• Accounts for leap years in interest calculations.
• Can account for proper rounding methods of interest and charges.
• Shows interest calculations per day.
• Supports various interest calculation methods.
• Supports various repayment calculation methods for fortnightly/weekly repayments.

The calculator is still in Beta and being tested. There might be some things I've overlooked.

I've verified accuracy of interest calculations with two major Australian banks, so I have some confidence in the results. I'll be collecting sample data from other lenders for further testing.

I'm not sure I'll ever be able to calculate to 100% precision, but I plan to get as close as possible. There are many nuances between Australian lenders that make this difficult.

Up next:

1. Partial offsets

   Support offset facilities that offer a discounted rate, rather than a full offset of the loan balance.

2. Interest rate fluctuation

   Visualise the effects of interest rate fluctuations for new loans. (Delayed - harder than I thought)

3. Fixed rates

Sometimes your loan may be repaid sooner than your loan term, even if you haven't made extra repayments.

Some possible reasons:

• You're making fortnightly or weekly repayments, so you're saving interest in reducing your loan balance more frequently.
• Some lenders calculate weekly or fortnightly repayments by splitting the monthly repayments by 2 fortnights, or 4 weeks. You end up paying roughly an extra 4 weeks of repayments per year.
• You entered a really high interest rate (over 15%), which can amplify inaccuracies between repayment calculations and interest.

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.

I saw a thread on Reddit where people were confused and couldn't agree whether weekly repayments saved interest compared to monthly repayments. Some quoted results of online loan calculators that showed minimal savings, and in some cases zero.

I looked into it and realised the calculators were all wrong. Their formulas are flawed because they charge interest at the same time as repayments. This provides inaccurate results for weekly or fortnightly repayments which are only charged interest monthly. I also discovered other errors in their algorithms which caused further inaccuracies...

For many people home loans are the biggest financial decisions and burdens of their lives. I thought it was silly that in this massive Australian home loan market, no lender provides an accurate calculator. I wanted to make a tool that provided more transparency where lenders haven't, and educate people about home loans in the process.

• I designed a temporal algorithm using date-fns.
• The algorithm is written in Javascript which provides a lot of flexibility. It could be wrapped into an API, exported as a node module, or in this case shipped directly to the client in my monorepo environment. Sounds obvious, but a few older calculators are written in different languages which put them at a disadvantage in a Javascript-powered world.
• The calculation occurs client-side, which I opted for rather than an external API which would affect performance.
• The algorithm is heavy, so it's offloaded to a separate thread using a web worker to clear up the main thread.
• The transaction schedule uses react-virtualized, which renders table rows on the fly as you scroll. This enables thousands of transactions to be displayed without affecting performance or memory.
• React.
• Typescript.
• Styled components for CSS.
• Website: Server-side rendered / statically generated with Next.js.
• Handmade components which I plan to release as an open-source design system. The amount of external dependencies used is minimal.