Cash ISA vs Stocks & Shares ISA

Cash ISA vs Stocks & Shares ISA: what the numbers show

Both ISAs let you save or invest up to £20,000 per tax year without paying tax on interest, dividends, or capital gains. The difference is what's inside the wrapper — and that difference matters enormously over long periods.

What a Cash ISA actually is

A Cash ISA is a savings account inside a tax wrapper. Your money earns interest at a fixed or variable rate, and that interest is tax-free. The headline rate looks good when interest rates are high — but two things often get missed:

• The Personal Savings Allowance means most basic-rate taxpayers can earn £1,000 of interest tax-free anyway, outside any ISA.
• Cash savings rarely keep pace with inflation. Over the last 25 years, UK Cash ISA rates have averaged below the rate of inflation for long stretches — meaning your money lost real purchasing power.

What a Stocks & Shares ISA actually is

A Stocks & Shares ISA holds investments — typically funds, ETFs, or shares — inside the same tax wrapper. Any growth, dividends, and gains are tax-free. You take on market risk in exchange for the chance of much higher long-term returns.

Over the last 120 years, a globally diversified equity portfolio has returned around 5% per year above inflation. That's roughly 7-8% nominal. The trade-off: it doesn't deliver that return smoothly. You'll see years where it drops 20-40%. The history says it recovers and continues higher — but only for investors who stay invested.

When a Cash ISA is the right choice

• Your emergency fund (3-6 months of expenses).
• Money you need within the next 1-5 years (a house deposit, wedding, planned big purchase).
• You genuinely cannot tolerate seeing your balance drop, even temporarily.

When a Stocks & Shares ISA is the right choice

• Long-term goals (10+ years away — retirement, financial independence, children's future).
• Money you can leave alone through market downturns without panic-selling.
• Building wealth that grows faster than inflation erodes it.

The most common mistake

Keeping large sums in a Cash ISA for decades because it feels safe. In real terms — after inflation — that's a near-guaranteed slow loss. The calculator above lets you see this directly: toggle inflation-adjusted values on, and watch what the Cash ISA actually buys you in 25 years.

The flip side: putting money you'll need in two years into the stock market and then watching helplessly as a crash arrives in month 18. That's a different kind of loss. Match the wrapper to the time horizon.