Mortgage Overpayment Calculator

How to use this calculator

1. Use the outstanding balanceNot the original loan. The balance on your latest statement is what interest is actually charged on.
2. Enter the remaining termYears left, not the original term. Both figures are on your annual statement.
3. Check the allowance rowMost lenders permit overpayments of 10% of the balance each year without penalty. The calculator flags when your annual overpayment exceeds it.
4. Compare against savingEnter your savings rate after tax. Overpaying effectively earns the mortgage rate tax free, so the comparison is direct.

Why overpayments compound

The saving from an overpayment is far larger than the amount paid, and the reason is worth understanding rather than taking on trust.

Interest each month is charged on the outstanding balance. Reduce that balance by £200 and the next month's interest falls by £200 times the monthly rate. But the balance stays £200 lower for every remaining month, so the saving repeats — and each saved pound of interest also stays in the balance reduction, compounding further.

Over a twenty-year remaining term at 4.5%, a single £200 overpayment saves considerably more than £200 of interest. Made monthly, the effect accumulates into tens of thousands.

This is why timing matters so much. An overpayment made in year one compounds for the whole remaining term; the same amount in year nineteen barely has time to work. If you are going to overpay, earlier is dramatically better than later.

It is also why overpaying is best understood as a risk-free, tax-free return at your mortgage rate. Paying down a 4.5% mortgage is equivalent to earning 4.5% on that money with no tax and no risk. A savings account would need to pay more than that after tax to beat it — and for a higher-rate taxpayer that means a headline rate well above 4.5%.

One crucial mechanical point: tell the lender to reduce the <em>term</em>, not the monthly payment. Many lenders default to recalculating the payment downward, which hands the benefit straight back and produces almost none of the saving shown here. Reducing the term is what converts the overpayment into interest saved.

The calculator amortises the loan twice, month by month, rather than applying a formula to approximate the difference. That matters because the two schedules diverge in a way a simple formula does not capture.

Worked example: £180,000 over 22 years, overpaying £200 a month

At 4.5% over 22 years, the contractual payment on £180,000 is £1,075.30 a month.

Without overpaying, the loan runs the full 264 months and costs £103,878 in interest. You repay about £283,878 in total for a £180,000 loan.

Now add £200 a month, taking the payment to £1,275.30.

The balance falls faster, so each month's interest is lower, so more of the next payment attacks the capital. The effect accelerates. The mortgage clears in 202 months instead of 264 — five years and two months early — and total interest falls to £76,734.

Interest saved: £27,144. Time saved: five years two months.

The total overpaid across those 202 months is £40,400. So each £1 overpaid saved 67p of interest — a 67% return on the money, achieved simply by paying it earlier.

Now the allowance check. £200 a month is £2,400 a year, against a 10% allowance on £180,000 of £18,000. Comfortably inside, so no early repayment charge arises.

And the comparison against saving. Putting the same £200 a month into a savings account at the same 4.5% earns £19,862 over the period — so overpaying wins by £7,282 even though the rates are identical.

That result surprises people, and the reason is worth understanding. The mortgage balance is far larger than the savings pot throughout, so the same percentage applied to a bigger number saves more than it earns. Equal rates do not mean equal outcomes.

And that is before tax. A higher-rate taxpayer's 4.5% savings account yields 2.7% after tax, which widens the gap further — overpaying is effectively tax free.

What different overpayments achieve

On £180,000 at 4.5% over 22 years. Note the last row: a single £1,000 lump sum saves £1,678 — it more than doubles itself over the remaining term, which beats any instant-access savings account. And note the saving is not proportional to the overpayment: doubling from £100 to £200 saves less than double, because the loan clears sooner and there is less remaining interest left to save.

When not to overpay

Overpaying is not automatically the right move, and several situations argue against it.

Expensive debt comes first. Credit cards, overdrafts and personal loans almost always carry higher rates than a mortgage, so clearing them returns more per pound. Paying down a 4.5% mortgage while carrying a 22% credit card balance is a straightforward loss.

An emergency fund comes second. Money paid into a mortgage is difficult to get back — you would need to remortgage or apply for a further advance, both of which take time and depend on circumstances at the time. Three to six months of expenses in accessible savings is worth more than the interest saved, because the alternative in a crisis is expensive borrowing.

Pension contributions frequently beat overpaying, particularly for higher-rate taxpayers. Relief at 40% means a £1,000 contribution costs £600, an immediate 67% uplift that no mortgage rate matches. Employer matching improves it further and is close to unbeatable.

Early repayment charges are the direct constraint. Most fixed-rate deals permit 10% of the balance a year penalty-free; beyond that a charge of typically 1% to 5% of the excess applies during the fixed period. Exceeding the allowance can wipe out several years of interest saving in one payment.

Interest-only mortgages need care. Overpaying reduces the balance, but the repayment vehicle still needs to cover what remains at term end, and some lenders treat overpayments differently on these.

And if your savings rate after tax exceeds your mortgage rate, saving simply wins. That situation has occurred in the past and can occur again.

Where overpaying does make sense, two mechanical points are worth getting right. Ask explicitly for the term to be reduced rather than the payment. And check whether your lender applies overpayments immediately or only at the annual anniversary — a lender that holds the money until year end gives you considerably less benefit than the calculation assumes, and the difference is worth asking about before setting up a standing order.

Assumptions and limitations

• Assumes a constant rate for the whole remaining term, whereas UK mortgages typically fix for two to five years
• Assumes overpayments are applied immediately — some lenders apply them only at the annual anniversary
• Assumes the term is reduced rather than the monthly payment recalculated downward
• Does not model early repayment charges beyond flagging when the allowance is exceeded
• Repayment mortgages only — interest-only works differently
• The savings comparison assumes a constant rate and an after-tax figure
• Not financial advice — clear expensive debt and build an emergency fund first

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