Initially we use your deposit and income to determine an amount we think you could be offered by a lender. We then use this result, together with the length of mortgage and interest rate you choose, to calculate a monthly figure.
Working out monthly mortgage payments is complicated. Both the loan amount and the interest payment change on a monthly basis, as you repay your mortgage and your loan amount decreases.
We use an industry standard called the amortisation formula to work out monthly payments. If you’d like to understand it in more detail, here’s Wikipedia’s explanation. Although this is an American source, mortgages function in the same way in the UK as they do in the US.