ISA millionaires set to more than treble over three years

• There could be as many as 17,600 ISA millionaires by April 2026, according to analysis of HMRC data obtained via FOI by Rathbones.
• This assumes an 8% annual return assumption and full use of the £20,000 ISA allowance each year.

The number of ISA millionaires looks set to more than treble in just three years to over 17,500 by April 2026, according to an analysis of recent HMRC data by Rathbones, one of the UK’s leading wealth and asset management groups.

The data, obtained via Freedom of Information request from Rathbones, to HMRC reveals that at the start the 2023/24 tax year, 5,070 people held ISA portfolios worth £1 million or more. A further 48,160 individuals had ISA holdings between £500,000 and £750,000, 9,430 had between 750,000 and under £950,000 and 1,210 were on the cusp of ISA millionaire status, worth between £950,000 and £1 million.

Rathbones’ analysis suggests the number of ISA millionaires could have risen to around 17,600 by April 2026 – from just 5,070 people in April 2023 – assuming 8% annual investment growth and full use of the £20,000 ISA allowance each year. Under higher return assumptions, the number could have grown further still.

The figures assume uninterrupted saving behaviour, with investors close to £1 million gradually crossing the threshold over the three‑year period.

Historically, the number of ISA millionaires has at least doubled every three years since 2016, and more than trebled during periods when MSCI World returns exceeded 15%. With global equities delivering annualised returns of 17.4% since April 2023*, conditions are similar to those previously associated with rapid growth in ISA millionaire numbers.

Angela Smith, Senior Investment Director at Rathbones, based in Glasgow, says: “Our analysis of the data suggests the number of ISA millionaires is likely to have risen significantly over a relatively short period, even on modest assumptions. This means many more investors are now benefiting from tax-free growth, income and dividends on seven-figure portfolios.

“Our experience with clients shows that the not-so-secret ingredients behind building an ISA millionaire portfolio are patience, time, consistently using ISA allowances, and avoiding interruptions to the power of compounding.”

Even if savers stopped contributing to their ISAs altogether after April 2023, an 8% annual return could still lift the number of ISA millionaires from 5,070 to around 7,100 by April 2024, nearly 9,510 by April 2025 and almost 13,100 by April 2026.

Under a more cautious assumption of 5% annual investment returns, the number of ISA millionaires would still rise to around 12,300 by April 2026 if the ISA allowance were fully used each year, or to 9,300 if no further contributions were made.

The growth reflects a powerful combination of investment growth, the long-term impact of compounding within a tax-free wrapper, and disciplined use of annual ISA allowances over many years.

“Even after investment gains, £1 million is no longer what it used to be,” Angela Smith continued. “To have the same £1m buying power as 20 years ago, investors today would need £1.7 million. Inflation steadily erodes purchasing power, which is why saving alone is rarely enough. Investing wisely – across assets such as shares and funds – can help deliver returns that outpace inflation and protect and grow wealth over the long term.”

Table 1: Individuals broken down by ISA market value band, as of 5 April 2023

Source: HMRC via Rathbones’ FOI. The number of individuals in each group has been rounded to the nearest 10.

* ISA Millionaires growth compared with MSCI World returns

Source: Rathbones. Performance data from 6 April 2023 – 28 February 2026.

Methodology: Starting from the 5,070 ISA millionaires recorded in April 2023, Rathbones estimated how many additional investors could cross the £1 million threshold over the following three tax years by applying an assumed annual investment return and assessing how much of each ISA balance band would grow beyond £1 million over time. ISA balances within each value band are assumed to be evenly distributed, meaning that as investment returns compound, the minimum starting balance required to reach £1 million falls each year. As a result, those closest to the threshold are assumed to cross first, followed by progressively larger proportions of lower balance bands moving into seven‑figure territory. Under this approach, virtually all investors with ISA balances above £950,000 cross within the first year, followed by increasing shares of those in the £850,000–£949,999, £750,000–£849,999 and £500,000 to £749,999.99 bands in subsequent years.