What the affordability calculator works out
The affordability calculator takes the monthly repayment you're facing, either what you're paying now or what you'd pay on a new rate once your fixed-rate deal ends, and checks it against what you have coming in each month. It gives you a straightforward answer: whether that repayment looks manageable against your income and your regular outgoings, based on the figures you enter.
It can only work with what you give it. It doesn't check your credit file, it doesn't know your spending habits beyond what you tell it, and it cannot confirm what any lender will actually offer you. The figures it produces are illustrative, built from your inputs, and they can be wrong if the numbers you put in don't hold up in practice. Use the result as a starting point for a conversation.
What it asks you for, and why
To produce a useful number, the calculator needs a small set of figures. Each one feeds directly into the result:
- Outstanding balance, the amount you still owe on your mortgage, because it sets the size of the loan being repaid.
- Remaining term, the number of years left to pay it off, because a longer term spreads the same balance into smaller monthly payments.
- An interest rate, either your current one or one you're considering, because it determines how much of each payment goes on interest.
- Your income and monthly outgoings, because affordability is a comparison.
Leave any of these rough and the result comes out rough too. If you know your outstanding balance from a recent mortgage statement, use that figure.
How a lender checks affordability differently
A lender doing its own affordability assessment goes further than this calculator can. It usually tests whether you could still afford the repayment at a higher interest rate than the one you're being offered, in case rates rise during your deal. It also looks at your credit history, any other debts, and how many people depend on your income. None of that is available to a tool that only has the numbers you type in.
If you want to see how a rate rise specifically would affect your payment, the Variable Rate Stress Test is built for that question, and the guide to it explains what lenders are actually testing for when they run this check.
Why your number here might not match what a lender offers
A few things commonly explain the gap between what this calculator suggests and what a lender is willing to lend. Your Loan to Value (LTV), the size of your mortgage compared with your home's value, affects which rates you're even eligible for, so a change here can move the rate a lender would actually apply. Your credit history and employment situation matter too, and this calculator has no way to see either. Lenders also apply their own stress-testing rules, which vary between them and are not published in a way this tool can reproduce.
If you want to check where your LTV currently stands, the LTV & Equity Position Calculator works from your balance and your property's estimated value, and its guide walks through what different LTV bands tend to mean for the rates on offer.
What to do with your result
Once you have a number, the practical next step is usually a conversation with a lender or a broker, because they can turn it into an Agreement in Principle (AIP), a lender's early indication of how much they'd lend you, based on a light credit check. This calculator cannot produce an AIP itself and cannot tell you which lenders would accept you.
The Six-Month Remortgage Checklist sets out what to do and when, working back from the date your fixed rate ends, and What to Do Next covers what happens once you're ready to apply or speak to someone. Both are the natural place to go after you've got your number here.