Compound Interest Explained

Written by Fraser Stewart
Reading time 3 minutes
Compound Interest Explained image

Albert Einstein famously called compound interest the "eighth wonder of the world," and for good reason. It's a financial force that, over time, can transform even modest sums into significant wealth. Understanding this concept is not just for financial experts—it's essential for anyone who wants to grow their money effectively.

Earning Interest on Interest

At its heart, compound interest is the process of earning interest not just on your initial investment (the principal) but also on the accumulated interest from previous periods.  In other words, your interest earns interest, and that earned interest then earns more interest, and so on, creating a snowball effect of growth.

How Compound Interest Works

  1. Initial Investment: You start by investing a certain amount of money, called the principal.
  2. Interest Calculation: The interest rate, often expressed as an annual percentage, determines how much interest is earned on your investment over a specific period.
  3. Compounding Period: The compounding period dictates how often the earned interest is added to your principal. This can be annually, semi-annually, quarterly, monthly, or even daily.
  4. Interest on Interest: Each time interest is compounded, it's added to your principal, creating a new, larger base on which to calculate future interest.

A Simple Example

Let's say you invest £10,000 with a 7% annual interest rate, compounded annually. Here's how your investment might grow over 10 years:

Year Starting Balance Interest Earned Ending Balance
1 £10,000 £700 £10,700
2 £10,700 £749 £11,449
3 £11,449 £801.43 £12.250.43
4 £12.250.43 £857.33 £13,107.96
... ... ... ...
10 £19,671.51 £1,377.01 £21,048.52

Notice how the interest earned increases each year, leading to an exponential growth curve.

The Sooner, the Better

The most critical factor in harnessing the power of compound interest is time. The longer your money compounds, the more substantial the gains.  Here's why:

The Mathematics of Compounding

To calculate compound interest, use this formula:

A = P (1 + r/n)^(nt)


Compounding Frequency

The frequency with which interest is compounded can significantly impact your returns:

The more frequent the compounding, the faster your investment grows because you're essentially earning interest on interest more often.

Applications of Compound Interest

Strategies to Maximise Compound Interest

Start Early and Invest Regularly: Prioritise consistent contributions over large, infrequent ones.

Consider High-Yielding Investments: Depending on your risk appetite, consider investments with the potential for higher returns.

Minimise Fees: Investment fees can eat into your gains. Opt for low-cost options.

Reinvest Your Earnings: Let your dividends and interest compound to maximise growth.

Consider Tax-Advantaged Accounts: In the UK, ISAs and pensions offer tax benefits that can boost your compounding power.

The Psychological Aspect of Compounding

Compound Interest Calculators

Online compound interest calculators are invaluable tools for visualising how your investments might grow over time. Experiment with different scenarios to see the impact of various interest rates, investment amounts, and compounding frequencies.

The Bottom Line: Your Path to Financial Freedom

Compound interest is not merely a financial concept; it's a wealth-building superpower. By understanding its mechanics and applying its principles, you can make informed investment decisions, accelerate your wealth accumulation, and secure a more comfortable financial future.

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