UK Calculator Platform
ISA & Tax WrappersISA2026/27 Tax Year

Stocks & Shares ISA Growth Calculator

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Disclaimer: Tax and take-home pay estimates are based on published 2026/27 UK, Scottish and Welsh statutory rates and allowances. This is not tax advice, but an annual mathematical model. For personal tax planning or complex affairs, consult HMRC or a qualified tax adviser.

Growing a stocks and shares ISA

Everything inside an ISA is free of Income Tax and Capital Gains Tax, and none of it ever needs reporting. The allowance is annual and cannot be carried forward.

Figures stated for the 2026/27 UK tax year. Content last checked against official sources on .

What this calculator does

  • Projects a stocks and shares ISA forward from a starting balance and annual subscriptions.
  • Applies an annual account charge, so the projection is net of costs.
  • Shows how much of the £20,000 allowance the subscription uses.
  • Reports the allowance still available in the current tax year.

How the calculation works

The projection compounds the existing balance and each year's subscription forward at the return you enter, net of the account charge. What makes an ISA distinct is not the arithmetic — it is what does not happen. Dividends inside the wrapper are not taxed, gains are not subject to Capital Gains Tax, and nothing has to be declared on a tax return however large the account grows. That matters more over time than it does at first, because a small unwrapped portfolio may fall within the dividend allowance and the annual exempt amount anyway, while a large one will not. The £20,000 limit is per tax year and use-it-or-lose-it: unused allowance cannot be carried into the next year, which is why the calculator reports what is left. Charges work against you in exactly the way returns work for you, compounding on a growing balance, so a difference of a quarter of a percent is worth considerably more than it looks.

The rule

Each year: balance = (balance + annual subscription) × (1 + return − charge). Allowance used = subscription ÷ £20,000.

Step by step

  1. Start from the current ISA balance.
  2. Add the annual subscription.
  3. Compound at the expected return, net of the account charge.
  4. Repeat for each year of the horizon.
  5. Compare the subscription against the £20,000 annual allowance.

Worked example

Someone with £10,000 in a stocks and shares ISA subscribing £12,000 a year for twenty years, assuming 6% growth and a 0.25% account charge.

What was entered

Inputs used in the worked example
Starting ISA balance£10,000
Annual ISA contribution£12,000
Expected annual return6%
Annual account fee0.25%
Investment horizon20 years

The arithmetic

  1. £12,000 a year uses 60% of the £20,000 allowance, leaving £8,000 unused each year.
  2. Over twenty years £240,000 is subscribed, on top of the starting £10,000.
  3. Compounded at 6% less the 0.25% charge, the projected value is about £470,726.
  4. So roughly £220,726 of the final balance is investment growth.
  5. All of that growth is free of Capital Gains Tax and Income Tax, and none of it needs reporting.
  6. The £8,000 of allowance left unused each year cannot be carried forward — it is simply lost.

What the calculator returns

Results produced by the worked example
Projected ISA value£470,725.78
Allowance still available this year£8,000.00

Key assumptions

  • Returns are steady at the rate entered, with no volatility.
  • The full subscription is made every year for the whole horizon.
  • The account charge is applied annually to the balance.

Limitations

  • Real investment returns are not steady, and a twenty-year average conceals years that are very good and very bad.
  • Figures are in nominal terms unless the return you entered is already net of inflation.
  • Charges are frequently layered — a platform fee plus a fund charge plus transaction costs — so a single headline figure may understate the true drag.
  • The £20,000 limit is shared across all ISA types you subscribe to in the same tax year, not per account.
  • Investments can fall as well as rise, and an ISA wrapper protects against tax, not against loss.

Common questions

Can I carry unused allowance into next year?
No. The ISA allowance is strictly per tax year and any part you do not use is lost on 6 April. That is why people subscribe before the end of the tax year rather than waiting.
Do I have to declare ISA income on a tax return?
No. Income and gains inside an ISA are not taxable and are not reportable, however large the account becomes. That administrative simplicity is a real part of the value, separate from the tax saved.
Is the £20,000 limit per ISA or in total?
In total, across every ISA you subscribe to in the same tax year. Putting £12,000 into a stocks and shares ISA leaves £8,000 to share between any cash, innovative finance or Lifetime ISA subscriptions.
Does a 0.25% charge really matter?
More than it looks. It is charged on the whole balance each year, so its cash cost grows as the account grows, and it compounds against you for the entire horizon. Over twenty years the difference between 0.25% and 1% is a large sum.

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Official sources

Every figure in this guide was checked against the sources below. Where a source could not confirm a figure, it is marked as requiring verification rather than presented as settled.

  • Individual Savings Accounts (ISAs) — GOV.UK (2026 to 2027 tax year)Overall ISA subscription limit £20,000, shared across cash, stocks and shares, innovative finance and Lifetime ISAs
  • Capital Gains Tax: allowances — GOV.UKAnnual exempt amount £3,000 for individuals
  • Tax on dividends — GOV.UK (2026 to 2027)Dividend allowance £500; dividend rates 10.75% basic, 35.75% higher, 39.35% additional