Cash ISA Interest Rates vs the FTSE 100: Who Is Winning in 2026?

With Cash ISA interest rates running between 3.5% and 4.5% this year, a £10,000 deposit made on 1 January 2026 has generated somewhere between £175 and £225 in interest so far — tax-free, with no market risk attached. That sounds reasonable until you set it against what UK and US equity indices have delivered over the same period.

What Cash ISA Interest Rates Are Actually Delivering

The arithmetic is straightforward. At those rates, the same £10,000 should produce roughly £350 to £450 in interest by December, depending on the account type and whether the rate is fixed or variable. The Bank of England expects inflation to average around 3% this year, which means the real return after price erosion is thin: positive, but not compelling.

Cash ISAs are not without merit. Investors who need a liquid buffer to cover unexpected expenses — a new car, a tax bill, a redundancy — are right to hold some cash outside the market. Forced selling of equity positions at the wrong moment is a more expensive mistake than accepting a low real yield on a cash account.

But the comparison with equities matters, because the government clearly thinks savers are holding too much in cash. As part of the Autumn 2025 Budget, HM Treasury confirmed a new annual cash ISA limit of £12,000 for savers under the age of 65, effective from 6 April 2027. The overall ISA allowance stays at £20,000; the remaining £8,000 will be reserved for stocks and shares. Savers aged 65 and over keep the full £20,000 cash ISA allowance.

The ISA Overhaul Goes Further Than the Cash Limit

The reform package extends beyond the cash cap. The Lifetime ISA is being withdrawn, and the government is consulting on a replacement: a First Time Buyer ISA, available to first-time buyers over 18 with a legal mortgage, provided the account has been open at least a year. The price cap and government bonus level are yet to be confirmed, pending a future fiscal event.

For investors under 65, the practical consequence of the cash limit reform is that some reallocation toward equities becomes structurally encouraged rather than optional. The question is where to put that incremental money.

Cash ISA Interest Rates Versus a FTSE 100 ETF: The 2026 Scorecard

All three major equity indices tracked in the original comparison — the FTSE 100, the S&P 500, and the Nasdaq-100 — have outpaced cash year to date. The Nasdaq-100 has been the standout: £10,000 invested at the start of the year would now be worth around £11,800. Once dividends are included, the FTSE 100’s total return rises above 7.5% for the year so far, comfortably clearing both Cash ISA interest rates and the inflation rate.

For those looking at the FTSE 100 specifically, the iShares Core FTSE 100 UCITS ETF (CUKX) is worth examining as a building block for a Stocks and Shares ISA. The accumulating share class reinvests dividends automatically, compounding the income rather than distributing it. On yield, the snippet puts the FTSE 100’s starting level at around 3.05%; DividendData.co.uk places the current index yield slightly lower at 2.99%. The two sources conflict on the precise figure, but both point to a yield that, combined with price appreciation, has already exceeded what Cash ISA interest rates are likely to return for the full year.

The fund’s top holdings, per the latest iShares fact sheet, reflect the index’s concentration: HSBC at 9.49%, AstraZeneca at 8.21%, Shell at 7.04%, and Rolls-Royce at 4.50%. These are global businesses with revenues drawn from across geographies and sectors, which provides a degree of insulation if a US technology correction materialises. The counterweight is that roughly 28% of the fund sits in financials, leaving performance and dividend reliability exposed to any sharp deterioration in global credit conditions.

The structural shift in the ISA regime takes effect on 6 April 2027. For savers under 65 who currently hold the full £20,000 in cash, that date represents a hard deadline for reviewing whether the allocation still fits. The yield gap between cash and equities has narrowed this year, but over longer periods the gap widens considerably, and the new rules will make the choice explicit rather than discretionary.