When there is enough in savings to cover a purchase outright, using it feels like the obvious choice: no interest to pay, no monthly commitment, no application process. But withdrawing from an ISA or savings account is not free. The withdrawn amount stops compounding, and the growth it would have produced over the following years is the real cost of the withdrawal. Whether that cost is higher or lower than the interest on a loan depends on the rates involved, the time horizon, and what the withdrawal does to the savings balance that remains.

This tool compares the foregone compound growth of an ISA withdrawal against the total interest cost of a loan across your chosen period. It also shows the impact on your emergency buffer, models a hybrid path that preserves a safety net while using part of the savings, and flags the ISA allowance implication where relevant. All figures are illustrative and depend on the inputs you provide.

About this tool

How to use the ISA vs loan cost comparison tool

The comparison is only meaningful when the rates and balances entered reflect your actual situation. Using a hypothetical savings rate or a representative loan APR rather than a personalised offer will skew the result toward one option or the other in ways that may not apply to your specific case.

How the comparison works across different time horizons

The ISA cost in this comparison is the compound growth foregone on the withdrawn amount over the chosen period. If £5,000 is withdrawn from an ISA earning 4% AER, the cost over two years is approximately £408: the growth that £5,000 would have produced if left in the account. The loan cost over the same period is the total interest paid on a £5,000 loan at the given APR over the chosen term. The lower figure is the less costly option, and the margin is the saving from choosing it.

The comparison changes over time because compound growth accumulates non-linearly. In the early years, the foregone ISA growth may be modest relative to the loan interest. Over longer periods, the compounding on the withdrawn amount becomes more significant, and the ISA cost can overtake or fall further behind the loan cost depending on the relative rates. The three-horizon panel captures this directly: it shows which option is cheaper at one year, at your chosen period, and at five years, making visible whether the comparison result is stable across time frames or whether it flips at some point. For decisions where the time horizon is genuinely uncertain, seeing the comparison at multiple points is more informative than a single figure. The dual-line chart shows the cumulative ISA foregone growth and cumulative loan interest simultaneously across seven years, with a vertical line at your chosen period.

The emergency buffer consideration

The pound cost comparison between the ISA withdrawal and the loan does not capture the full picture of what a savings withdrawal means for financial resilience. Savings held in an accessible account serve two functions: they may be the intended funding source for a specific purchase, but they also represent the emergency fund that covers unexpected income disruption or large unplanned costs. Withdrawing the full amount to avoid loan interest may be the cheaper option on paper while simultaneously leaving the emergency buffer dangerously thin.

The emergency buffer panel addresses this by showing how many months of essential expenses the remaining balance covers after a full withdrawal, alongside a plain-language status: solid (above six months), adequate (three to six months), thin (one to three months), or depleted (below one month). The hybrid path panel is specifically designed for cases where a full withdrawal would leave the buffer thin or depleted: it shows the cost of withdrawing as much as possible while preserving a three-month expense buffer, with the remainder borrowed. In most cases the hybrid path costs more than the pure ISA withdrawal but less than a full loan, and it preserves enough of the savings buffer to maintain basic financial resilience. Whether the extra cost of the hybrid versus the pure ISA withdrawal is worth paying for the buffer preservation is a judgement that depends on income stability and the availability of alternative credit if an emergency arose. The emergency fund builder can model how long it would take to rebuild a depleted buffer from future saving.

The hybrid path: partial withdrawal and partial loan

The hybrid path in the tool represents a middle ground that is often more practical than either extreme. A full ISA withdrawal funds the purchase at the lowest interest cost but may compromise the emergency buffer. A full loan preserves the savings buffer entirely but incurs the maximum loan interest. The hybrid path takes as much as possible from the ISA while keeping three months of expenses in the account, and borrows only the shortfall.

The cost of the hybrid path is a weighted average of the ISA cost (on the withdrawn portion) and the loan cost (on the borrowed portion). The three hybrid cards show the full ISA withdrawal cost, the hybrid cost, and the full loan cost side by side, with the lowest-cost option highlighted by a teal border. In cases where the ISA balance is large relative to the amount needed, the hybrid and pure ISA withdrawal may be identical because the three-month buffer requirement can be met even after a full withdrawal. In cases where the balance is close to the amount needed, the hybrid path may borrow a significant fraction, bringing its cost closer to the full loan option. The comparison makes the trade-off explicit rather than presenting only the two extreme options.

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Frequently asked questions

How does the tool calculate the cost of using savings versus a loan?

The ISA cost is the compound growth the withdrawn amount would have produced if left in the account over the comparison period: amount multiplied by (1 plus the annual rate) raised to the power of the comparison period in years, minus the original amount. This is the opportunity cost of the withdrawal: the growth that is given up by taking the money out rather than leaving it to compound. The loan cost is the total interest on a standard repayment loan: monthly payment multiplied by the number of months minus the original principal. The tool then compares these two figures directly to find the lower-cost option and the margin between them.

The comparison is sensitive to the relative rates: when the savings rate is high and the loan APR is also high, the outcome depends on which is higher. When the savings rate is below the loan APR, the loan costs more in interest than the ISA withdrawal costs in foregone growth, and the ISA withdrawal is typically cheaper. When the savings rate is above the loan APR (which is unusual for unsecured lending but possible for mortgage-rate borrowing), leaving the savings to compound and taking the loan may produce a better financial outcome. The three-horizon panel shows this across time frames, and the dual-line chart makes the crossover point visible if one exists within seven years.

If the ISA withdrawal is cheaper, does that mean I should always use savings first?

Not necessarily. The pound cost comparison is one input into the decision, not the only one. The emergency buffer consideration is often equally important: if withdrawing the savings would leave less than two or three months of expenses in accessible cash, the financial resilience cost of the withdrawal may outweigh the interest saving. A thin emergency buffer means that any unexpected cost during the loan repayment period would require additional borrowing, which could cost more in total than leaving the savings intact and taking the loan in the first place.

There are also behavioural considerations. Some people find it easier to maintain a savings habit when the account balance stays intact, rather than rebuilding from a lower starting point after a withdrawal. The ISA allowance point is relevant for larger withdrawals from a Cash ISA: funds withdrawn from a Cash ISA in the same tax year can typically be replaced within the current year’s allowance under a flexible ISA, but funds withdrawn from a non-flexible ISA permanently use up that year’s allowance unless replaced before the tax year ends. Whether any of these considerations change the decision depends on the individual’s circumstances, and the tool provides the cost comparison and buffer assessment as inputs to that judgement rather than as a definitive answer.

What is the ISA allowance opportunity cost panel showing?

The ISA allowance opportunity cost panel appears when two conditions are both true: the replacement toggle is set to “no” (the withdrawn funds cannot be replaced within the current ISA allowance), and a non-zero tax rate is selected (the saver has a taxable savings interest position above the Personal Savings Allowance). In these circumstances, the panel shows the annual and ten-year cumulative cost of holding the equivalent funds outside the ISA wrapper, subject to income tax on savings interest.

The calculation uses the same savings rate applied to the withdrawn amount and compares what that growth would be worth inside the tax-free ISA wrapper against the after-tax return on the same amount held in a taxable savings account. The difference compounds over time, and the ten-year figure makes the long-run cost of the lost wrapper space concrete. This panel is an additional consideration rather than the primary cost comparison: it is only relevant for savers with substantial savings balances where income tax on savings interest applies, and the amounts involved depend heavily on the total savings balance and the individual’s tax position.

Does the comparison account for flexible ISA rules that allow replacement within the same year?

The tool does not automatically distinguish between flexible and non-flexible ISA accounts. If your ISA is a flexible one, which allows withdrawn funds to be re-deposited in the same tax year without using additional annual allowance, the ISA allowance opportunity cost consideration does not apply to within-year withdrawals and replacements. The replacement toggle in the tool is designed to capture your specific situation: if you know your ISA is flexible and you plan to replace the funds within the current tax year, setting the toggle to “yes” reflects that accurately.

Not all ISAs are flexible: most fixed-rate cash ISAs and many investment ISAs do not allow replacement of withdrawn amounts within the same year. If you are unsure whether your ISA is flexible, checking with your provider before making a withdrawal is worthwhile, since the answer affects both the allowance opportunity cost and the practical question of whether the savings can be rebuilt tax-free after the withdrawal. The flexibility distinction also matters for timing: even a flexible ISA typically does not allow withdrawal and replacement across two different tax years, so a withdrawal made near the end of one tax year with planned replacement at the start of the next may not benefit from flexible ISA treatment.

Why does the comparison sometimes favour the loan even when the savings rate is above zero?

This can happen when the loan APR is lower than the savings rate, which means the cost of servicing the loan interest is less than the foregone growth on the savings. This is uncommon for unsecured personal loans, where APRs are typically well above savings rates, but it can occur for secured borrowing at mortgage rates, particularly during periods when savings rates are elevated relative to fixed mortgage rates. It can also occur when the comparison period is very short: over a period of a few months, the foregone compound growth on a modest savings balance at a standard AER may exceed the total interest on a small loan at a relatively high APR, because the loan interest calculation also accounts for the reducing balance each month while the foregone ISA growth compounds on the full withdrawn amount throughout the period.

The dual-line chart in the tool makes this relationship visible across seven years. If the lines cross within the chart range, it means the comparison result changes at that horizon: the ISA withdrawal may be cheaper in the short term but the loan becomes the lower-cost option over a longer period, or vice versa. The three-horizon panel captures this in three specific data points. The main result shows the comparison at your chosen period only; checking the chart and the three-horizon panel gives the fuller picture if the period is uncertain or if you want to understand how sensitive the result is to the time horizon chosen.

This tool is for illustrative purposes only and does not constitute financial, tax, or investment advice. ISA cost calculations represent foregone compound growth based on a constant savings rate applied to the withdrawn amount: actual ISA returns will vary over time. Loan cost calculations assume a standard fixed-rate repayment loan. The emergency buffer assessment uses a simplified monthly expenses figure and does not reflect the complexity of individual financial circumstances. ISA allowance opportunity cost calculations use simplified tax band assumptions based on 2025/26 rates and do not account for the Personal Savings Allowance, flexible ISA rules, or other individual tax circumstances. Tax rates, allowances, and ISA rules are subject to change. The comparison does not account for any early access penalties, exit charges, or transfer restrictions that may apply to specific savings products. Actual outcomes will depend on your individual circumstances.