At a Glance
- The tool models the impact of a regular monthly overpayment, a one-off lump sum, or both combined on a secured loan – how the tool works
- It shows interest saved, time cut from the term, and a projected debt-free date for both the standard and overpayment paths – about this tool
- Toggle the ERC switch to model whether an early repayment charge would offset the saving if the loan clears ahead of schedule – the ERC consideration
- The tool uses a month-by-month simulation rather than a closed-form formula, which is necessary to model a lump sum payment at a specific month accurately – how the maths works
- Overpaying may not always be the most effective use of spare funds – when overpaying makes sense
Overpayment Impact Calculator
Overpayment impact calculator – secured loans
See how much time and interest you could save by overpaying your secured loan – and whether an early repayment charge changes the picture. All figures are illustrative examples only.
Your loan
Overpayment
Drag to zero to model lump sum only
Select the ERC structure your lender uses – check your loan agreement
ERC applies at the point the overpayment path clears the loan ahead of the original term
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Balance and cumulative interest saved – month by month
Figures are illustrative only. The simulation assumes consistent payments throughout and does not account for rate changes, lender-specific ERC triggers, or administrative fees. Some lenders allow a fixed annual overpayment allowance (commonly 10% of the outstanding balance per year) before an ERC applies – check your specific loan agreement. This tool does not constitute financial advice.
About this tool
Most secured loan repayment calculators show what a loan costs on its standard terms. This tool asks a different question: what happens if you pay more than the minimum? By entering a monthly extra payment, a one-off lump sum, or a combination of both, you can see the direct impact on total interest paid, how many months you could cut from the remaining term, and whether an early repayment charge would change the picture.
The tool is designed for borrowers who already have a secured loan and want to understand the financial case for overpaying before speaking to their lender. Our guide to whether you can pay off a secured loan early covers the broader pros and cons, and the early repayment charge calculator lets you model ERC costs in more detail before committing.
What the tool calculates
Monthly interest saved, total interest saved over the remaining term, the revised debt-free date, and months cut from the original term. When the ERC toggle is active, it calculates the charge at the point the overpayment path clears the loan and shows the net saving after the charge. An ERC proportion bar shows how much of the gross interest saving the charge consumes.
What it does not do
The tool does not replicate a lender assessment, account for rate changes during the remaining term, or model lender-specific overpayment allowances. Some lenders allow a fixed percentage of the outstanding balance to be overpaid each year without triggering an ERC. The tool uses a single ERC rate applied at settlement; actual ERC structures vary. Always check your loan agreement before overpaying.
Inputs you will need
The current outstanding balance of the loan, the APR, and the remaining term in years. These are typically shown on your annual statement, your online account, or your original loan agreement. If you are unsure of the current balance, use the most recent statement figure. A slightly higher balance will produce a slightly conservative saving estimate, which is the more useful planning assumption.
Checking your ERC terms
ERC terms differ between lenders and loan products. Common structures are a percentage of the outstanding balance at the point of settlement (typically 1% to 5%) or a fixed number of months of interest. Some lenders permit annual overpayments of up to 10% of the outstanding balance without any ERC. Your loan agreement will confirm which structure applies. If you have misplaced the agreement, your lender can provide a copy.
How this tool works
Adjust the sliders and toggle the optional switches to model different overpayment scenarios. The tool updates all outputs instantly and the chart redraws to show the two balance trajectories side by side.
Enter your loan details
Set the current balance, APR, and remaining term. The tool will immediately show the standard repayment path as a baseline. The navy line on the chart represents what happens if you make no overpayments and simply continue to the end of the original term.
Add a monthly extra payment
Drag the extra monthly payment slider to set an amount above the standard payment. Even modest amounts have a compound effect over time because each extra pound reduces the balance on which interest accrues the following month. The default of £150 extra per month is illustrative; adjust it to reflect what is actually affordable for you.
Add a one-off lump sum (optional)
Toggle the lump sum switch to model a one-off payment. Set the amount and the month in which you expect to make the payment. Earlier lump sums have a larger effect because they reduce the balance for a longer period. The chart shows the step drop in balance at the month you select. You can combine a lump sum with a monthly overpayment to model both happening simultaneously.
Toggle the ERC if your loan has one
If your loan carries an early repayment charge and your overpayment path would clear the loan ahead of the original end date, toggle the ERC switch. Select whether your lender charges a percentage of the outstanding balance or a fixed number of months of interest, then set the applicable rate. The verdict banner and the ERC proportion bar show whether the net saving after the charge is still positive.
How the maths works
The tool uses a month-by-month simulation rather than a closed-form amortisation formula. This is necessary because a lump sum payment applied at a specific month in the future breaks the assumptions that allow closed-form formulas to work cleanly. Running the simulation month by month produces accurate results regardless of when the lump sum is applied.
Standard monthly payment
The standard payment is derived using the standard amortisation formula: M = P x r(1+r)^n / ((1+r)^n – 1), where P is the balance, r is the monthly rate (APR divided by 12), and n is the remaining term in months. This produces the fixed monthly payment that would clear the loan exactly on the original end date if no overpayments are made.
Overpayment simulation
For each month, interest is calculated on the opening balance at the monthly rate. The sum of the standard payment plus the extra monthly payment is then deducted. If a lump sum is set, it is applied at the start of the relevant month before interest accrues that month. The simulation continues until the balance reaches zero, recording the total interest paid and the number of months taken.
Interest saving
Gross interest saved is the difference between total interest on the standard path and total interest on the overpayment path. Because the overpayment path clears the balance sooner and on a lower trajectory throughout, both the number of interest-bearing months and the balance on which interest is charged are reduced. This is why even a relatively small monthly extra payment can generate a disproportionately large saving over time.
ERC calculation
When the ERC toggle is active and the overpayment path clears the loan before the original end date, the tool calculates the ERC at the point of settlement. For a percentage-of-balance structure, the ERC is the outstanding balance at that point multiplied by the ERC rate. For a months-of-interest structure, the ERC is the outstanding balance multiplied by the monthly rate multiplied by the number of months specified. The net saving is gross interest saved minus the ERC charge.
The ERC consideration
An early repayment charge is a fee a lender may apply when a borrower repays all or part of a loan ahead of schedule. Not all secured loans carry an ERC, and those that do often include a free overpayment allowance of up to 10% of the outstanding balance per year. Understanding the ERC structure of your loan is an essential first step before deciding whether to overpay significantly.
When an ERC applies
An ERC typically applies when a loan is fully repaid before the end of the agreed term, or when an overpayment exceeds the lender’s permitted annual threshold. Whether an ERC applies to a partial overpayment depends on the specific loan terms. Some lenders trigger the ERC only on full settlement; others apply it to any overpayment above the free allowance. Check your agreement to confirm which applies to you.
When an ERC does not apply
An ERC would not apply in the tool if the overpayment path does not clear the loan ahead of the original end date. This is relevant when the extra payment is modest enough that the loan still runs to approximately its original term. In these cases the tool shows gross interest saved with no ERC deduction, because no early settlement occurs. You can test this by reducing the extra monthly payment slider until the debt-free date roughly matches the standard path.
Staying within the free allowance
Many lenders allow borrowers to overpay up to 10% of the outstanding balance per year without triggering an ERC. On a £40,000 balance, this would permit up to £4,000 in overpayments per year, or roughly £333 per month, with no charge. If this applies to your loan, you may be able to capture a meaningful saving without any ERC exposure at all. The tool does not model a per-year allowance automatically, but you can approximate it by limiting the extra monthly payment to the threshold amount.
When the ERC outweighs the saving
The verdict banner turns red and shows a net extra cost when the ERC exceeds the gross interest saving at the inputs entered. This is most likely to occur when the ERC rate is high, the remaining term is short (so there is relatively little interest left to save), or the overpayment is so large that it clears the loan very quickly. In these cases, overpaying to a level just below the ERC threshold, or waiting until closer to the end of the term, may produce a better outcome.
When overpaying makes sense
Overpaying a secured loan is not always the optimal use of available funds. The decision depends on the interest rate on the loan, whether an ERC applies, and what the alternative uses of the money are.
Higher rate loans
The higher the APR, the more compelling the case for overpaying. Each pound of balance reduction on a 12% APR loan saves approximately 1% per month in interest charges. On a 5% APR loan, the monthly saving is smaller and the case for overpaying relative to other uses of the money is weaker. Secured loan rates vary considerably by product and credit profile; our guide to the APR on secured loans covers what determines the rate you pay.
Longer remaining terms
Overpaying early in a loan’s life has a larger effect than overpaying late. A lump sum paid in the first year of a ten-year loan reduces the balance on which interest accrues for the remaining nine years. The same lump sum paid in year nine has only one year of interest to save. If you have a significant remaining term and a rate above the return you would expect from savings, overpaying is likely to produce a better financial outcome than saving the equivalent amount.
No ERC or small ERC
When there is no ERC or the ERC rate is low, overpaying is almost always worthwhile if the funds are available. The interest saving is guaranteed and risk-free in a way that most investment returns are not. A 7% APR secured loan effectively offers a risk-free 7% return on any overpayment, since each pound of reduction avoids exactly 7% annual interest on that pound for the remaining term.
When to consider alternatives first
If you carry higher-rate unsecured debt alongside a lower-rate secured loan, repaying the unsecured debt first will typically save more total interest. If you have no emergency fund, building three to six months of essential expenses in cash before overpaying a loan is generally the more prudent approach. Our guide to whether secured loans are a good idea covers the relationship between loan management and broader financial planning.
Frequently asked questions
How do I find the current balance and remaining term of my secured loan?
Your annual statement is the most reliable source. Secured loan lenders are required to issue annual statements showing the outstanding balance, the remaining term, and a summary of payments made during the year. If you have not received one recently, your lender can provide a current balance and redemption statement on request. Your online account, if the lender provides one, will typically show the current balance and the number of months remaining. A redemption statement gives the exact figure needed to clear the loan on a specific date and will include any applicable ERC; this is different from the outstanding balance and will usually be slightly higher.
If you are using the tool for planning purposes rather than to model an imminent overpayment, an approximate figure from a recent statement is sufficient. The tool is sensitive to the balance input, so a materially outdated figure will produce a less accurate saving estimate. Using a slightly higher balance than the actual figure will produce a conservative (lower) saving estimate, which is the more cautious planning assumption.
Why does overpaying the same amount each month save more interest than one lump sum of the same total?
This is a common misconception. In most cases a lump sum paid at the start of the remaining term saves more interest than the same total spread across monthly overpayments, because the lump sum reduces the balance immediately and that reduction compounds over the entire remaining term. By contrast, monthly overpayments build up gradually, so the average balance reduction is lower in the early months.
Whether a lump sum or monthly overpayments save more depends on when the lump sum is applied and the size of the monthly extra payment. You can test both scenarios in the tool by setting the monthly extra to zero and modelling the lump sum alone, then switching to a monthly extra and setting the lump sum to zero, and comparing the gross interest saved in each case. In most scenarios involving a meaningful remaining term and a lump sum applied in month one, the lump sum wins. Monthly overpayments are often the more practical choice when a lump sum is not available.
Does overpaying reduce the monthly payment or shorten the term?
This depends on the lender and the loan product. Some lenders recalculate the monthly payment each time an overpayment is made, reducing the monthly obligation while keeping the end date unchanged. Others keep the monthly payment fixed and shorten the remaining term instead. A third approach keeps both fixed and simply builds up a credit balance on the account. The tool models the third and most common scenario for fixed-rate secured loans: the standard monthly payment is kept constant, and the overpayment reduces the term.
If your lender reduces the monthly payment rather than shortening the term, the interest saving will be lower than the tool shows, because the reduced payment means you effectively spread the saving over a longer period. Contact your lender to confirm which approach they apply before using this tool to plan a significant overpayment strategy.
What is the 10% overpayment allowance and how do I use it in the tool?
Many secured loan lenders permit borrowers to overpay up to 10% of the outstanding loan balance in any 12-month period without triggering an early repayment charge. This allowance resets each year, typically on the anniversary of the loan start date. Overpayments within this threshold are effectively free in ERC terms, making them a low-risk way to reduce the balance and save interest.
The tool does not automatically model a per-year free allowance, but you can approximate the position by setting the monthly extra payment to an amount that stays within the annual threshold. For example, on a £40,000 balance a 10% annual allowance permits £4,000 per year in overpayments, which is roughly £333 per month. Setting the monthly extra to £333 in the tool will show the interest saving achievable within the free allowance without any ERC risk. As the balance reduces over time, the 10% threshold also reduces, so the actual free allowance in later years will be smaller. For a precise figure, contact your lender directly.
Squaring Up
Overpaying a secured loan can save a meaningful amount of interest, especially on higher-rate loans and in the earlier years of the remaining term. The ERC toggle gives a clear picture of whether the saving survives any charge that applies on early settlement.
- Check your ERC terms before overpaying significantly. Your loan agreement will confirm whether a charge applies, what the free annual allowance is, and whether the ERC is calculated as a percentage of balance or months of interest.
- Earlier overpayments save more. The same extra payment applied in year one saves more total interest than the same amount applied in year five, because the balance reduction compounds over a longer remaining term.
- Lump sums typically outperform monthly extras of the same total. If a lump sum is available and the ERC position permits it, applying it early in the remaining term will usually produce the largest gross saving.
- The tool models the most common scenario. If your lender reduces your monthly payment rather than shortening the term when you overpay, the actual saving will differ from what the tool shows. Confirm the lender’s approach before planning.
The tools below cover related decisions on secured loan management.
Ready to see what you could borrow?
Checking won’t harm your credit score Check eligibilityAll figures produced by this tool are based on the information you enter and are illustrative only. The simulation assumes consistent payments throughout the remaining term and does not account for changes in interest rate, lender-specific ERC trigger points, administrative fees, or annual overpayment allowances. ERC structures and amounts vary by lender and loan product. Check your loan agreement before making overpayments. Your home may be repossessed if you do not keep up repayments on a mortgage or other debt secured on it. This tool does not constitute financial advice.
