Precis

Compound Interest / ISA Growth

A lump sum plus regular monthly contributions, compounded monthly. The 2026/27 ISA allowance is £20,000/year, tax-free.

  • 2026/27 tax year
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Contributing within the £20,000/year ISA allowance.
Projected value
£0
after 15 years
Total contributed£0
Growth earned£0
Projected value£0

How does compound interest work in an ISA?

Compound interest means you earn growth not just on what you put in, but on the growth itself — the earlier you start, the more time that compounding has to work. An ISA (Individual Savings Account) simply wraps your savings or investments so any growth is tax-free, up to an annual allowance of £20,000 for 2026/27. This calculator projects a starting lump sum plus regular monthly contributions forward at a chosen growth rate, compounded monthly, so you can see roughly where you'd land in 5, 10 or 20 years.

Frequently asked questions

What is the ISA allowance for 2026/27?

£20,000 per tax year, across all your ISAs combined. Contributions beyond that sit outside the tax-free wrapper.

What return should I assume for my ISA?

It depends entirely on what you're invested in — cash ISAs currently pay a few percent, while a stocks & shares ISA might average higher over the long run but with more risk. This calculator lets you set your own assumption.

Is compound interest calculated monthly or annually?

This calculator compounds monthly, which is standard for savings and investment accounts that add interest or growth on a regular schedule rather than once a year.

Related searches

Lifetime ISA (LISA)Your 25% government bonus, up to £1,000 a year, projected forward. Rule of 72How many years until your money — or your debt — doubles. ISA CalculatorTax-free growth projection within the £20,000 allowance.