Mortgage Overpayment Calculator

Mortgage overpayments reduce your outstanding balance faster, which means you pay less interest over the life of the loan. Most UK lenders allow you to overpay up to 10% of the outstanding balance per year without incurring early repayment charges. Overpayments can reduce either the monthly payment amount or the mortgage term.

Reducing the term keeps your monthly payments the same but clears the mortgage sooner. Reducing the payment lowers your monthly outgoing but keeps the same end date. Over the long term, reducing the term usually saves more in total interest because the principal decreases faster.

This calculator models both approaches. Enter your current mortgage balance, interest rate, remaining term and proposed overpayment to see the interest saved, the new payoff date and whether you would exceed the typical 10% annual overpayment limit.

Why overpaying early is so powerful. Mortgage interest is front-loaded — most of your early monthly payments are interest, not principal. £100/month overpayment in year 1 reduces the balance directly, so all future interest is calculated on a lower amount. The same £100 paid in year 24 saves only a few months of interest. Example: on a £200,000/5%/25-year mortgage, £100/month from day 1 saves £35,000+ in total interest and cuts the term by 4 years 7 months.

Overpayment limits and ERCs. Most UK fixed-rate mortgages allow 10% annual overpayment without penalty (some 5%, a few 20%). Above this limit, Early Repayment Charges (ERCs) typically apply: 1-5% of the overpayment amount, often tapering down each year of the fix (e.g. 5% in year 1, 4% in year 2, 3% in year 3). After your fix ends or once on the SVR, you can usually overpay unlimited amounts. Check your mortgage offer for exact limits and consequences.

Lump sum vs regular overpayments. A £10,000 lump sum on a £200,000/5%/25-year mortgage at year 5 saves ~£15,500 in interest and cuts the term by 14 months. The same £10,000 spread as £100/month over 8 years saves only ~£11,500 (because later overpayments have less impact). Lump sums from bonuses, inheritance, or savings are most efficient. Many lenders accept lump sum overpayments any time online, but check whether your annual 10% limit resets each anniversary.

Overpay vs invest — the maths. If your mortgage rate is 4.5%, every £100 overpayment 'earns' a guaranteed 4.5% return (saved interest). Investing the £100 in a Stocks & Shares ISA might return 7-10% long-term but with volatility and no guarantee. For mortgage rates below 4%, investing usually wins on expected return. Above 5%, overpaying typically wins on risk-adjusted return. Mixed strategy (overpay X%, invest Y%) often optimal — overpay to clear high-rate fixed deal, invest above that.

Why overpaying early matters most. Mortgage interest is front-loaded — most of your early monthly payments are interest, not principal. £100/month overpayment in year 1 reduces the balance directly, so all future interest is calculated on a lower amount. The same £100 in year 24 saves only a few months. £200k/5%/25-year mortgage with £100/month from day 1: saves £35,000+ in total interest, cuts term by 4 years 7 months.

Overpayment limits — typically 10% per year. Most UK fixed-rate mortgages allow 10% annual overpayment of balance penalty-free. Some 5%, a few 20%. Above the limit: Early Repayment Charge (ERC) of 1-5% of the overpayment. Limit usually resets on mortgage anniversary. After fix ends or on SVR: usually unlimited overpayments. Track your annual limit — don't waste capacity.

Reduce term vs reduce payment. When you overpay, choose: (1) Keep monthly payment same, reduce term — maximises interest savings; (2) Reduce monthly payment, keep term — improves cashflow. ALMOST ALWAYS choose option (1). Example: £200k/5%/25-year + £100/month overpayment. Option 1: pay off in 20yr 5mo, save £35k interest. Option 2: pay £1,069/month for full 25yr, save only £24k. £11k difference for the same overpayment.

Overpay vs invest — the maths. Mortgage rate 4%: every £100 overpayment 'earns' guaranteed 4% return. Investing in S&S ISA might return 7-10% but volatile. For mortgage rates below 4%: investing usually wins. Above 5%: overpaying wins on risk-adjusted return. Mixed strategy often optimal — overpay to clear high-rate fix, invest excess. Pension contributions (40% relief for higher-rate taxpayers) usually beat both for those eligible.