How Compound Interest Works: Why Starting Early Beats Saving More

Save £200 a month from age 25 to 35, then stop and never add another penny, and by 65 you could have around £283,000. Start at 35 and pay in that same £200 a month for the full 30 years, and you'd reach about £245,000, despite putting in three times as much money. Both assume a 7% average annual return, and the difference between them is compound interest.

Last updated: 25 July 2026

Growth on growth

Compound interest is the return you earn on your returns. Put £1,000 in an account paying 5% and you have £1,050 after a year. In year two you earn 5% on the whole £1,050, not just your original £1,000, so you gain £52.50 instead of £50. The extra £2.50 is tiny. Left alone for decades, that same effect is what turns modest saving into a large sum.

The early years feel slow because there's little growth to compound yet. The later years do the heavy lifting, once the returns you've already earned are big enough to generate serious returns of their own. This is why the length of time your money stays invested matters more than almost anything else.

The two savers in the intro show it starkly. The early saver pays in £200 a month for just 10 years, from 25 to 35, a total of £24,000, then stops and leaves it untouched. By 35 they have about £35,000. Over the next 30 years they add nothing, yet at a 7% return that pot grows to roughly £283,000 on its own. The late saver pays in £200 a month for the full 30 years to 65, £72,000 in total, and ends with about £245,000. Three times the money in, a smaller result out, because it started 10 years later.

Figures assume a 7% average annual return compounded monthly, with no charges or tax. Real returns vary from year to year; these use a steady average to show the shape of the effect.

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What regular saving actually becomes

Here's what £200 a month grows into over 10, 20, and 30 years, at three different rates. The 3% column is close to a good Cash ISA today, 5% is a cautious view of a mixed investment fund, and 7% is a long-run average for global shares before charges.

Time / paid in	3% a year	5% a year	7% a year
10 years (£24,000 in)	£28,000	£31,200	£34,800
20 years (£48,000 in)	£65,800	£82,500	£104,800
30 years (£72,000 in)	£116,800	£167,100	£245,400

Monthly contribution of £200, no starting balance, compounded monthly, no charges or tax. Approximate 2026 return assumptions. Use the calculator for your own figures.

Look at the 30-year, 7% figure. You pay in £72,000 across three decades and end with about £245,400. That means roughly £173,000 of it, close to 71%, is growth you never contributed. In the early years your own payments make up most of the balance. By the end, the growth dwarfs them.

Why an ISA makes compounding work harder

Tax is the quiet drag on compounding. Any tax you pay on interest, dividends, or gains is money taken out of the pot, so it never compounds again. An ISA (Individual Savings Account, a wrapper that shields your savings and investments from tax) removes that drag. Growth inside it is free of income tax and capital gains tax, so the full amount keeps working year after year.

You can pay in up to £20,000 across your ISAs in the 2026/27 tax year. For most people saving a few hundred a month, that's more room than they'll use, so the practical rule is to hold long-term compounding money inside an ISA rather than a taxable account. The £20,000 ISA allowance guide covers how to split it, and the compound interest calculator lets you test a Cash ISA rate against a Stocks and Shares ISA rate side by side.

Two levers you control, two that work against you

You control two things: how much you put in and how long you leave it. Time is the more powerful of the two. Saving £250 a month at 7% for 30 years reaches about £307,000. Wait five years and run it for 25 years instead and you get about £204,000. That five-year delay costs roughly £103,000, far more than the £15,000 of contributions you skipped.

Two forces pull the other way, and both are easy to underestimate.

Charges. A fund or platform fee comes off your return every year, so it compounds against you exactly as growth compounds for you. A 1% annual charge on a 7% fund leaves you with 6%, and over 30 years that gap is worth tens of thousands. Keep an eye on the total yearly cost of any investment.
Inflation. Your pot grows in pounds, but each pound buys less over time. £10,000 left at 7% for 20 years becomes about £40,000 on paper, worth around £24,000 in today's money after 2.6% inflation. The same £10,000 in cash at 4% becomes about £22,000 on paper, worth only around £13,000 today. That real-terms gap is the core reason long-term money is often invested rather than left in cash.

If you're starting late or small

The early-starter example isn't a reason to give up if you're already 40 or 50. It's a reason to start now rather than next year, because the same logic that rewarded the 25-year-old rewards you against your future self. Twenty years of compounding still roughly doubles your contributions at 7%, and a later start with more money each month can close much of the gap.

Small amounts still compound

£50 a month is not too little to bother. At 7% over 30 years it grows to about £61,000, of which £43,000 is growth. Regular saving beats waiting until you can afford a large amount, because the months you skip are the ones with the longest to compound.

How much you can realistically put aside comes down to what's left after your bills. If you're working that out from scratch, the 50/30/20 budget rule is a sensible way to decide how much of your take-home pay to direct at savings. Once you have a monthly figure, put it into the compound interest calculator and change the years and the growth rate to see what it becomes.

Frequently asked questions

How does compound interest work?

You earn a return on your money, and then next year you earn a return on the original money plus the return you already made. Growth starts earning its own growth. Over a few years the effect is small. Over decades it becomes the largest part of your pot, which is why time matters more than the exact amount you save.

What will £200 a month grow into?

At a 7% average annual return, £200 a month becomes about £34,800 after 10 years, £104,800 after 20 years, and £245,400 after 30 years. You would have paid in £24,000, £48,000, and £72,000 respectively, so the rest is growth. Lower the rate to 5% or 3% for a more cautious projection.

Is it better to start early or save more later?

Starting early usually wins, and by a wide margin. Save £200 a month from 25 to 35 and then stop, and at 65 you could have around £283,000 at a 7% return. Someone who waits until 35 and saves the same £200 a month for a full 30 years reaches about £245,000, despite paying in three times as much. The early money simply had longer to compound.

Does compound interest work in a normal savings account?

Yes, but the rate is lower. A cash savings account or Cash ISA compounds the interest you earn, currently around 4% a year on the best easy-access deals. A globally invested Stocks and Shares ISA has historically returned more, roughly 5% to 7% a year before charges, with the trade-off that its value falls in some years.

How does tax affect compound growth?

Tax on interest, dividends, or gains is money that stops compounding, so it drags on the result year after year. Inside an ISA there is no income tax or capital gains tax on the growth, so the full amount keeps compounding. You can pay in up to £20,000 across your ISAs in 2026/27, which is why filling an ISA before a taxable account usually makes sense.

Is this different in Scotland?

The compounding maths is identical, and ISAs are UK-wide, so the £20,000 allowance and tax-free growth are the same in Scotland as in England. The one difference is that Scotland taxes income above £43,662 at 42%, a lower threshold than the rest of the UK. More Scottish savers hit higher-rate tax on savings interest held outside an ISA, which makes sheltering investments inside the ISA wrapper marginally more valuable.