ISA Calculator
See how a UK Stocks and Shares or Cash ISA grows tax-free within the yearly allowance.
About the ISA Calculator
Enter your ISA's current value, how much you'll add each year, how many years and the return you expect after fees. The calculator projects the value, capping contributions at the £20,000 allowance.
You can put up to £20,000 a tax year into ISAs (6 April to 5 April), split across Cash, Stocks and Shares, Innovative Finance and Lifetime ISAs (Lifetime max £4,000). Interest, dividends and gains are tax-free. From April 2027, the government plans to cap Cash ISA contributions at £12,000 for under-65s within the overall £20,000.
The formula
Each year: value × (1 + r) + contribution
Worked example
£15,000 today, adding £6,000 a year for 15 years at 5% after fees
- Paid in: £15,000 + 15 × £6,000 = £105,000
- Projected value: about £163,892
- All of the £58,892 of growth is tax-free
Questions people ask
Do unused allowances carry over?
No; each year's allowance is use-it-or-lose-it.
Can I withdraw?
Yes; flexible ISAs let you replace withdrawals in the same tax year.
ISA or pension?
Pensions get tax relief going in but are locked until 55 (57 from April 2028); ISAs are flexible. Many people use both.