Mortgage tools & calculators
Mortgages can get surprisingly confusing once you get beyond the monthly payment. Start with the question you’re trying to answer and I’ll point you to the tool I’ve built for it.
What are you trying to work out?
You don’t need to know the name of the calculator. Pick the question that sounds most like yours.

Mortgage Payment Breakdown Calculator
See how your monthly payment splits between interest and paying down your mortgage balance, then explore how regular or one-off overpayments could change the interest you pay and how quickly the balance falls.

Mortgage Overpayment Allowance Calculator
Check how your lender works out its overpayment allowance, estimate how much you may have left and see whether a planned overpayment appears to fit within it.
Which mortgage calculator should I use?
Here are the kinds of questions each one is designed to answer.
I want to know how much of my payment is interest.Payment Breakdown Calculator →
My lender says I can overpay 10%, but 10% of what?Overpayment Allowance Calculator →
I want to understand how quickly my mortgage balance is actually reducing.Payment Breakdown Calculator →
I have money I want to overpay. Will it fit inside my lender’s allowance?Overpayment Allowance Calculator →
I want to see what an extra monthly or lump-sum payment could change.Payment Breakdown Calculator →
I’ve already overpaid this year. How much allowance might I have left?Overpayment Allowance Calculator →
I was already answering these questions anyway
I’ve somehow become the person friends and family tend to come to with money questions, particularly anything involving mortgages, overpayments and working out what the numbers actually mean.
I’ve spent a lot of time helping people work through these things over the years, alongside figuring them out for my own mortgage. Eventually I started turning some of the explanations I was already giving into simple tools for them and for me, then thought they might be useful to other people asking the same questions too.
I’m building Money With Clo in my spare time, so this mortgage toolkit will grow over time. If there’s a mortgage question you wish was easier to answer, I’d genuinely like to hear it.
What mortgage question should I tackle next?
These tools started with questions people were already asking me. Pick the problem you’d most like a tool for, or tell me what I’ve missed.
About these mortgage tools
You can use the calculators without creating an account or handing over your mortgage details.
The payment calculator explains the maths of your mortgage. The allowance calculator focuses on lender-specific overpayment limits and ERCs.
I use feedback from real questions to decide what to research and build next rather than adding calculators for the sake of it.
These tools explain mortgage calculations and lender rules. They don’t assess whether a particular mortgage decision is right for your circumstances.