Project tax-free growth for Cash ISA or Stocks and Shares ISA — with allowance tracker and yearly breakdown.
Calculate Your ISA Growth
Risk warning: Stocks and Shares ISAs invest in financial markets. The value of your investment can go down as well as up, and you may get back less than you put in. The growth rate you enter is an assumption only — past performance is not a reliable indicator of future returns. This calculator is for illustrative purposes and is not financial advice.
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Leave at 0 if starting from scratch
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Regular amount you will add each month
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Use the AER shown on your Cash ISA
yrs
Annual ISA contribution rate£0 / £20,000 allowance
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Final ISA Balance
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Growth Earned
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Total Contributed
ISA type
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Initial deposit
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Monthly contributions
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Total contributed
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Annual interest rate (AER)
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Investment period
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Total growth earned
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Tax-free wrapper: All interest, growth and dividends inside an ISA are completely free of Income Tax and Capital Gains Tax — no need to declare them on a self-assessment return. The annual allowance is £20,000 per person per tax year (2026/27). Unused allowance does not carry forward.
Year
Contributed
Growth
Cumulative Growth
End Balance
How the ISA Calculator Works
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 Growth Formula
The calculator uses the standard compound interest formula with monthly compounding:
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 Types at a Glance
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.
ISA Allowance Rules (2026/27)
The annual ISA allowance is £20,000 per person, running from 6 April 2026 to 5 April 2027. Key rules to know:
You can split the £20,000 across as many ISA types as you like
Since April 2024, you can contribute to multiple ISAs of the same type (e.g. two Cash ISAs) in the same tax year
Unused allowance is lost at the end of each tax year — it cannot be carried forward
Married couples and civil partners each have their own £20,000 allowance (£40,000 combined)
Withdrawals from a flexible ISA can be replaced in the same tax year without using additional allowance
Cash ISA vs Stocks and Shares ISA
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
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 free money per 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.
Why Starting Early Transforms ISA Outcomes
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.
Worked Examples
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.
Frequently Asked Questions
The ISA allowance for 2026/27 is £20,000 per person per tax year (6 April 2026 to 5 April 2027). This can be split across a Cash ISA, Stocks and Shares ISA, Innovative Finance ISA, and Lifetime ISA (capped at £4,000 within the £20,000 total). All interest, growth and dividends inside an ISA are completely free of Income Tax and Capital Gains Tax. Unused allowance is lost at the end of the tax year — it cannot be rolled over to the next year.
A Cash ISA earns tax-free interest on cash — it works like a standard savings account but the interest is always tax-free, regardless of how much you earn. A Stocks and Shares ISA invests in the stock market (funds, shares, bonds, investment trusts), with tax-free growth and dividends. Cash ISAs offer capital security but lower expected returns. S&S ISAs offer higher expected long-term returns but your capital is at risk. As a rule of thumb: use Cash ISA for money you may need within 5 years, and S&S ISA for longer-term goals where you can ride out short-term market falls.
A Lifetime ISA (LISA) is available to people aged 18–39. You can contribute up to £4,000 per tax year and the government adds a 25% bonus — up to £1,000 free per year. The £4,000 limit is a sub-limit within your overall £20,000 ISA allowance. LISA funds can only be used to buy a first home (worth up to £450,000) or accessed from age 60. Withdrawing for any other reason triggers a 25% HMRC penalty, which claws back the bonus and 6.25% of your own contributions. If you are saving for a first home or retirement, the LISA is one of the most generous government savings incentives available.
Yes — since April 2024 the rules changed to allow contributions to multiple ISAs of the same type in the same tax year. For example, you can hold two Cash ISAs with different providers and contribute to both in 2026/27. You can also hold a Cash ISA and a Stocks and Shares ISA simultaneously. The only constraint is that your total contributions across all ISAs must not exceed £20,000 in a single tax year. Each provider is responsible for tracking your contributions to them, and HMRC monitors compliance across providers.