The two ISAs at a glance

A Cash ISA works like a tax-free savings account: your money earns interest and never loses value in nominal terms. A Stocks & Shares ISA holds investments that can grow more over the long term, but can also fall. Both are tax-free on returns and withdrawals — the difference is risk and expected return, not tax.

This example assumes you start with £1,000 and add £250 a month — a typical starting point. A Cash ISA earns a conservative 4%; a Stocks & Shares ISA is modelled at a long-term 7%.

The numbers: Cash ISA vs Stocks & Shares ISA

TimeTotal investedCash ISA at 4%Stocks & Shares ISA at 7%Difference
10 years£31,000£38,303£45,281£6,978
20 years£61,000£93,916£134,270£40,354
30 years£91,000£176,826£313,109£136,283

The headline: over 30 years, the same £250 a month could be worth £176,826 in a Cash ISA at 4%, but £313,109 in a Stocks & Shares ISA at 7% — a gap of about £136,283. Of the £313,109, only £91,000 is money you actually put in; the rest is growth.

The gap is small after 10 years (£6,978) and explodes after 20–30. That is compounding doing its work — and why the biggest input is not the rate, but how long you stay invested.

How to choose (a framework, not advice)

People commonly choose like this:

  • Short-term money — Cash ISA. If you will need the money within about 5 years (emergency fund, house deposit), a Cash ISA keeps it safe and accessible, though inflation still eats into real value.
  • Long-term money — Stocks & Shares ISA. If the money is for 10+ years (retirement, long savings), investing has historically offered higher expected growth — but the value will go up and down along the way, and past performance does not guarantee future results.
  • Many people do both. A cash buffer for the short term, and an invested ISA for the long term.

The right split depends on your time horizon, your risk tolerance and your goals. There is no single answer — but there is a clear trade-off between safety and long-term growth potential.

What changes from 6 April 2027

UK ISA rules are changing. From the 2027/28 tax year, the Cash ISA annual limit drops from £20,000 to £12,000 for investors under 65 (those 65 and over keep £20,000), under the ISA (Amendment) Regulations 2026. The overall £20,000 ISA allowance is unchanged — the new rule limits how much of it can go into a Cash ISA each year.

This matters for this comparison: if you prefer cash, you may need to spread it across other ISA types or plan within the lower cash limit. Always check the latest rules before the tax year ends.

Risks to keep in mind

These projections assume a constant annual return with monthly compounding and no fees. In reality:

  • Investment returns vary — a 7% average does not mean 7% every year, and markets can fall sharply.
  • Cash interest rates change and can lag inflation, so a Cash ISA's real (inflation-adjusted) value can still fall.
  • Neither example includes platform fees, which reduce returns.

Frequently asked questions

Is a Cash ISA safer than a Stocks & Shares ISA?
In nominal terms, yes — cash does not lose value, though inflation can reduce its purchasing power. A Stocks & Shares ISA can fall in value, especially over short periods, but has historically offered higher expected long-term returns.
Can I have both a Cash ISA and a Stocks & Shares ISA?
Yes. You can pay into one Cash ISA, one Stocks & Shares ISA, one Innovative Finance ISA and one Lifetime ISA in the same tax year, as long as total contributions stay within your £20,000 annual allowance.
Are Stocks & Shares ISA returns guaranteed?
No. Investments can fall as well as rise. The 7% figure used here is a hypothetical long-term assumption for illustration — it is not a forecast or a guarantee.
What happens to my ISA allowance if I don't use it?
It does not carry over. The allowance resets every 6 April, and from 2027 the Cash ISA portion is capped at £12,000 for investors under 65.
Educational tools only — not financial advice. Example figures assume constant returns, no fees and no inflation; investment values can fall as well as rise. ISA rules and rates can change — always verify against the latest official guidance.