Are mortgage calculators accurate?

Mortgage calculators promise quick answers about your home buying power, but can you trust what they tell you?

These free, easy-to-use tools give you a quick estimate of how much you could borrow. But they don't show you the complete picture of all the costs involved. Use calculators as a starting point, then do your own calculations to understand the full financial commitment.

Here's how to use mortgage calculators wisely and understand what they can and can't do for you.

Mortgage calculators don't include all costs

You'll find many different mortgage calculators online, but none of them include all the costs of buying a house or remortgaging your home. When you enter basic information (mortgage amount, number of years and interest rate), a calculator will show you:

✅How much you'll pay each month
✅ The total cost over your entire mortgage
✅ How much money you could borrow

But these tools don't include important costs like stamp duty (from 1st April 2025, you'll pay 2% on the portion between £125,001 and £250,000, and 5% on anything above £250,000), legal fees, mortgage arrangement fees, and estate agent charges.

These are some of the biggest upfront costs when buying a home. You'll also need to factor in ongoing costs such as home insurance, council tax, and everyday bills.

If you want to work out what you can truly afford, make a list of every cost, even smaller ones like hiring removal vans and temporary storage. Use the mortgage calculator as your starting point, then add these extra costs.

Keep it realistic

Mortgage calculators only give you accurate figures based on the interest rate and deposit size you enter. Knowing what deposit you'll save up will give you a more realistic idea of your mortgage payments – as will knowing what interest rate you'll get. Many things affect your interest rate, including:

• Your credit score
• The LTV or loan to value ratio (the percentage you're borrowing of the total house value)
• Your deposit (usually, the bigger your deposit, the lower your interest rate)
• Interest rate type (fixed or variable)

The only way to know your actual interest rate is to speak with a mortgage advisor and start the application process. Lenders must give you a loan estimate when you apply, called a Key Facts Illustration.

While this is still an estimate, you should get this rate if you meet all the requirements discussed during your mortgage interview. It will also include fees, making it more accurate than just using a calculator.

Calculators are a helpful first step

This doesn't mean calculators aren't useful! In fact, online mortgage calculators should be the first tool you use when thinking about buying a house. Too many failed mortgage applications can hurt your credit score, so it's important to work out the numbers before you dive in.

Don't take chances – check your credit rating, do your research, and use a mortgage calculator to get an idea of costs and the price of house that's right for you.

As well as getting an idea of what your payments may look like, a mortgage calculator also allows you to see what your payments could look like in the future if rates changed. This process is referred to as a stress-test and can be helpful when trying to gauge affordability down the line.

Not all calculators are the same, though. Some are very simple, while others have advanced options so you can look at different financial situations. Our free mortgage calculator not only shows your monthly payments, the total amount you repay over time, and the cost of the loan itself, but it also helps you work out any extra payments you might make.

You can see how much faster you can pay off your mortgage by making higher monthly payments or by paying larger lump sums. This is really helpful if you expect a pay rise or an inheritance – or if you have extra savings you could use.