The UK State Pension is rising. And following the latest increase, pensioners now receive up to £12,547.60 a year from the government. But by planning ahead, it’s possible to secure a chunkier retirement income stream by leveraging the power of an ISA.

So how big does an ISA need to be to generate the same amount of money as the State Pension?

Let’s find out.

Running the numbers

Following the 4% withdrawal rule, an ISA would need to reach around £313,690 to generate that £12,547.60 a year.

Obviously, that’s a pretty chunky sum. Yet while it may feel out of reach, even a 40-year-old starting from scratch with just £500 to spare each month can hit this milestone before turning 60.

Here’s how. That £500 a month invested at a compounded annual return of 10% for 19 years will grow to £338,007.80. That means at the age of 59, this portfolio would generate around £1,000 more in passive income, landing at £13,520.31.

So which UK stocks should investors be looking at today to aim for that 10% annualised return?

Could this overlooked compounder do the job?

The UK stock market’s home to a growing list of proven compounders that have quietly outpaced the market for decades. And in 2026, one stock that’s started getting more institutional attention is Judges Scientific (LSE:JDG).

As a quick introduction, the company’s a scientific instruments group that acquires small, niche businesses making highly specialised laboratory and measurement equipment. Its customers include universities, research institutions, and industrial manufacturers worldwide.

So can it generate a 10% long term annualised return?

There are never any guarantees when it comes to investing. However, Judges Scientific’s bolt-on, acquisition-led growth model is a proven one used by many successful compounders.

By acquiring small founder-owned businesses at sensible prices, keeping existing management in place, and letting the earnings compound quietly over time, the results can be tremendous. And shareholders have already seen a glimpse of the returns this can generate.

Fun fact: over the last decade, Judges Scientific shares have averaged a 12.3% annualised return. The question now is, can it continue?

What could go wrong?

Despite its impressive track record, the company has had its fair share of challenges lately.

The freezing of US federal research grants due to budgetary concerns has taken a toll on organic growth. And with inflationary pressures on specialised components and skilled engineering talent driving up costs, margins have also gotten squeezed.

While frustrating, this does appear to be a temporary problem. But it nonetheless reveals a previously unexpected cyclical element and political sensitivity of Judges Scientific’s cash flows.

Fortunately, some early signs of improvement are emerging in 2026. However, even if the company gets back on track as expected, it nonetheless still needs to find new quality bolt-on acquisition targets to maintain long-term growth. And overpaying for acquisitions that underperform could meaningfully dent long-term returns.

Nevertheless, in my opinion, Judges Scientific offers a genuinely compelling case. It has a proven business model, a fragmented market full of acquisition targets, and a management team seemingly successfully steering the company through a difficult period.

So for a 40-year-old building an ISA to replace or piggyback the State Pension, this looks like the kind of quiet compounder that’s worth considering.

Should you invest £5,000 in Judges Scientific Plc right now?

When investing expert Mark Rogers and his team have a stock tip, it can pay to listen. After all, the flagship Twelfth Magpie Share Advisor newsletter he has run for nearly a decade has provided thousands of paying members with top stock recommendations from the UK and US markets.

And right now, Mark thinks there are 6 standout stocks that investors should consider buying. Want to see if Judges Scientific Plc made the list?

Zaven Boyrazian does not hold any positions in the companies mentioned.