About this tool

How to use the pay down debt vs save comparator

This tool compares two financial strategies across the same period using your specific numbers. Entering realistic figures for both the debt interest rate and the savings rate produces a more useful comparison than using round numbers or best-case assumptions.

How the comparison works: net worth as the measure

The tool uses net worth at the end of the chosen period as the single comparable measure for both strategies. Net worth in this context is savings balance minus outstanding debt balance at the end of the simulation. Both strategies start from the same position: the same debt balance, the same monthly allocation, and the same period. What differs is how the monthly amount is divided between debt repayment and saving, and that difference produces different savings balances and different remaining debt balances at the end of the period.

The reason the comparison does not always favour the same strategy is that two rates are competing. The debt interest rate is the cost of leaving debt outstanding; the savings rate is the return on accumulating savings. When the debt rate is higher than the after-tax savings rate, paying debt faster reduces total interest paid by more than saving faster would earn, which means the numbers tend to favour the debt-first strategy. When the savings rate after tax exceeds the debt rate, accumulating savings produces a higher net worth than clearing debt quickly. The comparison period affects which side wins because compounding on savings takes time to accumulate: a longer period gives the savings more time to outrun the debt cost at the same allocation. The split strategy panel shows where the crossover point is for your specific rates across four allocations.

Beyond the numbers: the case for clearing debt

The mathematical comparison between debt and saving is a useful frame but it is not the only relevant factor. There are situations where the numbers favour a savings-first approach but the practical case for prioritising debt is compelling regardless. The most straightforward is the behavioural one: accessible savings held in parallel with outstanding debt are, for many people, at risk of being spent rather than maintained. A savings account that is nominally growing while the debt runs at a higher rate produces a better net worth on paper only if the savings are genuinely left untouched for the full comparison period. If the reality is that accessible savings tend to be drawn on for non-emergency spending, the financial benefit of maintaining them alongside debt does not materialise.

There is also a simplicity argument. Carrying multiple financial commitments simultaneously increases cognitive load and the number of decisions that need to be managed each month. For some people, the value of clearing a debt and having one fewer outgoing to manage is real even if the interest rate comparison would suggest keeping the debt and saving. The peace of mind toggle in the tool acknowledges this openly: it does not pretend the non-financial factors do not exist or that the numbers are all that matters. Whether those factors are relevant depends entirely on the individual, and the tool does not make that determination either way.

How the savings tax wrapper affects the comparison

Savings interest is taxable as income for most savers above the Personal Savings Allowance: £1,000 per year for basic rate taxpayers and £500 for higher rate taxpayers as at 2025/26. When savings interest is taxable, the effective return on the savings side of the comparison is lower than the headline AER. A savings account paying 4.5% AER delivers an effective after-tax return of 3.6% to a basic rate taxpayer and 2.7% to a higher rate taxpayer on amounts above the allowance. The comparison between the debt rate and the savings rate changes when these adjustments are applied, and in some cases it changes which strategy the numbers favour.

The tax wrapper toggle in the tool applies this adjustment automatically: selecting basic rate or higher rate tax reduces the effective savings rate used in the simulation and recalculates the net worth outcomes under both strategies. A passive callout explains the difference between the gross rate and the effective after-tax rate, and notes whether applying the tax treatment changes the result. Selecting ISA in the toggle applies no tax reduction, since savings interest within an ISA wrapper is not subject to income tax. For savers with balances above the Personal Savings Allowance, the wrapper choice affects whether the tax adjustment changes the outcome, and the tool makes that visible directly rather than leaving it as an implicit assumption.

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Frequently asked questions

How does the tool calculate which strategy produces a better outcome?

The tool runs a month-by-month simulation for each strategy. Under Strategy A (debt first), the full monthly allocation goes toward the debt each month until it is cleared, at which point the full amount redirects to savings for the remainder of the period. Under Strategy B (savings first), the full monthly allocation goes into savings throughout the period and the debt is left to accrue interest unpaid. At the end of the chosen period, the tool calculates net worth under each strategy as savings balance minus remaining debt balance, and compares the two figures.

The split strategy cards apply the same month-by-month simulation but with fixed proportions of the monthly allocation going to each purpose simultaneously. At 75/25, £75 in every £100 of the monthly allocation goes to debt and £25 to savings throughout the period, adjusting as the debt is cleared. The net worth comparison across all four allocations shows where the best financial outcome sits for the specific combination of rates and period entered.

What is net worth in the context of this comparison, and why use it as the measure?

Net worth in this tool is defined simply as savings balance minus outstanding debt balance at the end of the simulation period. It is the same monthly amount and the same starting position applied over the same number of months, so the net worth figure reflects purely the effect of the allocation choice on the financial position at the end of the period. A higher net worth at the end means the strategy leaves the individual in a better financial position by that measure.

Net worth is used as the measure because it captures both sides of the ledger simultaneously. A strategy that maximises savings at the cost of a large remaining debt balance may show a high savings figure but a lower net position once the debt is subtracted. Conversely, a strategy that clears debt quickly may show a low savings figure but a higher net position because no interest is continuing to accrue. Comparing savings balances alone would favour the savings-first strategy regardless of the debt rate; comparing net worth shows the actual financial outcome accounting for both.

When does the comparison typically favour paying debt, and when does it favour saving?

The comparison tends to favour the debt-first strategy when the debt interest rate is materially higher than the after-tax savings rate. At a debt rate of 8% and a savings rate of 4.5% before tax (3.6% after basic rate tax), the cost of leaving the debt running exceeds the return on the savings for each month the debt persists, and the numbers tend to favour clearing it. The margin between the two rates is the key variable: the larger the gap, the more clearly the numbers favour debt repayment.

The comparison can favour the savings-first approach when the debt rate is low, the savings rate is high, and the comparison period is long enough for the compound savings growth to outrun the debt interest cost. A very low-rate fixed mortgage alongside a high-rate savings account held over a long period is the scenario where this dynamic most commonly applies. The tool shows the comparison for the specific rates entered rather than generalising: testing different combinations of rates is the most useful way to understand how sensitive the result is to each variable.

What does the peace of mind toggle actually change in the tool?

The peace of mind toggle does not change any of the underlying calculations. The net worth figures, debt-free dates, and split strategy cards all remain the same regardless of whether the toggle is on or off. What changes is the framing of the result: with the toggle on, the result card highlights the debt-first allocation regardless of what the numbers show, and a panel appears explaining the non-financial case for prioritising debt clearance.

The panel explains three aspects of the non-financial case. First, the behavioural risk: accessible savings held alongside outstanding debt are often drawn on for non-emergency spending, which means the savings-first strategy only delivers its mathematical benefit if the savings are genuinely maintained for the full period. Second, the psychological value of clarity: being debt-free removes a monthly obligation and reduces financial complexity, which has real value that the net worth comparison does not capture. Third, the interaction with motivation: for some people, the visible progress of a debt balance falling is more motivating than the visible progress of a savings balance rising, and sustained motivation matters to whether the plan is followed through. These factors are real but not quantifiable, which is why the toggle makes them explicit rather than embedding them invisibly in the result.

What happens if my monthly amount does not cover the monthly interest on my debt?

If the monthly allocation entered is less than the monthly interest accruing on the debt at the rate specified, the debt is growing faster than the allocation can address it. In this situation the results panel hides and a warning appears. The comparison between strategies is not meaningful in this scenario because under the debt-first strategy the debt would never be cleared, and under the savings-first strategy the debt balance would grow indefinitely.

If the warning appears, the first step is to check that the debt interest rate entered is correct: annual rates and monthly rates are sometimes confused, and entering a monthly rate in the annual rate field produces an inflated figure that triggers the warning incorrectly. If the rate is correct and the warning remains, the monthly allocation needs to be higher than the monthly interest charge before the comparison can run. The minimum needed to avoid the debt growing is the monthly interest charge alone; a meaningful reduction in the balance over any period requires the allocation to exceed that floor by enough to make material progress against the principal as well as covering the interest.

This tool is for illustrative purposes only and does not constitute financial advice. All projections depend on the inputs provided and apply simplified assumptions, including constant interest rates, consistent monthly contributions, and uninterrupted debt accrual, that may not reflect your actual circumstances. The peace of mind toggle changes result framing only and does not constitute a recommendation. Tax wrapper calculations use simplified 2025/26 rate assumptions and do not account for the Personal Savings Allowance, ISA subscription limits, or other individual tax circumstances. Actual outcomes will depend on your individual circumstances.