Isa Vs Savings Account: What The Numbers Actually Say Over 20 Years

The question of where to put your money is one most people spend far less time on than it deserves. It feels complicated, it feels risky, and cash in a savings account feels safe.

But the numbers over a 20-year period tell a story that cash savings simply cannot match, and it is worth understanding why before making any decisions.

The following uses illustrative figures based on historical averages. Past performance is not a reliable guide to future returns, and individual results will vary. These examples are designed to make the comparison concrete, not to predict outcomes.

The savings account

Suppose you put £500 a month into a savings account for 20 years. At a consistent interest rate of 4% per year, which broadly reflects the kind of rate a good easy-access account might offer today, you would end up with approximately £183,000 after 20 years.

That assumes you reinvest the interest each year. In practice, savings rates move over time.

The 4% rate available today reflects a higher interest rate environment than the near-zero rates seen between 2009 and 2021. Over a 20-year period, an average rate closer to 2% to 2.5% is historically more realistic for easy-access cash. At 2%, the same £500 a month would leave you with approximately £148,000.

The Stocks and Shares ISA

Now apply the same £500 a month to a Stocks and Shares ISA invested in a broad global index fund (a fund that tracks the performance of thousands of companies across the world).

Over the past 20 years, global equity funds have historically delivered annualised returns of around 8% to 10% before charges, though there have been significant short-term falls along the way.

Using a conservative 7% to reflect charges and variability, £500 a month over 20 years would grow to approximately £305,000.

At the full 20-year mark, that represents a difference of more than £150,000 compared with a 2% savings account.

That difference is not fees, not financial advice, and not complexity. It is time, and the way that returns compound when they are reinvested and sheltered from tax.

Why the ISA wrapper matters so much

Inside an ISA, your returns are completely free of income tax and capital gains tax.

If your investments were held in a general investment account instead, you would potentially owe CGT on any gains above £3,000 when you sell, and dividend tax on any income above £500.

Both allowances have been cut substantially in recent years. The ISA removes all of that entirely.

A note on risk

A Stocks and Shares ISA is not the same as a savings account. The value of your investments can fall as well as rise. Over short periods, particularly one to three years, stock markets can drop sharply.

The case for investing rather than saving is strongest over long timeframes, typically five years or more. For money you might need to access quickly, a cash savings account or easy-access cash ISA is a more appropriate home.

What to do next

Use our Stocks and Shares ISA calculator to model different scenarios for your own situation

Compare the best Stocks and Shares ISA providers by fee and features

Find the best savings account rates currently available

If you are new to investing, our beginner guide walks through the basics without jargon