ISA growth calculator, 2026/27

How this is calculated

The projection compounds your starting balance and monthly contribution at the annual growth rate you choose, applied monthly, over the number of years entered. It then checks your annual contribution against the current ISA allowance of £20,000, flagging any headroom remaining if you are contributing less than the maximum.

Because ISAs are entirely free of income tax and capital gains tax on growth, dividends and interest, there is no tax deduction to apply within the projection itself, unlike a pension where relief and eventual tax on withdrawal both need to be modelled.

This is a simplified compounding model. Real stocks and shares ISA returns depend on the specific investments held and will vary year to year rather than following a smooth flat line.

ISAs alongside company and pension planning

For a director who has already made efficient use of the personal allowance, dividend allowance and pension annual allowance, an ISA is often the next efficient home for further savings, since it removes the need to think about tax on withdrawal at all, unlike a pension.

Unlike a pension, ISA funds can generally be accessed at any time without a tax charge, which makes an ISA more flexible for medium-term goals, though pensions retain the advantage of tax relief on the way in, which an ISA does not offer.

Making the most of the allowance

The ISA allowance does not carry forward; unused allowance from a previous tax year is simply lost once the tax year ends on 5 April, so contributing steadily through the year, rather than trying to catch up all at once, is usually more practical.

Married couples and civil partners each have their own full ISA allowance, so a household can effectively shelter twice the individual limit in ISAs each year between them.

What is the ISA allowance for 2026/27?

The annual ISA allowance for 2026/27 is £20,000, which can be split between cash ISAs, stocks and shares ISAs, innovative finance ISAs and lifetime ISAs, subject to their own individual limits.

Does ISA growth need to be declared on my tax return?

No, income and gains within an ISA are entirely free of income tax and capital gains tax and do not need to be reported on a self-assessment return.

Can I contribute to an ISA as well as a pension?

Yes, they are separate allowances. Many directors use both: pension contributions for the up-front tax relief, and an ISA for tax-free growth with more flexible access before retirement age.

What happens if I do not use my full ISA allowance this year?

It is lost. The ISA allowance resets each tax year on 6 April and unused allowance cannot be carried forward to the following year, unlike the pension annual allowance which allows some carry forward.