UK ISA Calculator 2026/27

How It Works

An Individual Savings Account (ISA) is a tax-free wrapper for saving and investing. Everything inside it — interest on a cash ISA, and dividends and capital gains on a stocks and shares ISA — is free of UK income tax, dividend tax and capital gains tax, and you never have to declare it on a tax return. The projected value comes from the standard compound growth formula with monthly compounding: your current balance grows at the assumed annual return divided by 12, and each month’s contribution is added at month-end and compounds for the remaining months. In symbols, the projected value is P × (1 + r/12)^12t + M × [((1 + r/12)^12t − 1) / (r/12)], where P is your current balance, M is your monthly contribution, r is the annual return and t is the years.

The annual allowance is the key rule. In the 2026/27 tax year (6 April 2026 to 5 April 2027) you can pay up to £20,000 into ISAs in total — one combined limit across all your ISAs, not £20,000 per account. You can split it between a cash ISA, a stocks and shares ISA, an innovative finance ISA and a Lifetime ISA however you like, and you can pay into more than one ISA of the same type in a year. The allowance resets every 6 April and unused allowance cannot be carried forward: whatever you do not use by 5 April is gone for good. This calculator shows how much of the £20,000 your monthly contribution uses each year, and flags it if your contributions would exceed the limit.

Cash ISA vs stocks and shares ISA is the main choice, and it comes down to time horizon and attitude to risk. A cash ISA works like a savings account: your money earns interest, your capital is safe, and the interest is tax-free. A stocks and shares ISA invests your money in funds, shares and bonds: the value can fall as well as rise, but over long periods investments have historically grown faster than cash interest. As a rough guide, money you need within five years belongs in cash; money you can leave for ten years or more can reasonably go into stocks and shares. This calculator works for both: for a cash ISA, enter the interest rate as the return; for a stocks and shares ISA, enter an assumed investment return and try a cautious rate as well as an optimistic one.

Two other ISA types are worth knowing about. A Lifetime ISA is for 18 to 39-year-olds saving for a first home or retirement: you can pay in up to £4,000 a year (inside the £20,000 allowance) and the government adds a 25% bonus, up to £1,000 a year — but withdrawals for any other reason before age 60 carry a 25% charge. A Junior ISA is for under-18s, with its own separate £9,000 annual allowance on top of your own £20,000. And outside ISAs, the Personal Savings Allowance lets basic-rate taxpayers earn £1,000 of savings interest tax-free each year (£500 for higher-rate taxpayers, nothing for additional-rate taxpayers) — which is why a cash ISA matters most once your savings earn more interest than the allowance covers.

Worked example: a £1,000 starting balance in a stocks and shares ISA, £250 a month, a 5% assumed annual return and 20 years. Your total contributions are £1,000 + (£250 × 240 months) = £61,000. With monthly compounding at 5%, the projected value is £105,471.06. The tax-free growth is £105,471.06 − £61,000 = £44,471.06 — about 42% of the final balance — and none of it is taxed. Your £3,000 annual contributions use only 15% of the £20,000 allowance.

The table shows the power of time: at 5%, £200 a month reaches £31,056.46 after 10 years but £166,451.73 after 30 years — more than five times as much from only three times the contributions. Every figure assumes monthly compounding with contributions at month-end and no fees.

Assumptions

• Constant return, no fees. One annual return applies for the whole period, compounded monthly, with no fund charges, platform fees or withdrawals. Real stocks and shares returns vary year to year and charges reduce the balance.
• Contributions stay flat. The monthly contribution does not rise with pay or inflation, and no lump sums are added later.
• No Lifetime ISA bonus. The 25% government bonus on Lifetime ISA contributions is not modelled — this calculator projects a standard cash or stocks and shares ISA.
• Allowance is the 2026/27 limit. The £20,000 annual allowance is assumed to stay the same in future years for the allowance check; in practice it is set each year by the government.

Sources: GOV.UK — Individual Savings Accounts (ISAs); GOV.UK — Tax on savings interest: how much is tax-free. Figures checked 13 November 2026.

How to Use This Calculator

Enter your current ISA balance — the total across the ISAs you are projecting — and the monthly contribution you plan to make. Then choose the ISA type: cash ISA if you want the interest-rate view, stocks and shares ISA if you want the investment view. Set the assumed annual return and the number of years, and the results update as you type: the projected value, what you will have contributed, the tax-free growth, and how much of your £20,000 annual allowance your contributions use.

Try the return at different levels before you trust any single figure. For a cash ISA, use the actual interest rate your provider pays. For a stocks and shares ISA, try 3%, 5% and 7% and look at the spread: on £200 a month over 20 years, the difference between 3% and 7% is nearly £38,525. If your monthly contribution would push your annual payments over £20,000, the calculator flags it — the excess cannot go into an ISA that year.

Tips and Practical Advice

Use the allowance before 5 April. The £20,000 allowance resets every 6 April and unused allowance is lost forever, so if you have savings sitting in a taxable account and allowance left, moving money into an ISA before the tax year ends shelters its future growth. You do not have to invest it immediately — you can hold cash inside a stocks and shares ISA while you decide.

Start with whatever you can afford. The table above shows £200 a month growing to £82,206.73 over 20 years at 5%, but even £50 a month reaches £20,551.68 over the same period — and starting early beats starting big later, because compounding rewards time more than it rewards size. A monthly direct debit on payday makes it automatic.

Match the ISA to the time horizon. Money you need within five years — a house deposit, a car, an emergency fund — is safer in a cash ISA, where the capital cannot fall. Money you will not touch for ten years or more can reasonably go into a stocks and shares ISA, where short-term falls have time to recover. Many people hold both: cash for the near term, investments for the long term.

Check whether a cash ISA even helps you, and do not forget the Lifetime ISA if you are under 40. The Personal Savings Allowance lets basic-rate taxpayers earn £1,000 of interest tax-free outside an ISA (£500 for higher-rate taxpayers), so if your total savings interest is below that, a cash ISA saves you no tax today. And the Lifetime ISA’s 25% government bonus — £1,000 a year on the maximum £4,000 contribution — is an instant return no investment can match, for a first home or retirement at 60.

Frequently Asked Questions