An Open-Source Mortgage Overpayment Calculator

I built an open-source mortgage calculator because I wanted a straightforward way to answer a practical question: what difference would overpaying actually make?

It’s easy to look at a monthly payment or a total interest figure on its own. I wanted to compare the original repayment schedule with a couple of possible overpayment plans and see how the difference adds up over time.

Try it out

Open the mortgage calculator and enter the property price, deposit, interest rate, and mortgage term. Then try adding either a regular monthly overpayment, a one-off lump sum, or both.

The results compare the original schedule with the overpayment scenario. You can see the estimated interest saved and time taken off the mortgage, then explore how the balance and interest change over time in the charts. There’s also a yearly breakdown if you want to look beyond the headline numbers.

A few things I cared about

I wanted the comparison to be easy to understand, so both scenarios are shown together rather than making you work through separate results. Changing an input updates the estimates straight away, which makes it easier to try different amounts and timings.

Privacy mattered too. The calculations happen in your browser; the app doesn’t send mortgage figures to an external API or store them on a server. The figures are estimates for comparison, not financial advice, and real mortgage terms can vary.

The project is built with React and TypeScript. I’ve kept the calculation logic separate from the interface and added regression tests for payments, interest, regular and lump-sum overpayments, dates, and chart values—including what happens when a loan is paid off early.

The source is available on GitHub. If you try it, I hope it helps make the impact of an overpayment a little easier to see.