ISA Savings Calculator
Calculate how much your ISA could be worth over time with regular contributions and compound growth — and see exactly how much is tax-free gains.
Projected ISA value
£79,288.15
- Total contributed
- £61,000.00
- Tax-free growth
- £18,288.15
- Growth as % of contributions
- 29.98%
Returns compound monthly. The 2025/26 ISA allowance is £20,000 per tax year. Projections assume a constant rate and do not account for inflation, platform charges, or tax on Lifetime ISA withdrawals.
How to use this calculator
Enter your initial deposit, your planned monthly contribution, the annual return rate you expect, and how many years you plan to invest. The results update as you type. For a Cash ISA use a rate of 4–5%; for a Stocks & Shares ISA a long-run average of 7–8% is commonly cited (though past performance is no guarantee).
How the calculation works
Uses the standard future value formula for an annuity with an upfront lump sum: FV = P·(1+r)ⁿ + PMT·((1+r)ⁿ − 1) / r, where P is the initial deposit, r is the monthly rate (annual rate ÷ 12), n is the total number of months, and PMT is the monthly contribution. Returns compound monthly.
Worked example
Starting with £1,000, contributing £500 per month at 5% annual return for 10 years: the initial £1,000 grows to £1,647; the £60,000 of monthly contributions compound to £77,641; total projected value = £79,288. Total contributed = £61,000, tax-free growth = £18,288.
Frequently asked questions
How much can I put in an ISA each tax year?
The ISA allowance for 2025/26 is £20,000 per person. You can split this across a Cash ISA, Stocks & Shares ISA, Innovative Finance ISA, and/or Lifetime ISA (limited to £4,000 of the £20,000). Unused allowance cannot be carried forward to the next tax year.
What annual return rate should I use?
For a Cash ISA, current easy-access rates are around 4–5% (May 2025). For a Stocks & Shares ISA, the FTSE All-World index has returned roughly 8–10% annually over the past 30 years before charges; after a typical 0.2–0.5% platform fee, 7–8% is a reasonable long-run assumption. These are not guarantees — equity returns vary significantly year to year.
Is growth inside an ISA really tax-free?
Yes. Any interest, dividends, and capital gains earned inside an ISA are entirely free of UK Income Tax and Capital Gains Tax. Withdrawals are also tax-free (except from a Lifetime ISA used for anything other than first home purchase or retirement, which incurs a 25% government withdrawal charge).
Does this calculator account for inflation?
No — the projected value is in today's money terms only if you subtract inflation from your return rate. To see real purchasing power, reduce the rate you enter by the expected inflation rate (e.g. enter 3% rather than 5% if you expect 2% inflation). The raw figure shown is the nominal future pound value.
Can I contribute more than £500 per month?
The calculator accepts any monthly figure, but HMRC limits total ISA contributions to £20,000 per tax year (roughly £1,666/month). If your monthly contribution times 12 exceeds this, you would need to split the excess into a General Investment Account (GIA), where growth above the annual CGT allowance is taxable.
What is the difference between a Cash ISA and a Stocks & Shares ISA?
A Cash ISA works like a savings account — your capital is protected up to £85,000 by the FSCS and you earn a fixed or variable interest rate. A Stocks & Shares ISA invests in equities, bonds, or funds — higher long-run return potential but your capital is at risk and can go down. Both shelter all returns from UK tax. This calculator models both using the same compound-interest formula; the difference is the rate you choose to enter.