Finance & Business
ISA Calculator UK
Project your ISA growth over time. Works for Cash ISA, Stocks & Shares ISA, and Lifetime ISA. All growth is completely tax-free.
Enter your ISA details to project your tax-free growth over time.
Related to ISA Calculator UK
An Individual Savings Account (ISA) is a UK government-approved savings wrapper that shelters all growth, interest, and income from income tax and Capital Gains Tax — permanently, not just temporarily. This calculator projects the future value of your ISA using the standard compound growth formula, accounting for your starting balance, annual contributions, expected growth rate, and chosen time horizon.
The annual ISA allowance for 2025/26 is £20,000 per person. This is the maximum you can pay into all your ISAs in a single tax year — it is a use-it-or-lose-it allowance that resets each 6 April and cannot be carried forward. The compound growth formula used here is:
FV = P × (1 + r)ⁿ + C × ((1 + r)ⁿ − 1) / r
Where FV is the future value, P is the current balance, r is the annual growth rate as a decimal, n is the number of years, and C is the annual contribution. This formula assumes contributions are made at the start of each year and that returns compound annually. In practice, Stocks & Shares ISAs compound more frequently and Cash ISAs typically credit interest monthly, so actual results may differ slightly.
Lifetime ISA Bonus
The Lifetime ISA (LISA) has a maximum annual contribution of £4,000, which counts towards your overall £20,000 allowance. The government adds a 25% bonus on everything you contribute, up to a maximum bonus of £1,000 per tax year. In this calculator, when you select Lifetime ISA, your annual contribution is multiplied by 1.25 to reflect the bonus being added — so a £4,000 contribution effectively becomes £5,000 inside the LISA each year. Over 20 years of maximum contributions, the government would add £20,000 in bonuses on top of your £80,000 of personal contributions. The LISA can only be used to buy a first home worth up to £450,000 or to fund retirement from age 60.
The key difference between a Cash ISA and a Stocks & Shares ISA is the nature of the growth rate. A Cash ISA offers a fixed or variable interest rate — typically 4–5% in 2025 given current Bank Rate levels — which is guaranteed and protected by the Financial Services Compensation Scheme (FSCS) up to £85,000 per institution. A Stocks & Shares ISA invests in equities, bonds, or funds, with historical long-run returns averaging 5–7% per year for diversified global equity funds, but with significant year-to-year volatility. The tax-free status amplifies the benefit over time: an investment returning 5% per year outside an ISA at the higher-rate income tax band on dividends and CGT would compound far more slowly than the same investment inside an ISA where all returns reinvest in full.
The growth chart shows two lines: the solid blue line represents the projected ISA value including compound growth or interest, while the dashed green line shows what the same contributions would look like with no investment return at all. The widening gap between these two lines over time is the visual representation of compounding — the longer the investment horizon, the more dramatic the divergence. For a 20-year projection at 5% growth with £5,000 per year, that gap typically represents tens of thousands of pounds of tax-free gain.
For a Cash ISA with a 4.5% rate, the projection is relatively predictable and the growth line rises steadily. For a Stocks & Shares ISA with a 6% assumed return, actual year-by-year performance will be volatile — markets may fall significantly in some years — but the long-run historical data supports the 5–7% range for globally diversified equity portfolios before fees. Always apply a modest discount for ongoing annual charges (typically 0.15–0.75% per year for index trackers or active funds), which this calculator does not deduct automatically.
ISA Allowance Reset Date: 6 April each tax year
The ISA allowance resets at the start of each new tax year on 6 April. Any unused allowance from the previous year is permanently lost — it cannot be carried forward. To maximise your ISA value, contributing as early as possible in the tax year (ideally on 6 April) gives your money the maximum time to compound within the tax-free wrapper. This is sometimes called "ISA season" and is when many providers offer promotional rates or cashback incentives for early contributions.
Comparing an ISA to a pension is a common question. A pension benefits from upfront tax relief — basic-rate taxpayers get 20% relief, higher-rate taxpayers get 40% — making it more tax-efficient for contributions. However, pension withdrawals are taxed as income (except for the 25% tax-free lump sum), whereas ISA withdrawals are completely tax-free at any time with no restrictions. The LISA bridges this gap by offering upfront government bonuses like a pension, while retaining the tax-free withdrawal benefit of an ISA — but only for qualifying purposes (first home purchase or retirement).
For the Lifetime ISA, the 25% withdrawal penalty for unauthorised purposes is important to understand: if you withdraw money from a LISA for any reason other than buying a qualifying first home (up to £450,000) or at age 60+, you lose not only the government bonus but also a portion of your own contributions, as the penalty is calculated on the total pot including growth. This makes the LISA unsuitable as a general emergency fund and it should only be opened if you are confident in using it for its intended purpose.
1. What is the ISA annual allowance for 2025/26?
The ISA annual allowance for 2025/26 is £20,000 per person. This is the total you can pay into all your ISAs in a single tax year — so if you have both a Cash ISA and a Stocks & Shares ISA, your combined contributions across both cannot exceed £20,000. The allowance has been £20,000 since 2017/18. Junior ISAs (JISAs) have a separate, lower allowance of £9,000 per year for children under 18. The Lifetime ISA sub-limit is £4,000 per year, which counts as part of the overall £20,000 limit. There is no minimum — you can contribute any amount from £1 upwards (subject to individual provider minimums). Unused ISA allowance cannot be carried forward to the next tax year; it expires on 5 April each year.
2. Which type of ISA gives the best return?
Over long time horizons (10 years or more), Stocks & Shares ISAs have historically delivered higher returns than Cash ISAs, with global equity markets returning approximately 7–10% per year in nominal terms before fees over the past century, compared to Cash ISA rates that rarely exceed the Bank of England base rate for extended periods. However, Stocks & Shares ISA returns are variable and can be negative in any given year — your capital is at risk and there is no guarantee of a positive return. Cash ISA returns are lower but predictable and FSCS-protected. For money you might need within five years, a Cash ISA is generally safer. The Lifetime ISA offers the best effective return for qualifying uses because the 25% government bonus is equivalent to a 25% instant return on your contribution — no investment strategy can reliably match that, making the LISA the most mathematically attractive ISA for eligible first-time buyers and retirement savers.
3. Can I have more than one ISA?
Yes — from 6 April 2024, the rules were relaxed to allow you to open and contribute to multiple ISAs of the same type in the same tax year. Previously you could only open one ISA of each type per year. So in 2025/26 you could contribute to two different Cash ISAs with different providers, as well as a Stocks & Shares ISA, as long as your total contributions across all of them do not exceed £20,000. You can also hold ISAs from previous years with different providers indefinitely — there is no requirement to consolidate. Some providers offer a flexible ISA, which allows you to withdraw money and replace it in the same tax year without it counting towards your annual allowance again — this is a significant advantage for managing cash flow.
4. What happens to my ISA when I die?
When an ISA holder dies, the ISA continues to earn interest or investment returns tax-free until it is either closed, transferred, or three years have passed — whichever comes first. This continuing ISA is called an "Additional Permitted Subscription" (APS) or a continuing account managed by the estate. The deceased's surviving spouse or civil partner is entitled to an APS allowance equal to the value of the ISA at the date of death, allowing them to subscribe this amount to their own ISA in addition to their normal £20,000 annual allowance. This effectively means the ISA wrapper is preserved for the surviving spouse. However, the ISA does not pass automatically outside of the estate for Inheritance Tax purposes — the value of the ISA is included in the estate and subject to IHT if the estate exceeds the available nil-rate band and residence nil-rate band.
5. What is the scientific source for this calculator?
This calculator applies the standard future value of an annuity formula as set out in financial mathematics and used in actuarial practice. The ISA rules — including the £20,000 annual allowance, the Lifetime ISA £4,000 sub-limit, and the 25% government bonus — are drawn from the Individual Savings Account Regulations 1998 (SI 1998/1870) as amended, most recently by the Individual Savings Account (Amendment No. 2) Regulations 2023. The LISA withdrawal penalty rules are set out in the Individual Savings Account (Amendment No. 3) Regulations 2017. The Annual Exempt Amount for Junior ISAs is confirmed in HMRC's ISA technical guidance for 2025/26. Historical equity market return data is sourced from the Barclays Equity Gilt Study (2024 edition) and the Credit Suisse Global Investment Returns Yearbook, both of which show long-run real returns for UK and global equities in the range of 4–7% per year after inflation.