Everything you need to know about the most powerful force in personal finance.
Beginner Guide
What Is Compound Interest and How Does It Work?
Compound interest is the process by which interest is earned not only on your original investment (the principal), but also on the interest that has already accumulated. This creates an exponential growth effect that becomes more powerful the longer your money is invested.
For example, if you invest £10,000 at a 7% annual return, you earn £700 in year one. In year two, you earn 7% on £10,700 — which is £749. Each year the interest amount grows, even if your rate stays the same. Over 30 years, that original £10,000 grows to over £76,000 without adding a single penny more.
This is why financial experts consistently say that starting early is the single most important decision you can make. A 25-year-old who invests £200 per month will retire with significantly more money than a 35-year-old who invests £400 per month — even though the older investor puts in twice as much each month.
Key Concept
The Rule of 72: How to Estimate Doubling Time
The Rule of 72 is a simple mental shortcut for estimating how long it takes for an investment to double. Divide 72 by your annual interest rate and the result is the approximate number of years to double your money.
At 6% annual return: 72 ÷ 6 = 12 years to double. At 8%: 72 ÷ 8 = 9 years. At 10%: 72 ÷ 10 = 7.2 years. This means that at a 10% return, £10,000 becomes £20,000 in 7 years, £40,000 in 14 years, and £80,000 in 21 years — without any additional contributions.
The Rule of 72 also works in reverse to illustrate the damaging effect of inflation. At 3% inflation, the purchasing power of your cash savings halves every 24 years — which is why leaving large sums in low-interest accounts is a long-term wealth destroyer.
Strategy
How to Maximise Compound Growth in the UK
For UK investors, the most tax-efficient way to benefit from compound interest is through a Stocks and Shares ISA. You can invest up to £20,000 per year and all growth — dividends, interest, and capital gains — is completely free from UK tax. This means compound growth is not eroded by tax year after year, which makes a significant difference over decades.
Within your ISA, a globally diversified index fund such as the Vanguard FTSE Global All Cap or the iShares MSCI World ETF gives you exposure to thousands of companies worldwide. Historically, global equity markets have returned approximately 7–10% per year before inflation over long periods.
Set up a monthly direct debit into your ISA on payday and reinvest all dividends automatically. This approach — known as pound-cost averaging — removes emotion from investing and ensures you buy more shares when prices fall and fewer when prices rise.
Watch Out
The Hidden Cost of High Fees on Compound Returns
While compound interest works powerfully in your favour as an investor, it works equally powerfully against you when it comes to fees. An actively managed fund charging 1.5% per year versus a passive index fund charging 0.2% may seem like a small difference, but over 30 years the impact is staggering.
On a £200 monthly investment over 30 years at 7% gross return, a 1.5% annual fee reduces your final portfolio from approximately £227,000 to around £163,000. That is £64,000 lost purely to fees — money that went to the fund manager rather than compounding in your account.
Always check the Total Expense Ratio (TER) of any fund before investing. For passive index funds, look for TERs below 0.25%. Vanguard charges 0.15% platform fee, and InvestEngine charges 0% for ETF portfolios, making them among the most cost-effective options for UK investors.
Worked Example
A Real-World Example: Two Investors, One Surprising Result
Consider two investors — Emma and James. Emma starts investing £300 per month at age 22 and stops at 32, having invested for just 10 years. James starts at 32 and invests £300 per month all the way until he is 62, investing for 30 years. Both earn a 7% annual return. Who ends up with more money at 62?
Emma contributes £36,000 over 10 years and then leaves her money to grow untouched. By age 62, her portfolio has grown to approximately £430,000. James contributes £108,000 over 30 years — three times as much — but ends up with only around £340,000. Emma wins by £90,000 despite investing for a third of the time, simply because she started a decade earlier.
This example illustrates the most important truth about compound interest: time in the market is worth more than the amount invested. Use the calculator above to run your own numbers and see how powerful starting early can be.
£430k
Emma — 10 years investing
£340k
James — 30 years investing
10 yrs
The advantage of starting early
How to Use This Calculator
Getting the Most Accurate Results
Our compound interest calculator gives you an instant projection of how your investments grow over time. To get meaningful results, use realistic inputs based on your actual financial situation.
Initial investment: Enter the lump sum you plan to invest today. This could be existing savings, an inheritance, a bonus, or any capital you have available to invest now.
Monthly contribution: This is often the most impactful input. Even modest regular contributions dramatically increase your final portfolio value. A common recommendation is to invest at least 15% of your take-home pay each month.
Annual return rate: For a globally diversified index fund, a commonly used planning figure is 7% nominal (before inflation) or 4–5% real (after inflation). For cash savings accounts, use the current rate offered by your provider. For bonds or mixed portfolios, 4–6% is a reasonable assumption.
Compounding frequency: Most investments compound monthly or annually. Index funds effectively compound continuously as dividends are reinvested. For savings accounts, check your provider's terms — some compound daily, others monthly or annually. More frequent compounding always produces slightly higher returns at the same headline rate.