I was tired of using fairly lacklustre mortgage calculators online, or having to dig out a spreadsheet every time I wanted to sanity-check a number, so I thought I’d make my own web version with the help of ChatGPT. Quite often I don’t just want one result, I want to see what happens if the inputs move around slightly, so I added sliders that let you tweak the values and see the result recalculate instantly.

Unlike a lot of the calculators online, this one isn’t trying to sell you anything or plaster the page in ads.

I’ve included stamp duty, total repaid and total interest too, because the monthly figure on its own only tells part of the story. Two options can look quite similar month to month but work out very differently over the full term.

It’s meant as a simple, transparent tool for exploring those “what if?” questions, rather than a substitute for proper advice or a lender’s affordability checks.

Note that with the interest-only calculation, you obviously still owe the original loan balance at the end of the term.