Compound interest is the mechanism that quietly does most of the work in savings and investments. It is the reason people who start saving in their twenties usually have bigger pots than people who start in their forties, even when the later savers contribute more each month.
The maths is simple, but the effect over decades feels almost magical.
What compound interest actually means
If you put £1,000 in an account paying 5 per cent a year, you earn £50 in the first year and now have £1,050. The next year, the 5 per cent is earned on the new balance, not the original deposit. So year two earns £52.50, year three earns £55.13, and so on.
Over time, the interest earns interest, and the growth line starts to curve upward sharply.
Worked example: a regular saver
Imagine you save £200 a month for 30 years at 6 per cent a year — a reasonable long-term stock market return after costs.
- Total you pay in: £72,000.
- Final pot after 30 years: around £201,000.
- Pure interest earned: around £129,000.
Nearly two-thirds of the final pot comes from compounded growth, not your own contributions. Use our compound interest calculator to run your own numbers.
The power of starting early
Now compare two savers:
- Early saver: saves £200 a month from age 25 to 35, then stops. Leaves the pot to grow until age 65 at 6 per cent. Final pot: around £170,000.
- Late saver: starts at age 35 and saves £200 a month all the way to 65 at the same 6 per cent. Total contributions £72,000. Final pot: around £201,000.
The early saver pays in only £24,000 but still ends with nearly as much as the late saver who paid in three times as much. That is compound interest doing its job.
Where UK savers get this growth
- Cash ISA: tax-free interest at bank rates (usually 3 to 5 per cent in 2026).
- Stocks and Shares ISA: tax-free growth on investments, with long-term returns typically 5 to 7 per cent.
- Pension: tax relief on the way in and compound growth inside the wrapper — the most efficient long-term vehicle for most people.
- Premium Bonds: prize-based rather than compound, but some people prefer the structure.
What slows compound growth
- Fees: a 1 per cent annual fund fee can wipe out a quarter of the final pot over 30 years.
- Tax: savings interest above the Personal Savings Allowance and dividends above the £500 Dividend Allowance drag on returns outside an ISA or pension.
- Inflation: your £200,000 pot is less impressive in real terms if prices double in the same period.
- Gaps in saving: missing a few years of contributions in your twenties can cost tens of thousands by retirement.
The bottom line
Compound interest rewards consistency more than size. A modest amount saved regularly and left untouched for decades almost always beats a large amount saved late. If you are in your twenties or thirties, the single best move is often to start — even with small amounts — and let the maths do the rest.