UK ISA Calculator (Cash & Stocks and Shares)

ISA Guide

What should I know about ISA Allowance 2026/27?

The annual ISA allowance is £20,000 per person. You can split this between a cash ISA, stocks and shares ISA, innovative finance ISA, and Lifetime ISA (capped at £4,000). Unused allowance cannot be carried over to the next tax year. Couples can invest £40,000 per year combined — one of the most efficient tax shelters available to UK residents.

What's the difference between Cash ISA and Stocks and Shares ISA?

Cash ISAs offer certainty — your capital is protected and you earn a fixed or variable interest rate. Stocks and shares ISAs invest in the market — historically returning 7–10% annually over long periods, but with year-to-year volatility. For time horizons under 5 years, cash is generally safer. For 10+ years, the historical evidence strongly favours equities despite short-term risk.

What's the key thing to understand about The Lifetime ISA (LISA)?

The Lifetime ISA lets you save up to £4,000 per year and receive a 25% government bonus (up to £1,000 per year). It can only be used to buy your first home (under £450,000) or for retirement from age 60+. Withdrawing for any other reason incurs a 25% penalty — which effectively means losing your bonus plus some of your own contributions.

What's the key thing to understand about Power of ISA Compounding?

Because ISA gains are never taxed, the compounding effect is significantly greater than a taxable account over long periods. A basic rate taxpayer investing £10,000 per year at 7% for 30 years: in an ISA — approximately £944,000. In a taxable account — approximately £755,000 (assuming gains are taxed each year). The difference grows with time and is even larger for higher rate taxpayers.

What's the key thing to understand about Flexible ISAs?

Not all ISAs are 'flexible', but many cash ISAs and some stocks and shares ISAs now are. In a flexible ISA, money you withdraw during the tax year can be replaced later in the same tax year without it counting again against your annual allowance — provided the replacement happens before the tax year ends. For example, in a flexible ISA, if you've paid in your full annual allowance and then withdraw £2,000 for an emergency, you can pay that £2,000 back in later in the same tax year without breaching the limit; in a non-flexible ISA, that replacement would count as a brand new contribution against an allowance you'd already used up, potentially triggering an excess-contribution issue. This feature makes flexible ISAs useful as an emergency fund location without permanently sacrificing tax-free allowance space, but it's provider-specific — not every ISA offers it, and the flexibility resets at the start of each new tax year, so money withdrawn one tax year cannot be replaced in a later one without it counting as a fresh contribution.

What should I know about Transferring Between ISA Providers?

You can move an existing ISA from one provider to another — for a better interest rate, wider fund choice, or lower platform fee — without it affecting your current year's allowance or being treated as a withdrawal, but only if done correctly through an official ISA transfer process. Withdrawing the money yourself and depositing it into a new ISA counts as a withdrawal and a brand new contribution, immediately losing the tax-free wrapper on that money for CGT and income tax purposes and using up allowance space unnecessarily. Instead, request the transfer directly through the new provider, who will handle moving the funds (and, for cash ISAs, this is generally required to complete within 15 working days). You can transfer a current tax year's ISA in full or in part, and you can transfer previous tax years' ISA savings freely between providers with no limit, as long as the transfer process — not a manual withdrawal and redeposit — is used throughout.

Why Tax-Free Growth Compounds Into a Bigger Advantage the Longer Money Stays Invested

The core benefit of an ISA is that all growth, interest, and dividends within the wrapper are entirely free of UK income tax and Capital Gains Tax, for as long as the money remains inside the ISA. In the early years, this tax-free status saves a relatively modest amount compared to a taxable account, simply because the balance and the annual growth on it are both still small. But because the tax saved each year would otherwise have been lost and unable to compound, the cumulative advantage of an ISA over an equivalent taxable account grows disproportionately larger the longer the money stays invested — which is why ISAs are often described as most powerful as a long-term, buy-and-hold vehicle rather than a short-term trading account.

Cash ISA vs. Stocks and Shares ISA — Matching the Wrapper to the Time Horizon

A Cash ISA functions much like a regular savings account but shelters the interest from tax, making it most useful for money that needs to stay accessible and stable in value — an emergency fund, or savings earmarked for a near-term goal where capital preservation matters more than growth. A Stocks and Shares ISA invests the money in the market, offering higher long-term growth potential at the cost of short-term volatility, making it generally better suited to money with a longer time horizon — commonly cited as five years or more — where there's time to ride out market fluctuations. Using the same annual ISA allowance across both types according to each pound's own time horizon, rather than defaulting everything into one or the other, lets savers match the right wrapper to the right purpose.

Cash ISA vs. Stocks & Shares ISA vs. LISA vs. Innovative Finance ISA

All four ISA types share the same tax-free wrapper and sit within the same combined £20,000 annual allowance, but they differ sharply in what they hold, how accessible the money is, and who they suit.

Many savers use more than one type in the same tax year, splitting the £20,000 allowance between, say, a Cash ISA for an emergency fund and a Stocks & Shares ISA for long-term growth — the only ISA-specific restriction on combining types is the £4,000 LISA sub-limit, which counts toward, not on top of, the overall £20,000 allowance.

Worked Example: Growth Inside an ISA vs. a Taxable Account Over Time

Assume £10,000 invested every year into a diversified portfolio growing at 7% a year before tax. Inside an ISA, none of that growth is ever taxed, so the full 7% compounds. In an equivalent taxable account, ongoing dividend tax and periodic Capital Gains Tax on rebalancing or eventual sale typically create a 'tax drag' that often works out to roughly 1 percentage point of effective annual return lost for a basic-rate taxpayer with a reasonably tax-inefficient portfolio — reducing the effective growth rate to around 5.9% in this simplified comparison.

The advantage looks modest after five years — only around £1,200 — but balloons to nearly £168,000 after thirty years, because the tax saved each year compounds on top of itself for every year that follows, rather than being a one-off saving. This is the core reason ISAs are usually described as a long-term compounding tool rather than a short-term tax dodge: the real value only shows up fully after many years of uninterrupted, untaxed growth.