ISA

An Individual Savings Account (ISA) is a tax-free wrapper available to UK residents for saving or investing. Any interest, dividends, or capital growth earned inside it is completely free of Income Tax and Capital Gains Tax, no matter how large the balance grows.

There are several ISA types — Cash, Stocks and Shares, Innovative Finance, and Lifetime — and you can hold more than one, as long as your total new contributions across all of them stay within the annual allowance. Project your own growth with the UK ISA Calculator by entering a lump sum, monthly contribution, and expected return.

ISA growth compounds like any investment, just without a tax deduction applied to the result:

Lump sum growth: FV = P × (1 + r)^t

Monthly contribution growth: FV = PMT × (((1 + i)^n − 1) ÷ i) × (1 + i)

Where P is the initial lump sum, PMT is the monthly contribution, r is the annual return rate, i is the monthly rate (r ÷ 12), t is years, and n is the total number of months.

A £5,000 lump sum plus £500 a month at a 6% annual return over 10 years grows to roughly £91,300. Because it's inside an ISA wrapper, that entire figure is yours to keep — there's no further tax to subtract, unlike a general investment account where the growth portion would be liable for Capital Gains Tax.

• The allowance doesn't carry forward. Whatever you don't use by 5 April is gone for good, which is why many savers front-load contributions early in the tax year rather than leaving it to the deadline.
• Withdrawals are always tax-free, making an ISA fundamentally different from a pension, where tax relief comes on the way in but withdrawals are usually taxed later.
• A Lifetime ISA's 25% bonus counts towards the £20,000 limit, not on top of it, so maxing out a LISA at £4,000 leaves £16,000 of headroom for other ISA types in the same tax year.
• Pair ISA saving with a clear view of what you can actually afford monthly by checking PAYE and National Insurance deductions on the UK Take-Home Pay Calculator.
• If you're saving towards a house deposit inside an ISA, also check the Stamp Duty cost on the property you're targeting via the UK Stamp Duty Calculator — it's easy to forget this on top of the deposit itself.