What compound interest actually is
Simple interest pays you a fixed amount on your original deposit and nothing more. Compound interest pays you interest on your interest. The difference sounds academic, but it's the single most important idea in personal finance, because it turns time into your biggest asset.
Here's the mechanism. Put £10,000 in at 6% and after year one you have £10,600. In year two you earn 6% not on £10,000 but on £10,600 — so you earn £636, not £600. That extra £36 is interest on last year's interest. It's tiny at first, but it grows every year, and eventually the growth-on-growth dwarfs the original deposit. Left for 30 years at 6%, that £10,000 becomes roughly £57,000 — and around £37,000 of that is compounding, not your money.
Two levers control the outcome more than anything else: time and rate. Starting ten years earlier often beats contributing more later, because those early pounds have the longest to compound. This is why “start now” is the most valuable — and most ignored — piece of investing advice.
The tax drag on a normal account
In a standard savings or investment account, HMRC takes a cut of your returns as you go, and that cut compounds against you. Three taxes apply:
- Income tax on interest. Savings interest above your Personal Savings Allowance (£1,000 for basic-rate taxpayers, £500 for higher-rate, £0 for additional-rate) is taxed at your income tax rate.
- Dividend tax. Dividends above the £500 dividend allowance are taxed — at 8.75% for basic-rate taxpayers and more above that.
- Capital gains tax. Profits when you sell, above the annual exempt amount of around £3,000, are taxed as a capital gain.
None of these are huge on a small pot, and a modest saver may stay within the allowances. But as your investments grow, the returns rise above the allowances and the tax starts to bite — and crucially, every pound taken in tax is a pound that can no longer compound. It's not a one-off cost; it's a leak that widens over time.
How an ISA changes the maths
An Individual Savings Account (ISA) is a wrapper you put savings or investments inside. Everything within it — interest, dividends and capital gains — grows completely free of UK tax, and you never have to declare it. There's no tax when it grows and no tax when you withdraw.
The one constraint is the annual allowance: you can pay in up to £20,000 per tax year (2026/27), across any mix of Cash ISAs, Stocks & Shares ISAs and other ISA types. The allowance resets each 6 April and can't be carried forward — an unused allowance is simply lost, which is why regular savers try to use as much of it as they can each year.
Because there's no annual tax leak, the compounding inside an ISA runs at full speed. Over one or two years the gap versus a taxable account is small. Over twenty or thirty, it becomes large — often tens of thousands of pounds on a serious pot — purely because the ISA let every pound of growth keep compounding instead of handing some to HMRC each year.
A worked example: ISA vs taxable over 20 years
Imagine two higher-rate taxpayers, each investing £10,000 and adding £200 a month for 20 years at a 6% annual return. One uses a Stocks & Shares ISA; the other uses a general taxable account and pays tax on dividends and gains along the way.
Illustrative — £10,000 + £200/mo, 6% for 20 years
| Total contributions | £58,000 |
| Value inside an ISA (tax-free) | ≈ £120,000 |
| Value in a taxable account (after tax drag) | ≈ £110,000 |
| Extra kept by using the ISA | ≈ £10,000 |
The exact figures depend on the return, your tax band and how much of your allowances you use — this is an illustration, not a promise. But the direction never changes: the tax-sheltered pot ends up ahead, and the longer the horizon, the wider the gap. Run your own numbers, with your own contributions and rate, in the compound interest calculator, which includes an ISA-vs-taxable comparison built in.
Cash ISA or Stocks & Shares ISA?
Both shelter your money from tax; the difference is what's inside. A Cash ISA holds savings at a set interest rate — safe, predictable, and sensible for money you'll need soon or can't afford to lose. A Stocks & Shares ISA holds investments that can rise and fall in value but have historically produced higher long-term returns. For a multi-decade goal like retirement, the higher expected return of investing usually matters more than short-term wobbles; for a house deposit you'll need in two years, the certainty of cash usually wins. Many people use both, for different goals.
Past performance is not a guide to the future, and the value of investments can fall as well as rise. An ISA doesn't change the risk of what's inside it — it only removes the tax.