Mortgage Calculator — Monthly Payment, Interest & Overpayments
Estimate your monthly payment including tax and insurance, and see what overpaying saves.
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20% of the price · LTV 80%
Add running costs (property tax, insurance, fees)
Principal — Interest —
| Year | Principal paid | Interest paid | Balance |
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Work out your monthly mortgage payment from the property price, deposit, rate and term. Add property tax, insurance and service charges for a realistic total, and see exactly how much time and interest a monthly overpayment would save.
How to use Mortgage Calculator
- Enter the property price and your deposit. The loan-to-value ratio updates as you type.
- Add the interest rate and term.
- Open “running costs” to include tax, insurance and fees.
- Try an overpayment — the tool shows how many years it removes.
Why loan-to-value matters so much
LTV is the loan as a percentage of the property value. Lenders price in bands, and the difference between them is substantial: dropping from 90% to 85% LTV, or from 85% to 80%, often unlocks a meaningfully lower rate. If your deposit is close to a band boundary, finding a little more can save far more over the term than the extra deposit itself.
In the US, an LTV above 80% typically also triggers private mortgage insurance, an ongoing cost that disappears once you cross below that line.
Overpayments are the biggest lever you control
An overpayment goes entirely against the principal, so it reduces every future interest charge. The effect compounds, and it is much larger than most people expect — on a typical 25-year mortgage, a modest monthly overpayment can remove several years from the term and tens of thousands in interest. Enter a figure above and the tool quantifies it for your numbers.
Two caveats. Many mortgages cap penalty-free overpayments (commonly 10% of the balance per year) — check your terms. And overpaying is not automatically the best use of the money: if your mortgage rate is lower than what you could earn elsewhere, or you have higher-interest debt, the maths may point somewhere else.
What this estimate does not include
Arrangement and valuation fees, legal costs, stamp duty or transfer tax, mortgage insurance, and maintenance are all outside this calculation. The rate is also assumed fixed for the whole term — most mortgages are fixed only for an initial period and then revert, so your real payment will change at that point.
This is an illustration to help you compare scenarios, not a lending decision. Get a formal quote before committing.
Frequently asked questions
Does this include tax and insurance?
Only if you enter them. Open the "running costs" section to add annual property tax, insurance and monthly service charges for a realistic total monthly figure.
How much does overpaying actually save?
More than most people expect, because every overpayment reduces all future interest. Enter a monthly figure and the tool tells you exactly how many years it removes and how much interest it saves for your numbers.
What is LTV and why does it matter?
Loan-to-value is the loan as a percentage of the property price. Lenders price in bands, so crossing below 90%, 85% or 80% can unlock a noticeably better rate. In the US, going below 80% also typically removes private mortgage insurance.
Will my payment really stay the same for the whole term?
Usually not. Most mortgages fix the rate only for an initial period and then revert to a variable rate. This calculator assumes one fixed rate throughout, so treat longer projections as an illustration.
Are there limits on overpaying?
Often yes — many mortgages allow penalty-free overpayments up to about 10% of the balance a year, with early repayment charges beyond that. Check your terms before committing to a plan.