Mortgage Rate-Watch & Overpayment Calculator
See exactly how much overpaying saves you — then get a reminder six months before your fixed deal ends, the single moment that saves the most money.
1 · How much could overpaying save you?
Adjust the figures — the result updates instantly.
2 · When should you remortgage?
Tell us when your fixed deal ends. The biggest money mistake is forgetting — and rolling onto the lender's Standard Variable Rate.
Alert me 6 months before my fixed deal ends
We'll email you about 6 months before your fixed deal ends — the moment to start remortgaging. Free.
We store your deal-end date so the reminder lands at the right time. Privacy-first; unsubscribe anytime.
The two decisions that move the most money on a mortgage
For most UK homeowners, the mortgage is the single largest financial commitment they'll ever make — and two decisions decide how much it ultimately costs. The first is whether and how much to overpay. The second, and arguably more important, is remembering to remortgage before your fixed deal ends. This tool handles both: it shows the exact saving from overpaying, and it sets a calendar reminder for the one date that quietly costs people the most money when missed.
How the overpayment calculation works
A repayment mortgage uses the standard amortisation formula for the contractual monthly payment:
M = P · r / (1 − (1 + r)−n)
where P is the balance, r is the monthly interest rate (annual rate ÷ 12 ÷ 100), and n is the number of months remaining. We then run a month-by-month simulation twice: once on your normal payment, and once with your overpayment added on top. Each month, interest is charged on the outstanding balance, the rest of the payment reduces the principal, and the balance falls. Because overpayments cut the balance early, they reduce the interest charged on every subsequent month — which is why a modest monthly overpayment compounds into thousands saved.
A worked example
Take a £200,000 balance at 5.0% with 25 years left. The contractual payment is about £1,169 a month. Add a £200 monthly overpayment and you'd save roughly £41,800 in interest and clear the mortgage around six years early. The maths is the same whatever your numbers — change the inputs above to see your own figures. (Figures are illustrative and rounded; your lender's exact method may differ slightly.)
Why six months before your deal ends is the moment that matters
When a fixed-rate deal ends, your lender automatically moves you onto its Standard Variable Rate (SVR). SVRs are set by each lender and have frequently sat well above the best new fixed deals — many UK SVRs have ranged between roughly 7% and 8.5% in recent years. On a £200,000 mortgage, the gap between a 4.5% fix and a 7.5% SVR is well over £300 a month. Yet a mortgage offer is typically valid for three to six months, and most lenders let you switch to a cheaper rate for free if rates fall before you complete. That asymmetry is why starting around six months out is the sweet spot: you lock in protection against rises while keeping the upside if rates fall. Leave it to the last week and you risk rolling onto the SVR by default.
Overpay, or save?
Overpaying isn't always the right call. If an easy-access or fixed savings account pays more than your mortgage rate (after any tax on the interest), parking the money in savings can leave you better off — and keeps it accessible. As a rule of thumb: keep an emergency fund first, then compare your mortgage rate against the best savings rate you can get. When your mortgage rate is higher, overpaying usually wins; when savings pay more, saving usually wins. With rates having moved a lot, it's worth re-checking this each year.
The honest caveats
Most fixed deals cap penalty-free overpayments at 10% of the outstanding balance per year. Exceed that during the fixed period and you may trigger an Early Repayment Charge, often 1–5% of the amount repaid — which can wipe out the benefit. Always check your specific mortgage offer or ask your lender before making large overpayments. This tool gives you the numbers and the timing; it does not account for your full circumstances and is not financial advice. For a recommendation tailored to you, speak to a qualified mortgage broker or adviser.
Frequently asked questions
Does overpaying my mortgage actually save money?
Yes. Every overpayment reduces the balance that interest is charged on, so you pay less interest for the rest of the term and clear the mortgage sooner. On a £200,000 mortgage at 5% over 25 years, overpaying £200 a month saves roughly £41,800 in interest and clears it about six years early.
When should I start looking to remortgage before my fixed deal ends?
About six months before. A mortgage offer is normally valid for three to six months, so locking in a new rate early protects you if rates rise — and you can usually switch to a better rate for free if they fall before completion.
What happens if I do nothing when my fixed rate ends?
Your lender moves you onto its Standard Variable Rate (SVR), which is typically much higher than a new fixed deal. Many UK SVRs have sat between 7% and 8.5%, so doing nothing can add hundreds of pounds to your monthly payment overnight.
Are there limits on how much I can overpay?
Most fixed-rate deals let you overpay up to 10% of the outstanding balance each year without penalty. Overpay more than that during a fixed period and you may trigger an Early Repayment Charge (ERC). Always check your mortgage offer or ask your lender.
Should I overpay the mortgage or put the money in savings?
Compare your mortgage rate to the rate on an easy-access or fixed savings account after tax. If a savings account pays more than your mortgage rate, saving may win; if your mortgage rate is higher, overpaying usually wins. Keep an emergency fund first.
Is this tool financial advice?
No. This is an information tool to help you see the numbers and remember the right moment to act. It does not account for your full circumstances. Overpayment caps and Early Repayment Charges may apply. Consider speaking to a qualified mortgage broker or adviser.