Stocks & Shares ISA Tax-Free Returns: How Much Can You Really Make?

What a Stocks & Shares ISA actually does

A Stocks & Shares ISA is a tax wrapper provided for in UK law (Individual Savings Accounts Regulations 1998). Inside it, you can hold:

• Listed shares (UK and overseas).
• Investment funds — OEICs, unit trusts, ETFs.
• Investment trusts.
• Corporate and government bonds.
• Cash held temporarily before investing.

Everything inside the wrapper escapes UK income tax on dividends and interest, and UK capital gains tax on profits when you sell. There is no reporting requirement, no upper lifetime cap, and no clawback when you withdraw.

The contribution allowance is £20,000 per tax year, shared across all ISAs you hold (Cash, Stocks & Shares, Innovative Finance, Lifetime ISA — the LISA has a separate £4,000 sub-limit).

What "tax-free returns" actually means in 2025/26 cash terms

The value of the ISA wrapper has gone up sharply over the last few years because of allowance cuts outside the ISA:

• Capital gains tax annual exemption cut from £12,300 (2022/23) to £3,000 (2024/25 onwards).
• Dividend allowance cut from £2,000 to £500 (2024/25 onwards).
• Personal allowance, basic-rate band, dividend rates and CGT rates are all frozen.

This matters because money held outside an ISA increasingly bumps into these shrinking allowances.

Outside the ISA, a higher-rate taxpayer with a £500,000 portfolio of global equities yielding ~1.5% in dividends would pay:

• Dividends: ~£7,500 — minus £500 allowance = £7,000 × 33.75% (higher-rate dividend rate) = £2,362/year tax.
• CGT on rebalancing 5% of the portfolio at a 7% paper gain: £1,750 of gain, minus £3,000 allowance = £0 charge — but only because gains are spread.

Over 20 years, repeated dividend tax and occasional CGT compound into very large numbers. Inside an ISA, the bill is always zero.

Worked example — Maxing the ISA for 20 years

Assume:

• Contribution: £20,000 per year, paid in equal monthly instalments.
• Nominal return: 7% per year (a mainstream long-run global equity assumption — not guaranteed).
• Fees: ~0.3% per year (typical for a low-cost global tracker on a cheap platform).
• 20-year horizon: 2026/27 to 2045/46.

Approximate values at the end of each 5-year period (rounded to the nearest £1,000):

So roughly £500,000 of growth on top of £400,000 contributed. All of it shielded from CGT and dividend tax.

For a higher-rate taxpayer holding the same portfolio in a General Investment Account (GIA), conservative back-of-envelope tax saved over 20 years:

• Dividend tax (1.5% yield × growing pot × 33.75%): roughly £35,000–£45,000 depending on assumptions.
• CGT on accumulated gains realised at retirement: £500,000 of gain, allowance assumed frozen at £3,000, basic-rate marginal CGT 18% (or higher-rate 24%): potentially £75,000–£110,000.

The wrapper saves a typical investor somewhere between £100,000 and £150,000 in tax over a 20-year accumulation. That dwarfs almost any other lever in retail investing.

What can go wrong

The tax wrapper does nothing about market risk. Realistic ranges around the 7% expected return over 20 years (based on historical equity volatility, ~16% standard deviation):

• Optimistic (top quartile): ending pot ~£1.4m+.
• Middle: ~£900k as in the table above.
• Pessimistic (bottom quartile): ~£600k.
• Severely bad sequence (e.g. 1929-style crash early on): under £450k.

Two key risks to plan around:

1. Sequence risk — a big crash in year 1 or 2 is much less damaging than one in year 18 or 19. The pot has time to recover.
2. Behaviour risk — selling out in a bear market locks in losses. Most retail investors underperform the funds they hold by 1–2% a year because of trading at the wrong time. The wrapper helps by making rebalancing tax-free.

Stocks & Shares ISA vs Cash ISA — when each makes sense

Short-term money belongs in cash because equity drawdowns can take 5+ years to recover. Long-term money belongs in equities because inflation will erode cash value. See our full comparison: Cash ISA vs Stocks & Shares ISA 2026.

Stocks & Shares ISA vs Pension

Both are tax-advantaged. Both have annual limits. They have different shapes:

• ISA: Contributions from post-tax money. Withdrawals tax-free, any time, any age.
• Pension: Contributions get tax relief at marginal rate (effectively pre-tax). Locked until age 55 (rising to 57 in April 2028). 25% tax-free on withdrawal, rest taxed as income.

For higher-rate taxpayers planning to be basic-rate in retirement, pensions usually beat ISAs on pure tax efficiency. For basic-rate taxpayers, an ISA is often as good or better because the marginal rate of relief now equals the marginal rate paid in retirement. Most people should use both — pension for the long-term tax arbitrage, ISA for accessible mid-life money. See SIPP vs Workplace Pension for the pension side.

Practical tips for 2025/26

• Open and contribute early. Funding on 6 April rather than 5 April gives you a full extra year of tax-free growth — over 30 years, roughly an extra year's contribution in value.
• Use direct debits. Pound-cost averaging removes timing decisions.
• Pick a low-cost global tracker. Vanguard FTSE Global All Cap, HSBC FTSE All-World Index, Fidelity Index World — all under 0.25% TER.
• Keep platform fees in check. Below ~£100k, percentage fees are usually cheaper. Above ~£200k, flat-fee platforms (e.g. Interactive Investor, iWeb) often win.
• Don't transfer your ISA by withdrawing. Always use the provider-to-provider transfer form — withdrawing cash and re-depositing into a new ISA wastes that year's allowance.
• Flexible ISA: some Stocks & Shares ISAs are "flexible" — you can withdraw and replace within the same tax year without losing the allowance. Check before withdrawing.

What happens on death

Since April 2018, an ISA stays in a special "continuing ISA" status until probate is granted (up to three years), preserving the tax wrapper during administration. Surviving spouses also inherit an Additional Permitted Subscription (APS) equal to the value of the deceased's ISA — effectively a one-off extra allowance, on top of the £20,000.

So a couple with maxed ISAs of £400,000 each can pass £400,000 to the surviving partner in an ISA wrapper plus give them a £400,000 APS — meaning the surviving partner can shelter the full £800,000 over time.

How big can your ISA really get?

For perspective, the highest reported ISA balances in HMRC's annual statistics now exceed £8 million. Those investors maxed allowances from PEP days (pre-1999), reinvested everything and rode the equity bull market. The mechanic still exists — anyone starting today and maxing for 40 years could realistically end with £3m+ in real terms.

The compounding is the magic. The wrapper just makes sure HMRC never breaks the compounding.

Try the calculator

To project your own ISA pot at your contribution rate and expected return:

For raw compound-growth modelling:

Sources

• HMRC: Individual Savings Accounts statistics
• HMRC: Individual Savings Account rules — gov.uk
• HM Treasury: Spring Budget 2023 — dividend allowance and CGT exemption cuts
• ONS: Wealth and Assets Survey