Project tax-free growth for Cash ISA or Stocks and Shares ISA — with allowance tracker and yearly breakdown.
You can pay in up to £20,000 across all your ISAs in the 2026/27 tax year. Interest, dividends and capital gains inside an ISA are free of tax and need not be reported to HMRC. Unused allowance cannot be carried into the next year. The £20,000 can be split between a Cash ISA and a Stocks and Shares ISA; a Lifetime ISA is capped at £4,000 of that total and a Junior ISA has its own separate £9,000 limit.
| ISA type | — |
| Initial deposit | — |
| Monthly contributions | — |
| Total contributed | — |
| Annual interest rate (AER) | — |
| Investment period | — |
| Total growth earned | — |
| Year | Contributed | Growth | Cumulative Growth | End Balance |
|---|
This calculator projects how money grows inside an Individual Savings Account (ISA) using compound interest. The same mathematical formula applies to both Cash ISAs (interest) and Stocks and Shares ISAs (investment growth) — the difference is tax treatment and risk level, not the underlying projection method.
The calculator uses the standard compound interest formula with monthly compounding:
Balance = P × (1 + r/12)12t + PMT × [(1 + r/12)12t − 1] ÷ (r/12)
Where P is the initial deposit, r is the annual rate, t is years, and PMT is the monthly contribution. Contributions are treated as end-of-period payments. For Stocks and Shares ISAs, the rate represents an assumed annual growth rate — not a guaranteed return.
| ISA type | Tax treatment | 2026/27 limit | Risk | Best for |
|---|---|---|---|---|
| Cash ISA | Tax-free interest | £20,000* | Low | Short–medium term goals |
| Stocks & Shares ISA | Tax-free growth & dividends | £20,000* | Medium–high | Long-term wealth building (5+ yrs) |
| Lifetime ISA (LISA) | Tax-free + 25% bonus | £4,000 (sub-limit) | Low–high | First home or retirement |
| Junior ISA | Tax-free | £9,000 | Low–high | Children's savings (under 18) |
| Innovative Finance ISA | Tax-free interest | £20,000* | High | Peer-to-peer lending |
* Total contributions across all ISA types cannot exceed £20,000 per tax year.
The annual ISA allowance is £20,000 per person, running from 6 April 2026 to 5 April 2027. Key rules to know:
Cash ISA: Works like a standard savings account but sheltered from tax. Interest rates track the Bank of England base rate and are typically lower than investment returns over the long term. Suitable for emergency funds, short-term goals, or anyone who cannot tolerate investment risk. Current best rates in 2026 range from 4–5% AER.
Stocks and Shares ISA: Invests in the stock market, funds, bonds and investment trusts. Returns are variable — the market value rises and falls. The UK and global stock markets have historically returned around 7–8% per year on average over the long term (including reinvested dividends), but this is not guaranteed and individual years can show significant losses. Generally only suitable if you can commit funds for at least 5 years.
The Lifetime ISA (LISA) is a powerful option for eligible savers. You must be aged 18–39 to open one. You can contribute up to £4,000 per year and receive a 25% government bonus added automatically — up to £1,000 a year. The LISA allowance counts within your £20,000 overall ISA limit. Funds can only be used to purchase a first home (worth up to £450,000) or from age 60. Withdrawing early incurs a 25% HMRC withdrawal charge — this claws back the bonus and 6.25% of your own money.
Compound growth inside an ISA accelerates dramatically over time. £400/month in a Stocks and Shares ISA at 7% over 25 years produces roughly 5× more growth than over 10 years — not 2.5×. The ISA wrapper means none of that growth is eroded by tax, making it especially powerful for long-term wealth building. Maximising your allowance in early tax years delivers the greatest compounding benefit.
The same growth arithmetic works for a Junior ISA (JISA), with two differences. First, enter the number of years until the child turns 18, when the account becomes theirs. Second, the allowance bar above tracks the adult ISA allowance. A Junior ISA has its own, lower annual limit, shown in the ISA Types table above, so check that your monthly contribution × 12 stays within it.
For example, £100 a month is £1,200 a year. Over 18 years that is £21,600 paid in. Using the same rates as the worked examples below, a Cash JISA at 4.5% AER would reach about £33,187 (£11,587 interest), and a Stocks and Shares JISA at an assumed 7% a year would reach about £43,072. The 7% figure is an assumption, not a forecast: invested JISAs can fall in value.
If you are deciding between investing and holding cash, the stocks and shares ISA guide covers risk and charges, and the savings calculator projects growth outside an ISA for comparison.
On 23 June 2026 the government published its 2026 Tax Update and an ISA reform factsheet confirming a package of changes to adult ISAs. These changes take effect from 6 April 2027 and are subject to detailed regulations still to be laid. The current 2026/27 rules above remain unchanged until 5 April 2027 — this calculator continues to use those current rules.
The overall £20,000 ISA allowance is unchanged. From 6 April 2027 it is split as follows:
| Age | Maximum into a Cash ISA | Maximum into other ISA types | Overall allowance |
|---|---|---|---|
| Under 65 | £12,000 | Up to £8,000 (subject to LISA £4,000 sub-limit) | £20,000 |
| 65 or over | £20,000 | Up to £20,000 instead of cash | £20,000 |
The higher £20,000 Cash ISA limit applies from the start of the tax year in which the person turns 65, not from the birthday itself. For example, someone turning 65 in November 2027 would have the £20,000 Cash ISA limit available for the whole of 2027/28.
The remainder of the £20,000 allowance can be subscribed to a Stocks and Shares ISA, a Lifetime ISA (still capped at £4,000), an Innovative Finance ISA, or any combination, subject to the existing rules for each product.
From 6 April 2027 a flat 22% charge will apply to interest and alternative-finance returns paid on cash held within a Stocks and Shares ISA or an Innovative Finance ISA. Key points:
Money market funds may continue to be held in a Stocks and Shares ISA as part of a wider portfolio, but an account cannot consist entirely of money market funds.
No changes to the Lifetime ISA, Junior ISA or Personal Savings Allowance have been confirmed as part of this package. The Lifetime ISA continues at £4,000/year with a 25% government bonus, a £450,000 property-price cap and the existing 25% withdrawal penalty. The Junior ISA allowance remains £9,000.
Alongside the 2027 reforms, the government launched a consultation on a new First Time Buyer ISA. This product is proposed and not yet available. Final rules will only be known once the consultation concludes and legislation is laid. Nothing on this page should be read as suggesting the First Time Buyer ISA is available now.
UKCalc reads each source directly rather than relying on press summaries. This page is for information only and is not financial advice — speak to a regulated adviser for advice on your circumstances.
Last reviewed: 7 September 2026.
Three realistic UK ISA scenarios — projected with monthly compounding and end-of-period contributions.
Stocks and Shares ISA projections assume a fixed annual growth rate maintained throughout the period. Actual returns will fluctuate with market conditions. Cash ISA projections assume the stated interest rate is maintained — variable rates will change over time. Not financial advice.
ISA limits and the projection assumptions used here. Official figures last checked against GOV.UK on 7 September 2026.
Official figures this calculator applies
UKCalc assumptions — not official figures
Tax treatment. Income and gains inside an ISA are free of income tax, dividend tax and capital gains tax, and need not be reported. ISA tax rules can change, and their benefit depends on your circumstances.