When you’re searching for a home in Melbourne for a family of five, the school catchment question is only the beginning. Once you’ve found properties in the right zones—which is why I built School Zone Homes VIC—the next question hits harder: can we actually afford this? And not just “can we scrape together the deposit” afford it, but the full picture. The stamp duty in Victoria. The Lenders Mortgage Insurance if we’re under 20% deposit. What the repayments look like at current rates versus what happens if rates climb another percent. Whether paying fortnightly instead of monthly actually saves meaningful money over thirty years. I found myself running the same calculations over and over, across multiple websites, none of which quite gave me the complete view I needed.
So I built AU Loan Calculator, a mortgage calculator designed specifically for the Australian property market. It started as a tool for my own use—another spreadsheet-replacement born from the frustration of not finding exactly what I needed—and evolved into something comprehensive enough to share. The app handles stamp duty calculations across all Australian states, including the first home buyer exemptions that can save tens of thousands of dollars in Victoria and NSW. It calculates LMI so you know your true upfront costs before you fall in love with a property you can’t quite reach. And it models offset accounts properly, with daily interest calculations on the net balance, because that’s how offset accounts actually work and it makes a real difference to the numbers.
What surprised me during development was how many edge cases exist in Australian property finance that generic mortgage calculators miss entirely. Stamp duty isn’t just a percentage—it’s a tiered system that varies by state, with different thresholds for first home buyers, different concessions for different property values, and rules that change more often than you’d expect. LMI isn’t a simple lookup either; it depends on your loan-to-value ratio, the loan amount, and the insurer’s risk appetite. Most online calculators give you a rough estimate at best. I wanted something that felt closer to what a mortgage broker would run through with you, except available at midnight when you’re lying awake wondering if that house you inspected on Saturday is actually within reach. The numbers are still estimates—actual costs depend on fees, your specific lender, and factors only a mortgage broker or financial advisor can assess for your situation—but they’re close enough to know whether a property is in the ballpark or a fantasy.
The payment frequency comparison became one of my favorite features because it reveals something counterintuitive about how mortgages work. Switching from monthly to fortnightly payments doesn’t just feel more manageable—it actually shortens your loan term and reduces total interest, sometimes by years. The math is straightforward once you see it: fortnightly payments mean you’re making the equivalent of thirteen monthly payments per year instead of twelve, chipping away at principal faster. But knowing that abstractly is different from seeing your specific numbers, your loan amount, your interest rate, showing you’ll pay off four years earlier and save $87,000 in interest. That’s the kind of insight that changes decisions.
The app also includes an offset account simulator, which matters more in Australia than many other markets because offset accounts are genuinely powerful here. The concept is simple—money sitting in your offset account reduces the principal your interest is calculated against—but modeling it properly requires daily interest calculations on the net balance. Most calculators either ignore offset accounts entirely or simplify them in ways that don’t reflect reality. I built the simulator to handle monthly contributions to your offset balance over time, so you can see how building up that buffer affects your total interest paid and loan term. For families trying to balance mortgage payments against maintaining an emergency fund, understanding this trade-off matters.
I wrapped the web version with Capacitor, the same approach I used for School Zone Homes VIC. Both the Android app and iOS app are now live. Everything runs locally on your device—no data sent anywhere, no accounts required, works offline. That last point matters more than you’d think when you’re standing in a property and want to quickly run numbers without hunting for signal.
Building tools along the way of our own home search has been an unexpected outcome of this process. The school catchment finder solved one problem; the mortgage calculator solves the next. Each tool exists because I needed it, couldn’t find exactly what I wanted, and had the skills to build it myself. That’s the privilege of being a dev—when the tool you need doesn’t exist, you can make it. And in sharing these tools, maybe other families navigating Melbourne’s property market find them useful too. House hunting with kids, school zones, and mortgage math is complicated enough without fighting inadequate tools.
Try it yourself:
Or read about the earlier part of this journey: Building a School Catchment Property Finder in a Weekend.