How Much Could Your ISA Be Worth? A Plain-English Guide for UK Savers

What is an ISA, really?

ISA stands for Individual Savings Account. The short version: it's a wrapper that sits around your savings or investments and shields any interest, dividends, or capital gains inside it from UK tax. You don't pay income tax on the interest. You don't pay dividend tax. You don't pay capital gains tax when you sell. And — unlike a pension — you can take the money out whenever you like without penalty (with some exceptions below).

That tax-free status is what makes ISAs powerful. Over a long enough timeline, the tax you don't pay quietly compounds alongside your returns.

The four main types of ISA

There are several kinds of ISA, each designed for a slightly different goal.

Cash ISA. Works much like a regular savings account, but the interest is tax-free. Good for short-term savings and emergency funds where you don't want to risk your capital. Returns are typically modest and won't always beat inflation.

Stocks and Shares ISA. You invest in funds, shares, bonds, or ETFs through this wrapper. Returns aren't guaranteed and your balance can fall as well as rise, but historically equity markets have produced higher long-run returns than cash. Best suited to money you won't need for at least five years.

Lifetime ISA (LISA). Available to UK residents aged 18 to 39. You can pay in up to £4,000 per tax year, and the government adds a 25% bonus — up to £1,000 a year free. The catch: you can only withdraw the money to buy your first home (worth up to £450,000) or after age 60. Take it out for any other reason and you'll pay a 25% withdrawal penalty, which actually leaves you with less than you put in.

Innovative Finance ISA. Holds peer-to-peer loans and similar investments. Niche, higher-risk, and not the right starting point for most people.

You can pay into multiple types of ISA in the same tax year, as long as you don't exceed the overall annual allowance.

How much can you put in?

For the 2025/26 tax year, the total ISA allowance is £20,000 per person. That's the combined limit across all your ISAs — so if you put £15,000 into a Stocks and Shares ISA, you've got £5,000 left for any other type.

The Lifetime ISA has its own sub-limit of £4,000, which counts toward the overall £20,000. The Junior ISA (for under-18s) sits outside this with its own separate allowance.

Two things worth knowing: the allowance resets every 6 April and you can't carry unused allowance into the next year. If you don't use it, it's gone. Couples each get their own £20,000 allowance — a household with two adults can shelter £40,000 a year between them.

The thing most people underestimate: time

The single biggest driver of how much your ISA will be worth isn't the size of your contributions — it's how long the money has to grow. This is compounding, and it does most of the heavy lifting quietly in the background.

A quick illustration. Say you contribute £200 a month to a Stocks and Shares ISA and average a 5% annual return after costs:

• After 10 years, you'd have paid in £24,000 and your pot would be worth roughly £31,000.
• After 20 years, you'd have paid in £48,000 and your pot would be worth around £82,000.
• After 30 years, you'd have paid in £72,000 and your pot would be worth around £166,000.

Notice the pattern: doubling the time triples (and then doubles again) the end balance, even though your contributions only doubled. That's compounding. The growth on your growth eventually overtakes the contributions themselves.

These numbers aren't predictions — markets don't return a smooth 5% every year. But they show why starting earlier, even with smaller amounts, usually beats starting later with larger ones.

Cash ISA vs Stocks and Shares ISA: which one?

The honest answer is "it depends on your timeline."

If you'll need the money within the next two or three years — a house deposit, a wedding, a planned move — a Cash ISA is usually the better home. You won't get rich on the interest, but you also won't be forced to sell investments at a loss if markets dip just before you need the cash.

For genuinely long-term money — retirement, a child's future, or just building wealth over decades — a Stocks and Shares ISA has historically done better. UK and global equity markets have averaged around 5–7% real returns over long periods, well above what cash typically pays. The trade-off is volatility: your balance will fall in some years, sometimes sharply, and you need to be willing to ride that out.

Many people use both. Cash ISA for the short-term fund, Stocks and Shares ISA for the long-term pot.

Common mistakes to avoid

A few things trip people up.

Leaving the money in cash inside a Stocks and Shares ISA. Opening the account isn't the same as investing — your contributions sit as cash until you actually buy funds or shares. Check this if you've opened one recently.

Picking a LISA without realising the lock-in. The 25% bonus is genuinely generous, but the 25% withdrawal penalty for non-qualifying withdrawals means a LISA only really makes sense if you're confident the money is for a first home or retirement.

Chasing last year's best Cash ISA rate. Introductory rates often drop after 12 months. Set a reminder to review and switch if needed.

Forgetting about platform fees. Stocks and Shares ISA platforms charge different fees — usually a percentage of your pot, sometimes a flat fee. On a small pot, percentage fees are cheaper. On a large pot, flat fees usually win. Over decades, the difference can be thousands. Use our platform fee comparison to see how they stack up.

Waiting for "the right time" to invest. Markets are unpredictable in the short term but tend upward over the long term. Most evidence suggests that putting money in regularly — monthly, in fixed amounts — beats trying to time entry points.

How to work out what you'll actually have

The honest way to plan an ISA is to model your own situation rather than trust a generic figure. Three numbers matter most:

Monthly contribution. What can you realistically put in, every month, without it derailing the rest of your finances?

Expected return. For a Cash ISA, this is the interest rate. For a Stocks and Shares ISA, a sensible long-run assumption is 4–6% after inflation and costs, though any individual decade can come in well above or below that.

Time horizon. When do you actually need the money? Be honest — "as long as possible" tends to give the most useful answer.

Try your numbers with a few different return assumptions — 3%, 5%, 7% — to see the range of outcomes. That range is usually more useful than a single number, because it reflects the genuine uncertainty in long-term investing.

A few final things worth knowing

You can transfer ISAs between providers without losing your tax-free status — just use the provider's official ISA transfer process rather than withdrawing and re-depositing, which would count against your allowance.

ISAs pass to a surviving spouse or civil partner with an Additional Permitted Subscription, which effectively lets them inherit the tax-free wrapper. Worth knowing for couples.

Junior ISAs are owned by the child but locked until they turn 18. Even £50 a month from birth compounds into something substantial — use the JISA calculator to see the numbers.

The bottom line

An ISA is one of the simplest, most flexible tax breaks available to UK adults, and most people don't use it as well as they could. The two biggest levers you control are how much you contribute and how long you leave it alone. Get those broadly right and the tax savings, plus compounding, do most of the rest.

This article is for general information only and isn't personal financial advice. Tax rules can change and depend on your individual circumstances.