This New Zealand calculator tackles one of the biggest financial decisions homeowners face: should spare money go towards paying down the mortgage faster, or into investing instead? You enter your mortgage details and an expected investment return, and the tool compares the guaranteed return of paying off mortgage interest against the potential, but uncertain, return of investing the same money over the same period. It accounts for the certainty of debt reduction versus investment risk, and lets you see the outcome across different return assumptions. The breakdown shows the interest saved, the projected investment value, and the difference over time, helping you weigh a decision that has no single right answer, with the calculator making the trade-offs visible rather than pretending to predict the future.
Paying extra off your mortgage earns a guaranteed return equal to your mortgage rate. Investing the same money earns your expected return, but with risk and possible tax, so use a realistic after-tax figure. This compares the value built by the same monthly amount under each path. Estimate only, not financial advice.
Every extra dollar you put on your mortgage saves interest at your mortgage rate, so it effectively earns that rate, guaranteed and risk free. The same dollar invested earns your expected investment return, which is uncertain. The calculator grows your monthly spare amount at the mortgage rate and at the investment return over your chosen period, then compares the two totals. If the after-tax investment return beats your mortgage rate, investing builds more, but the mortgage path carries no risk.
Putting $500 a month towards a 6.5% mortgage builds value as if earning 6.5% guaranteed. Investing the same $500 at an expected 7% may build slightly more over 15 years, but only if that return is actually achieved after tax and fees. The gap is often small, so certainty and your comfort with risk matter as much as the numbers.
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