How to Use a Mortgage and Stamp Duty Calculator

A mortgage calculator only needs four numbers to work out your monthly payment: the price, your deposit, the interest rate, and the term. Stamp duty is a separate calculation on top, and it's the one first-time buyers most often get wrong.

How does a mortgage calculator work?

A repayment mortgage calculator works out a fixed monthly payment that clears the loan (the "principal") over the term, at a set interest rate, using standard amortising-loan maths. Every payment is part interest, part principal, and the split shifts more towards principal as the term goes on. That's different from an interest-only mortgage, where the payment covers interest alone and the principal is repaid separately at the end.

The four numbers that change your monthly payment

• Price: the property price you're comparing against your budget.
• Deposit: usually shown as a percentage. A bigger deposit means a smaller loan, and often a better interest rate too.
• Interest rate: the annual rate on the loan, expressed as a percentage.
• Term: how many years you're repaying over. A longer term lowers the monthly payment but increases the total interest paid.

Move any one of these and the monthly payment changes immediately. That's the whole point of using a calculator instead of a rough mental estimate.

How is UK stamp duty calculated?

Stamp Duty Land Tax (SDLT) is charged in bands, not as one flat percentage of the price. In England and Northern Ireland:

First-time buyers, on a property up to £500,000:

• 0% on the first £300,000
• 5% on the portion between £300,000 and £500,000

Above £500,000, first-time buyer relief is lost entirely, and standard rates apply to the whole price, not just the amount over £500,000.

Standard rates (home movers, or first-time buyers above £500,000):

• 0% up to £125,000
• 2% on £125,001 to £250,000
• 5% on £250,001 to £925,000
• 10% on £925,001 to £1.5m
• 12% above £1.5m

Additional properties (a second home, or a buy-to-let) pay a 5% surcharge on top of the standard rate for every band.

Cash needed to complete is more than the deposit

The number that actually determines whether you can complete on a purchase is deposit plus stamp duty, plus whatever else you're paying up front. Stamp duty alone can run to tens of thousands of pounds on a mid-range home, and it's due on completion alongside the deposit, not spread across the mortgage.

Try it with your own numbers

The mortgage and stamp duty calculator works out all of this from four sliders, no login needed. If you're buying with someone else, it's also worth reading how buying together actually gets split, since a mortgage calculator alone won't tell you what either of you can afford. Once you know the monthly payment, the bill split calculator picks it up automatically and works out who pays what alongside the rest of your bills.