Last Updated on May 12, 2026
How compounding grows your money
Compound interest is a key component of personal finance, yet it is frequently underestimated. Attributed as the ‘Eighth Wonder of the World’ by Einstein, compound interest can turn modest, regular amounts of savings into substantial wealth over time. Whether you are saving money in a savings account or investing in the stock market, compound interest and returns grow your money exponentially year on year. With compounding, your savings earn interest on the original money saved plus the accumulated interest. While any investments you have benefit from compounding on capital growth and any reinvested dividend income.
What is compound interest, and why is it important for your savings?
Interest is the money you earn on your savings, or the charge you pay to borrow money. It can be simple or compound, and the difference between the two dramatically impacts how your money grows over time. Simple interest is calculated on the original amount of savings only. Compound interest is important for your savings as it pays interest not only on the original money saved but also on the accumulated interest earned. This means your money grows exponentially over time, because every period you are earning interest on a larger pot of funds.
How compound interest and returns grow wealth with time
To illustrate how compound interest works for savings accounts, let’s assume you have savings of £5,000 in a savings account earning 4.5% interest, compounded monthly.
In the first month, you earn interest of £18.75 (annual interest of £225). In month two, interest is calculated on the higher balance of £5,018.75, which is £18.82. It is only a small increase in interest, but over time, the effect becomes significant. After five years, your balance increases to £6,259, which is an extra £1,259. With simple interest, the balance would only be £6,125, or £134 less. After ten years, the balance will be around £7,835.
If you add a monthly contribution of £100 to the savings account, after ten years, the balance will be approximately £23,100. This is made up of £17,000 of your contributions and £6,000 in compound interest. This is how regular saving, combined with compound interest, steadily grows wealth.
How compounding grows stock market investments
With stock market investments, you can benefit from the capital growth on your investments plus any dividend income. Be aware that investing in the stock market carries risk as share prices can rise and fall, and dividends are not guaranteed. You should also invest using the tax-free wrapper of a stocks and shares ISA. You need to take a long-term approach to investing and let your money benefit from years of compound returns.
To illustrate the returns on investments, let’s look at Emma and James, who both invest £200 a month into a stocks and shares ISA. Their investments return an average of 7% a year. Emma starts investing at age 25, while James starts at 35. By the time they are 60, they will have the following amounts in their ISA investments:
- Emma has invested for 35 years and will have approximately £360,000 in her ISA.
- James has invested for 25 years and will have approximately £162,000 in his ISA.
| No. of years | Total invested | Future value | Growth |
|---|---|---|---|
| Emma – 35 years | £84,000 | £360,000 | £276,000 |
| James – 25 years | £60,000 | £162,000 | £102,000 |
They both invested the same monthly amount into their ISAs, but Emma’s additional ten years of compounding have more than doubled her investment outcome, compared with James’s. This highlights why starting early, even with a modest contribution, returns a significant advantage over the long term.
A quick way to calculate the impact of compounding interest
The rule of 72 provides a quick way to calculate the impact of compound interest. It is a quick and easy tool for calculating the time to double an investment amount. For example, an investment earning an annual return of 10% would grow as follows:
- Divide 72 by the percentage return on the investment (72/10 = 7.2).
- This means it will take roughly 7 years for the investment amount to double.
The table below illustrates the rule of 72, assuming an initial investment of £10,000. With an annual return of 10%, it takes around 7 years to double the initial investment from £10,000 to £20,080.
| Year | Opening balance | Annual interest | Balance including interest |
|---|---|---|---|
| 1 | 10,000 | 1,047 | 11,047 |
| 2 | 11,047 | 1,157 | 12,204 |
| 3 | 12,204 | 1,278 | 13,482 |
| 4 | 13,482 | 1,412 | 14,894 |
| 5 | 14,894 | 1,560 | 16,454 |
| 6 | 16,454 | 1,723 | 18,177 |
| 7 | 18,177 | 1,903 | 20,080 |
The maths formula behind compounding returns
If you are curious about the maths behind compound returns and the future value of an amount of money, the formula to calculate this is: A = P(1+r/n)^nt
Where:
- A = the future value of the investment plus interest
- P = the initial or capital amount of the investment
- r = the annual rate of interest expressed as a decimal value
- n = the number of times interest is compounded over a period
- t = the amount of time the money is invested for
- ^ = to the power of
Assuming an investment of £5,000, invested for 5 years, earning 5% annual interest, compounding monthly, the figures are:
| A = | 5,000(1+0.05/12)^12*5 |
| A = | 5,000(1.004167)^60 |
| A = | 5,000 x 1.283359 |
| A = | 6,416.79 |
If you want to calculate the amount using Excel, enter the data as follows:
=5000*(1+0.05/12)^(12*5)
How does compound interest impact debt?
Einstein is attributed to calling compound interest the ‘Eighth Wonder of the World’ and described it as follows:
Compound interest is the eighth wonder of the world. He who understands it, earns it, he who doesn’t, pays it!
Albert Einstein
As Einstein’s quote infers, those who don’t understand compound interest will pay it on debt. While compound interest provides potentially great returns on savings and investments, it works in reverse on debt. Compound interest will steadily increase your debt balance if loan repayments are only paying off interest and not reducing any of the capital balance. You can learn more about this by reading our article on credit cards.
In conclusion
Compound interest is not complicated, and when you use it to your advantage, it can provide meaningful financial returns. The earlier you start saving or investing, the more time your money has to grow and the easier it is to build sustainable wealth. Even small, regular contributions can snowball into significant sums over the long term. The key is to start early, be consistent, and let your money and its returns compound over time. Use our free future value calculator below to see exactly what compound interest could do for your money.