Maxing out a Stocks and Shares ISA every year for 20 years would build a substantial portfolio.

But the more important lesson for investors is that the same principle applies even to much smaller contributions. What matters most is consistency over time.

It’s not the starting amount that drives the outcome, but the combination of time and compounding.

So what could a consistently funded ISA actually become after 20 years?

Time in the market

The chart below shows the outcome of consistently investing into a Stocks and Shares ISA over 20 years, assuming different annual return profiles: 5% (balanced), 7% (equities), and 10% (strong equity markets).

In all three scenarios, the total amount invested is the same — £400,000 over 20 years. The difference in outcomes comes entirely from returns and compounding.

What stands out is how powerful that compounding effect becomes over time. At 7% and 10% returns, more than 50% of the final portfolio value is generated not from contributions, but from investment growth on previous gains.

Chart generated by author

That distinction matters. It shows that long-term outcomes are not driven purely by how much an investor can afford to contribute each year, but also by how effectively those contributions are allowed to compound.

For many investors, that is an important point of reassurance. Even if someone cannot max out their ISA allowance, consistent investing combined with time in the market can still do much of the heavy lifting.

Consistent performer

Halma  (LSE: HLMA) is a good example of how consistent compounding can quietly build long-term income.

At first glance, it doesn’t stand out as a typical income stock. The dividend yield is less than 1%, far below many of the high-yielding names in the FTSE 100.

But that misses the more important point. Over the long term, Halma has focused on steady growth in earnings and reinvestment across a decentralised group of niche businesses. That has supported both capital growth and rising dividends.

Over the past decade, total shareholder returns have increased by around 374%, while the dividend itself has compounded at roughly 7.2% annually. That combination is what turns a modest starting yield into a much more meaningful income stream over time.

What stands out is that this is not driven by a single cycle or one-off boost. Instead, it reflects a repeatable model of acquisition, reinvestment and organic growth across multiple specialist markets, from safety equipment to healthcare and infrastructure technology.

For ISA investors, that matters. It shows how income can build gradually without relying on chasing the highest starting yield.

The risk is valuation. Halma currently trades on a price-to-earnings ratio of around 40, which leaves little room for disappointment. If earnings growth slows or falls short of expectations, the shares could come under pressure.

Bottom line

Halma is not a high-yield income stock in the traditional sense. But that’s not really the point.

Whether an investor is contributing the maximum ISA allowance each year or building positions more gradually, the principle is the same: consistent investment into high-quality compounders can do much of the heavy lifting over time.

In that sense, even modest and regular ISA contributions can build towards a meaningful second income over time — provided the underlying investments are doing the compounding work.

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

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

Andrew Mackie does not hold any positions in the companies mentioned.