How to use the ISA Calculator
- 1 Enter your starting balance, if you already have an ISA.
- 2 Enter how much you will deposit each month.
- 3 Enter the interest rate for a cash ISA or an expected growth rate for a stocks and shares ISA.
- 4 Enter the number of years and read your final ISA value.
Worked example
£5,000 to start, £300 a month for 10 years at 4.5%. You pay in £3,600 each tax year, well within the £20,000 allowance, and £41,000 in total. With monthly compounding the ISA grows to about £52,907.55, including £11,907.55 of tax-free interest.
Features
Tax-free growth
All interest, dividends and gains in an ISA are free of UK tax.
Allowance check
Warns you if monthly deposits go over the £20,000 yearly limit.
Monthly compounding
Growth added each month for a realistic projection.
When to use it
Cash ISAs
See how much interest a fixed or easy-access ISA could earn.
Stocks and shares ISAs
Project long-term growth from regular investing.
Lifetime ISA planning
Compare saving in a LISA with a standard ISA.
Using the allowance
Work out the monthly amount that fills your allowance.
Frequently asked questions
What is the ISA allowance for 2026/27?
£20,000 a tax year, split however you like between cash, stocks and shares, innovative finance and Lifetime ISAs. Up to £4,000 can go into a Lifetime ISA.
Is ISA interest taxed?
No. Interest, dividends and capital gains inside an ISA are tax-free, and you do not need to report them to HMRC.
Can I have more than one ISA?
Yes. You can pay into several ISAs of different or the same type in a tax year, as long as the total stays within £20,000.
What happens to unused allowance?
It is lost at the end of the tax year on 5 April. It cannot be carried forward.
Is the cash ISA limit changing?
The government has announced that from April 2027 the amount under-65s can pay into cash ISAs will be capped at £12,000, with the overall £20,000 limit unchanged. This does not affect 2026/27.
What rate should I use?
Use the current rate for a cash ISA. For stocks and shares, many people assume 4% to 6% a year, but returns go up and down and are not guaranteed.
Can I transfer an ISA?
Yes. Use the new provider's transfer service so the money keeps its tax-free status. Withdrawing and re-depositing uses up new allowance unless the ISA is flexible.
Is my money protected?
Cash ISAs with UK-authorised banks and building societies are protected by the FSCS up to £120,000 per person per institution. Investments can fall in value, but the FSCS covers you if a provider fails.
Can I open an ISA for my child?
Yes. A Junior ISA has a separate allowance of £9,000 a year. The money belongs to the child and they can take control at 16 and withdraw at 18.
Results are estimates based on the official rates linked above. Confirm important figures with the issuing authority or a qualified adviser before you act on them.
Reviewed by ToolsUnlocked Team · Last updated