There is an endless supply of online mortgage calculators. Most of the big personal finance sites have. The best one, in my opinion, is one you’ve probably never heard of: Karl’s Mortgage Calculator.

You’re probably asking who the heck is Karl? I have absolutely no idea. Well, that’s not entirely true. I’ve emailed Karl from time-to-time. I thought his mortgage calculator was so great, I wanted to buy it to add to a website of my own. His reply was a very polite “no thank you.”

And with good reason. It’s a great calculator. You can find Karl’s Mortgage Calculator here.

Use our free mortgage calculator to estimate your monthly mortgage payment

So what makes this mortgage calculator the best? It’s not its looks. It was built in the 90′s (I think). It’s not flashy and sleek like some of the mortgage calculators you find today.

But there are two things I like about it. It’s very easy to use, and it has a ton of options. This isn’t just a basic calculator where you stick in the amount you’re going to borrow and the interest rate and it spits out your monthly mortgage payment.

Yes, it will generate your monthly payments. And it also generates an amortization table to show you how your balance will come down over time.

But it does even more useful things. For instance, it allows you to figure out how paying extra on your mortgage will reduce your balance, how quickly you’ll be able to pay it off, and how much interest you’ll save. It looks like this:

Karl’s Mortgage Calculator does a whole lot more than just this. So let’s look at the ins and outs of using it.

Options on the Calculator

When you look at the calculator towards the bottom, you’ll see a row of buttons. When you click on any of these, it brings up more options.

The prepayment option is the one that’s really of interest to me, particularly as my wife and I think about paying down our mortgage, but the calculator has a number of options.

For example, let’s say you’re in a 30-year mortgage that you’ve had for five years. With a little extra cash, you’re thinking about paying an extra $50 a month. How much interest will that save you and when will the mortgage be paid in full? These questions are easy to answer with this calc. You can actually put that prepayment in the calculator. Just click the “Prepayment” button, and add $50 into the “Monthly prepayment” field and the month you’ll start making that extra payment (e.g., month 61 of the loan).

Of course, you can change the assumptions to see how different amounts will affect the life of your loan.

But what if you don’t want to pay out a little extra each month but want to make extra payments every now and again sort of like a one-time prepayment? Or what if one month you just pay an extra $1,000 on your mortgage?

Well, the calculator has got you covered. You can enter this date, including the month that you’re going to make that one-time prepayment. The calculator will show you how your loan term will shorten and how much interest you’ll save.

And Karl, if you happen to be reading this, kudos to you. For my other readers, this is an excellent calculator, and I hope you get some use out of it. For my money, it’s one of the best mortgage calculators out there – and I’ve looked at a ton of them.

Give it a try and let me know what you think.

Related: Ways to Calculate How Much House You Can Afford


  • Rob Berger

    Rob Berger is the founder of Dough Roller and the Dough Roller Money Podcast. A former securities law attorney and Forbes deputy editor, Rob is the author of the book Retire Before Mom and Dad. He educates independent investors on his YouTube channel and at