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How compound interest makes you money

Jasmine Birtles 10th Mar 2024 3 Comments

Reading Time: 7 minutes

The trouble with maths class is they don’t teach you how compound interest can make you rich without you needing to lift a finger.

If our maths teacher had started with that as an opener, everyone would’ve paid attention!

Compound interest is the secret to people getting rich with even small investments. If you’re like us and didn’t pay attention in maths, don’t panic: here’s the lowdown on compound interest.


How compound interest makes your money grow

Simply put, investing involves turning money into more money – without you having to work at it. It’s making money while you sleep – nice concept!

How does this money turn into more money? Through compound returns – i.e. the miracle of compound interest.

It works by adding interest on your interest. When you’re in debt it’s your worst enemy, but when you’re investing it’s your best friend.

It’s like snowballs!

Go to the top of a mountain in Gstaad in the middle of winter, make a snowball in your hands and roll it down the hill. As the snowball rolls down it picks up snow as it goes. Every revolution is bigger than the one before. At the bottom you’ve got yourself a snowball the size of a hot air balloon. (It’s a really high mountain, okay?)

Compound interest works like a snowball. You put money into something that gives you interest or other returns each year and it grows by a greater amount annually.

Think of putting £100 into something that gives you a 10% return (wow) each year. After a year you’ll make £10. If you keep that extra in the fund at the end of the next year you’ll make £11 (10% of £110). At the end of the next year you’ll make £12.10 (10% of £121) and so on. Each year you make more than the last. Nice!


the golden rules…

1. The more you save and the longer you save for, the more you’ll end up with. You start off just getting interest, but then you earn interest on that interest and then you earn interest on the interest on the interest, and so on. Over a long time it really adds up.

2. Small differences in the interest rate can make a big difference in the long term so try to go for the biggest return possible on long-term investments. Also cut out tax where possible by using investments like ISAs and having a mixed portfolio..

Here’s how compound interest makes you money

Let’s say your granny decides to give you £1,000 on your birthday. (Well, she always was your favourite granny). You decide that instead of blowing it on a new wardrobe down at Gap, you’re going to invest it in a simple index-tracking ISA.
For the purposes of our example, let’s say your £1,000 appreciates at a rate of 12% a year (before taking account of inflation) – a rate we’d all love right now! After five years, the numbers should look like this:
• Year 1 – £1,120
• Year 2 – £1,254
• Year 3 – £1,405
• Year 4 – £1,574
• Year 5 – £1,762

So, without doing anything at all, you’ve just made yourself a profit of £762! If you’d spent your £1,000 down at Primark, where would your ‘investment’ be now?

Now let’s introduce you to Kylie, a young woman who, on her 20th birthday, decides to invest £100 a month into an index-tracking ISA. At the age of 30 she marries Wayne, stops work to have children and cancels her direct debit into her ISA, having contributed a total of £12,000. For argument’s sake, let’s say the interest rate stays 12% a year – although this would, in reality, fluctuate over time.
Wayne, meanwhile, who has frittered away his money and his twenties on pastimes too terrible to mention here, decides on his 30th birthday to start contributing the same £100 a month into the same scheme and continues until he is 60, meaning he has invested a total of £36,000. The numbers pan out like this:

                           Kylie              Wayne

  • Age 20    £0                       £0
  • Age 30    £22,403              £0
  • Age 40    £69,580              £22,403
  • Age 50    £216,105            £91,983
  • Age 60    £671,186            £308,088

Ouch! Extraordinary, isn’t it? Kylie only contributed for 10 years and yet she’s ended up with more than twice as much as her husband. Not only that, overall Kylie contributed a total of £12,000 while Wayne paid in £36,000 – three times as much for only half the return!

The lesson here is to start investing your money into a savings plan as early as you possibly can. You don’t need to save hundreds or thousands of pounds each year, either. Even £25 a month works out at £300 a year saved. If you paid in £25 a month for five years, on an interest rate of 5% you’ll have £1702 – not bad for an investment of £1500!

Even better, if you stopped paying into the fund, the interest would still build over time. Five years later, you’ll have £2172 for your initial investment of £1500. That’s a profit of £672 for doing absolutely nothing!


The first law of compound returns: start early!

Let’s look at yet another example. Assume a number of women at the age of 20. All appreciate the importance of long-term regular investment but disagree about the best method. For simplicity, we’ll assume that they have each chosen methods that return different annual growth rates and they each contribute £100 per month until they’re 60.

Let’s look at the numbers:

                    Fennella       Felicity     Ffyona

                           5%            8%              10%

  • Age 20    £0               £0              £0
  • Age 30    £15,499     £18,128      £20,146
  • Age 40    £40,746     £57,266      £72,399
  • Age 50    £81,870     £141,761    £207,929
  • Age 60    £148,856   £324,180    £559,461


                       Frances        Freda          Faith

                           12%            15%               20%

  • Age 20    £0               £0                  £0
  • Age 30    £22,404      £26,302         £34,431
  • Age 40    £91,986      £132,707       £247,619
  • Age 50    £308,097    £563,177       £1,567,625
  • Age 60    £979,307    £2,304,667    £9,740,753

Take a look at the bottom line for each woman. It speaks for itself, doesn’t it? Over long periods of time a difference of only one or two percentage points can have a huge impact.


the second law of compound returns is: small differences in the rate of return matter. A lot!

It’s obvious that the type of investment you put your money into has a big influence on the return on investment but there’s another thing that needs to be taken into consideration. Any ideas? No? Give up? Oh, all right then – charges!

Give yourself a pat on the back and treat yourself to a HobNob if you got it right.

Imagine Frances with her 12% growth rate is actually investing in a fund that has annual maintenance charges of 2%. She hasn’t thought about it much but, each time her investment fund takes its cut, her cash pile reduces. By the time she’s 60 she’ll only have the same as Ffyona – little more than half as much as she would have accumulated without the charges.

The same goes for initial charges. Imagine Freda with her 15% rate of growth is paying initial charges of 5%, reducing her monthly input to £95. That extra £5 could have added about £100,000 to her fund over the 40-year period if she’d been able to invest it.

Charges of any kind are extremely painful. Annual maintenance charges of much above 1% and initial charges, which can be as high as 6% for some unit trusts, are quite simply, excruciating. So, the third law of compound returns is: watch the charges!


How to Maximise Your Investment Returns

If you want to find out how to invest your money in a diverse portfolio, you’re in the right place.

Charges build up quickly when you have large or complex investments – but there are ways to keep them low. For example, you could maximise your annual ISA allowance to avoid any charges. While these interest rates are typically lower than other investment types, you have easy access to your cash if you need it and there are no charges cutting into profits.

When you invest in funds and stocks, you can keep charges low by doing more of the legwork yourself instead of getting a fund manager to do it for you. This’ll save an awful lot of cash in the long-term. You can also start your own pension fund at any age: managing your investments in the SIPP wrapper helps you to keep control and make the most of tax reliefs available.

However, any type of investment needs a lot of research before you put your money into it. Check out our investment guides to find the right mix of options to maximise your long-term profit potential!

Are you keen to learn more about investing? Sign up for our free fortnightly MoneyMagpie Investing Newsletter.

*This is not financial or investment advice. Remember to do your own research and speak to a professional advisor before parting with any money.

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Leslie Austin
Leslie Austin
3 months ago

Actually, I found as a kid that a rolling snowball produces a big snow disc rather than a big snowball, unlike what I’d seen on TV cartoons.

10 years ago

phffft yeah right, Try and find interest rates nowadays of 12%! dream on!

10 years ago

how do i do this???????????

Jasmine Birtles

Your money-making expert. Financial journalist, TV and radio personality.

Jasmine Birtles

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