Savings Guide: Cash ISA vs Savings Account — Which Pays More After Tax?

A cash ISA is a tax-free — see GOV.UK for current allowances (gov.uk/income-tax-rates) savings wrapper. Any interest you earn inside a cash ISA is completely free from income tax — no matter how much you earn or how much interest you receive. The annual ISA allowance for 2025/26 is £20,000, which can be split across up to four types of ISA (Cash, Stocks & Shares, Innovative Finance and Lifetime), but the total across all types cannot exceed £20,000. You must be 18 or over to open a cash ISA (with a narrow exception for 16- and 17-year-olds born between 6 April 2006 and 5 April 2008), and you must be a UK resident.

A standard savings account — whether easy access, notice or fixed-rate bond — is not tax-free. Interest earned is added to your taxable income. However, most savers benefit from the Personal Savings Allowance (PSA), introduced in April 2016. Basic rate taxpayers (income up to £50,270) can earn up to £1,000 of savings interest tax-free each year. Higher rate taxpayers (income between £50,271 and £125,140) get a £500 allowance. Additional rate taxpayers (income above £125,140) receive no PSA at all — GOV.UK explains the full rules — every penny of interest is taxed.

There is also a lesser-known starting rate for savings. If your total income (from employment, pensions, and other sources excluding savings) is below £17,570, you may be entitled to earn up to £5,000 of savings interest at 0% tax. This is particularly relevant for part-time workers, retirees with modest pensions, or those relying mainly on savings income. The £5,000 band reduces by £1 for every £1 of non-savings income above £12,570, disappearing entirely once non-savings income reaches £17,570.

Let us put real numbers to it. Suppose you have £20,000 in a savings account paying 4.5% — broadly in line with the best easy access rates available in early 2026. That produces £900 of interest per year.

If you are a basic rate taxpayer, your PSA covers £1,000 of interest. Since £900 is below that threshold, you pay no tax at all. The cash ISA offers no advantage here — your standard savings account delivers the full £900 tax-free, and you have not used any of your ISA allowance. If you are a higher rate taxpayer, your PSA covers only £500. Of your £900 interest, £400 is taxable at 40%, costing you £160 in tax. Your effective return drops from £900 to £740 — equivalent to an after-tax rate of 3.70% instead of 4.50%. A cash ISA paying the same 4.5% would give you the full £900 with no tax to pay. If you are an additional rate taxpayer with no PSA, all £900 is taxed at 45%, costing you £405. Your net return is just £495 — an effective rate of 2.48%. The cash ISA advantage is stark.

The chart makes the point clearly. For basic rate taxpayers with modest savings, the PSA does the heavy lifting and a cash ISA adds nothing. But for higher and additional rate taxpayers, the ISA wrapper delivers materially better returns — £160 and £405 more per year respectively on a £20,000 pot.

The critical question is: at what savings balance does your interest exceed your PSA, making a cash ISA the better choice? The formula is straightforward — divide your PSA by the interest rate on offer.

For a basic rate taxpayer with a £1,000 PSA and a savings rate of 4.5%, the breakeven point is £1,000 ÷ 0.045 = approximately £22,222. Below that balance, all your interest falls within the PSA and there is no tax to pay. Above it, every additional pound of interest is taxed at 20%. If you have less than around £22,000 saved, a standard savings account is perfectly efficient. For a higher rate taxpayer with a £500 PSA at 4.5%, the breakeven is £500 ÷ 0.045 = approximately £11,111. That is a much lower threshold. If you have more than about £11,000 in savings, you will start paying 40% tax on the excess interest — and a cash ISA starts delivering genuine value. For an additional rate taxpayer, the breakeven is £0. With no PSA, every pound of interest is taxable from the first penny, making the cash ISA the obvious choice regardless of balance.

These breakeven figures move with interest rates. If rates fall — as many economists expect over 2026 — the breakeven balances rise, because you earn less interest per pound saved. At 3.5%, for example, a basic rate taxpayer's breakeven would climb to roughly £28,571. Conversely, if you hold savings across multiple accounts and the combined interest exceeds your PSA, a cash ISA becomes more valuable even at modest individual balances. The key is your total interest across all accounts, not just one.

Historically, cash ISA rates lagged behind standard savings accounts by a noticeable margin — sometimes 0.5% or more. Providers knew savers valued the tax-free wrapper and could afford to offer lower headline rates. In 2026, that gap has largely closed. The best easy access cash ISAs are paying around 4% to 5%, broadly in line with the top standard easy access accounts. Fixed-rate cash ISAs over one or two years are similarly competitive.

This matters because the ISA-versus-savings-account decision used to involve a rate penalty. If a cash ISA paid 4.0% and a standard account paid 4.5%, you had to weigh the tax saving against the lower gross rate. Today, with rates closely matched, the comparison is simpler: if your interest would exceed your PSA in a standard account, the cash ISA wins outright. If it would not, you might still prefer the ISA for long-term planning — interest earned in an ISA remains tax-free in future years even if your circumstances change, rates rise, or you accumulate more savings.

One practical consideration: flexibility. Many of the best standard easy access accounts allow unlimited withdrawals. Some cash ISAs — particularly fixed-rate versions — restrict access or impose penalties for early withdrawal. If you might need your money at short notice, check the terms carefully. An easy access cash ISA avoids this issue, though rates may be marginally lower than the very best fixed-rate options.

Related reading: ISA guide.

Start by estimating your total savings interest for the current tax year. Add up the balances across all your savings accounts (excluding ISAs) and multiply by the average interest rate. If the total interest is comfortably below your PSA — £1,000 for basic rate, £500 for higher rate — a standard savings account is fine and you lose nothing by not using an ISA. If your interest is approaching or exceeding your PSA, move the excess into a cash ISA before 5 April 2026 to shelter it from tax. Remember, the £20,000 ISA allowance is use-it-or-lose-it — any unused allowance cannot be carried into the next tax year.

Consider your future trajectory too. If you are building savings steadily, you may breach your PSA next year even if you are within it now. Opening a cash ISA this year — even with a modest deposit — starts building a tax-free pot that compounds without any tax drag in future years. This is especially relevant if interest rates remain elevated or your income rises, pushing you into a higher tax band with a smaller PSA.

Finally, do not overlook the starting rate for savings if your income is below £17,570. Combined with the PSA, this can shelter up to £6,000 of savings interest from tax (£5,000 starting rate plus £1,000 PSA), making a cash ISA unnecessary for most people in this income bracket. For everyone else, the decision comes down to a simple calculation: total interest versus PSA. Run the numbers, and the right answer will be clear.

Capital at risk. Tax treatment depends on your individual circumstances and may change. This article is for informational purposes only and does not constitute financial advice. If you are unsure about your tax position, consult an FCA-regulated financial adviser.