UK ISA Calculator
Finance & InvestmentProject your Individual Savings Account (ISA) growth in seconds. Enter your lump sum, monthly contribution, and return rate to see tax-free returns.
🇬🇧This tool is specific to United Kingdom
Final ISA Value
£91,304
Total Invested
£65,000
Tax-Free Growth
£26,304
Annual Allowance Used
30.00%
Breakdown
How the total splits
Total Invested
£65,000
Tax-Free Growth
£26,304
What is a UK ISA?
The UK ISA Calculator projects how your Individual Savings Account balance grows over time, combining any lump sum you start with, regular monthly contributions, and compound growth at a rate you choose. Because everything inside an ISA is shielded from Income Tax and Capital Gains Tax, the figures this tool shows you are the actual amount you'd keep — there's no tax deduction to subtract afterwards.
The £20,000 annual allowance is the ceiling most savers bump into, and it resets every tax year rather than accumulating. If you're deciding how to split money between an ISA and other tax-free accounts, it's worth checking the UK Take-Home Pay Calculator first to see what you can realistically commit monthly once tax and National Insurance are accounted for.
How to use this UK ISA calculator
- Enter your Initial Lump Sum if you're starting with existing savings — leave it at zero if you're beginning from scratch.
- Set your Monthly Contribution using the slider or input field.
- Adjust Expected Annual Return to match your ISA type — lower for cash, higher (with more uncertainty) for stocks and shares.
- Move the Investment Period slider to see how the projection changes over different timeframes, from a few years to several decades.
- Check the Final ISA Value result for your projected balance, and Annual Allowance Used to confirm you're within the £20,000 yearly limit.
Formula & Methodology
The calculator compounds two components separately and adds them together: Lump sum growth:FV = P × (1 + r)^t, where P is your initial lump sum, r is the annual return rate, and t is the number of years. Monthly contribution growth:FV = PMT × (((1 + i)^n − 1) ÷ i) × (1 + i), where PMT is the monthly contribution, i is the monthly rate (annual rate ÷ 12), and n is the total number of months (years × 12). The final(1 + i)factor assumes each contribution is invested at the start of the month. Worked example: a £5,000 lump sum plus £500 a month at a 6% annual return over 10 years grows to a lump sum value of roughly £8,954 and a contributions value of roughly £82,349, for a projected Final ISA Value around £91,304. Total Invested over that period is £65,000 (£5,000 + £500 × 120 months), meaning close to £26,300 of the final balance is tax-free growth.
Frequently Asked Questions
An Individual Savings Account (ISA) is a tax-free wrapper for your savings or investments, available to UK residents. Any growth, interest, or dividends earned inside it are completely free of Income Tax and Capital Gains Tax, no matter how large your balance grows over time.
The annual ISA allowance is £20,000 per tax year (2024/25), which you can split across different ISA types — cash, stocks and shares, innovative finance, or Lifetime ISA — in any combination you like. This calculator's Annual Allowance Used figure shows what percentage of that limit your monthly contributions account for.
A Cash ISA works like a tax-free savings account, paying interest with no growth risk but typically lower long-term returns. A Stocks and Shares ISA invests your money in funds or shares, which historically outpaces cash over long periods but can fall in value in the short term — this calculator's return rate input lets you model either scenario.
No. Withdrawals from an ISA are completely tax-free, unlike a pension where withdrawals are usually taxed as income. That's what makes an ISA different from a general investment account — the tax advantage applies both while your money grows and when you take it out.
Start by entering any lump sum you're investing now, then set your planned monthly contribution and your expected annual return. Adjust the Investment Period slider to see how the Final ISA Value, Total Invested, and Tax-Free Growth figures change over different timeframes.
Yes — because it invests in the market, its value can fall as well as rise, and there's no guarantee you'll get back what you put in. The return rate you enter into this calculator should reflect a realistic long-term average for your chosen investments, not a guaranteed outcome.
No, it doesn't. If you don't use your full £20,000 allowance by the end of the tax year on 5 April, the unused portion is lost — it can't be carried forward or backdated. That's why many savers front-load contributions early in the tax year rather than leaving it to the last few weeks.
A Lifetime ISA (LISA) is a specific ISA type for first-time home buyers or retirement saving, where the government adds a 25% bonus on contributions up to £4,000 a year. That £4,000 counts towards your overall £20,000 allowance rather than sitting on top of it, and this calculator doesn't model the LISA bonus specifically — it's built for general ISA growth projections.
The calculator compounds your lump sum and monthly contributions separately at your chosen annual rate, then adds them together. Monthly contributions are assumed to be invested at the start of each month, so each one gets the full month's growth before the next one is added.
Yes — you can hold ISAs from previous tax years with multiple providers, and since April 2024 you can also open and pay into multiple ISAs of the same type within a single tax year. The £20,000 limit applies to your total new contributions across all of them combined, not per account.
No, it's a projection based on the annual return rate you enter, not a guarantee. Actual returns depend on which ISA type and underlying investments you choose, and past performance is never a reliable predictor of future results — treat this as a planning tool, not a promise.
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