How Mortgage Overpayment Works
The Mortgage Overpayment Calculator is free to use and all calculations run entirely in your browser. When you overpay, the extra money goes directly to reducing the principal balance — not the interest. Because interest is calculated on the outstanding balance each month, every unit of overpayment reduces all future interest charges for the remainder of the term. The earlier in the mortgage you overpay, the bigger the saving, because that lower balance carries through every remaining month.
Worked Example: a £200,000 Mortgage at 5%
Using the calculator's default figures — a £200,000 balance at 5% with 25 years remaining — the standard monthly payment is about £1,169 and the total interest over the full term is £150,754. Adding overpayments changes the picture dramatically:
| Overpayment | Paid off in | Interest saved |
|---|---|---|
| None (baseline) | 25 years | — |
| £100 / month | ≈ 21 years 6 months | ≈ £24,500 |
| £200 / month | ≈ 18 years 10 months | ≈ £41,800 |
| £200 / month + £10,000 lump sum | ≈ 17 years 4 months | ≈ £56,500 |
A £200/month overpayment alone clears the mortgage more than six years early and saves roughly £41,800 in interest. Change the currency, balance, rate and overpayment above to model your own mortgage — the figures recalculate when you press the button.
Reduce Term vs Reduce Payment
Most UK lenders let you choose what to do with overpayments:
- Reduce the term — keep paying the original monthly amount, finish years earlier, save the most interest
- Reduce monthly payment — pay less each month, keep the original end date, save less interest but lower immediate cash flow
For maximum savings choose to reduce the term. Some lenders automatically apply overpayments as a payment reduction unless you specifically ask otherwise — call them to confirm. This calculator models the reduce-term approach, which is why the payoff date moves earlier.
The 10% Limit and Early Repayment Charges
Before overpaying a large amount, check your mortgage terms. Most UK 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 usually trigger an Early Repayment Charge (ERC) of 1–5% of the amount — which can wipe out the interest you would have saved. Variable, tracker and discount mortgages typically have no overpayment cap. If you are mid-fix, spreading a large lump sum across two years, or waiting until the fix ends, can avoid the charge entirely.
Should I Overpay or Save / Invest?
Overpaying is effectively a risk-free, tax-free return equal to your mortgage rate. Whether it beats saving or investing depends on your circumstances:
- If your mortgage rate is higher than your guaranteed savings rate after tax, overpaying usually wins
- If you have no emergency fund of 3–6 months of expenses, build that first — overpayments are hard to get back
- If you have high-interest debt such as credit cards or personal loans, clear those before overpaying the mortgage
- If your employer matches pension contributions, max those first — it is effectively free money
- Above the £85,000 FSCS protection limit per institution, consider a mix of overpayment and tax-free ISAs
Frequently Asked Questions
How does mortgage overpayment save money? Overpaying reduces the outstanding balance immediately. Because mortgage interest is charged on the remaining balance, every overpayment saves you interest for every remaining month of the loan. A £100/month overpayment on a £200K, 25-year, 5% mortgage saves around £24,500 of interest and cuts about 3.5 years off the term.
What is the 10% overpayment limit? Most UK fixed-rate mortgages let you overpay up to 10% of the outstanding balance per year without penalty. Overpaying more during the fixed period typically triggers Early Repayment Charges (ERCs) of 1–5% of the overpayment amount. Variable and tracker mortgages usually have no overpayment cap.
Should I overpay or invest the money instead? It depends on your mortgage rate vs. your expected after-tax investment return. If your mortgage rate is 5% and you can earn a guaranteed 5%+ in a cash ISA, investing the money is roughly equivalent. If your mortgage is at 6%+ and you'd invest in volatile assets, overpaying typically wins on a risk-adjusted basis. Also consider tax — ISA returns are tax-free; mortgage savings effectively are too.
Lump sum vs monthly overpayment — which is better? A lump sum saves more total interest because it reduces the balance immediately and for the entire remaining term. Monthly overpayments build up gradually but are easier to budget. Many people do both — annual bonus as lump sum + small monthly overpayment.
Does overpaying reduce my monthly payment or shorten the term? Most UK lenders give you the choice. "Reduce term" keeps your monthly payment the same and shortens the mortgage — this saves the most interest. "Reduce monthly payment" lowers each payment but keeps the original end date. For maximum savings, choose reduce term.
When is the best time to overpay a mortgage? As early as possible. Interest is charged on the outstanding balance, so an overpayment in year 1 saves interest across every remaining year, while the same overpayment in year 20 saves very little. Overpaying early in the term — and early in each year — delivers the largest interest saving.
Can I get my overpayments back if I need the money? Usually not automatically. A standard overpayment permanently reduces your balance and is not a savings pot you can withdraw. Some lenders offer an "overpayment reserve" or "borrow back" facility, but it is not guaranteed. Keep an emergency fund separate before overpaying so you are not left short of accessible cash.
Does this overpayment calculator work for any country or currency? Yes. The maths of overpaying — extra payments reducing the principal and cutting future interest — is the same worldwide. Choose your currency at the top, and the calculator works for any repayment (amortising) mortgage. Only the lender rules, such as the UK's 10% cap and Early Repayment Charges, vary by country, so check your own agreement.
Last reviewed: June 2026. Estimates only — confirm exact figures with the relevant authority or a qualified adviser before acting.