What Is Compound Interest?
Compound interest is interest earned on both your original investment and on the interest that has already been added. This is different from simple interest, which is only ever calculated on your original sum.
The result is exponential growth — your money grows faster and faster over time, because every year you are earning returns on a larger and larger pot. This is why Albert Einstein is often (perhaps apocryphally) credited with calling it "the eighth wonder of the world."
"Compound interest is the eighth wonder of the world. He who understands it, earns it; he who doesn't, pays it."
Simple Interest vs Compound Interest: The Difference
The difference between simple and compound interest might seem small at first, but it becomes dramatic over time. Here is a direct comparison using £10,000 at 7% annual interest over 30 years:
- Simple interest: £10,000 + (£700 × 30) = £31,000
- Compound interest (annual): £10,000 × (1.07)³⁰ = £76,123
That is a difference of over £45,000 from the exact same starting amount, the exact same interest rate, and the exact same time period. The only difference is whether interest is compounding on itself or not.
How Compound Interest Works: The Formula
The formula for compound interest is:
A = P × (1 + r/n)^(n×t)
- A = the final amount
- P = the principal (your starting amount)
- r = the annual interest rate (as a decimal, so 7% = 0.07)
- n = the number of times interest compounds per year
- t = the number of years
You do not need to memorise this formula — that is what our free calculator is for. But understanding the variables helps you see which levers you can pull to increase your returns.
The four levers that determine your final wealth are: how much you start with, how much you add regularly, your annual return rate, and most importantly — how long you leave it invested. Time is the most powerful lever by far.
Compounding Frequency: Does It Matter?
Interest can compound at different frequencies — annually, quarterly, monthly, or even daily. The more frequently interest compounds, the more you earn, because you start earning interest on your interest sooner.
On £10,000 at 7% over 10 years:
- Annual compounding: £19,672
- Monthly compounding: £20,097
- Daily compounding: £20,136
As you can see, the difference between monthly and daily is small (£39 in this case). The difference between annual and monthly is more meaningful (£425), but in practice, for long-term investments in index funds, compounding happens effectively continuously as dividends are reinvested.
The Rule of 72: A Quick Mental Shortcut
The Rule of 72 is a simple way to estimate how long it takes for an investment to double in value. Just divide 72 by your annual return rate:
- At 4%: 72 ÷ 4 = 18 years to double
- At 6%: 72 ÷ 6 = 12 years to double
- At 8%: 72 ÷ 8 = 9 years to double
- At 10%: 72 ÷ 10 = 7.2 years to double
This rule also works in reverse to show the damage inflation does to your purchasing power. At 3% inflation (roughly the UK long-run average), the real value of cash sitting in a non-interest-bearing account halves every 24 years. This is why investing is not optional — it is a defence against the silent erosion of inflation.
Why Starting Early Beats Investing More
The single most important decision you can make as an investor is to start as early as possible. This seems obvious, but the numbers are genuinely shocking when you see them laid out.
Consider two people — Ava and Ben — both investing in a global index fund returning 7% annually:
- Ava invests £300/month from age 22 to 32, then stops. Total invested: £36,000.
- Ben invests £300/month from age 32 to 62. Total invested: £108,000.
By age 62, Ava has approximately £430,000. Ben has approximately £340,000. Ava invested three times less money but ends up with £90,000 more — purely because she started 10 years earlier and let compound interest do the work.
The best time to start investing was ten years ago. The second best time is today. Even investing a small amount now — £50 or £100 per month — will dramatically outperform a larger amount started later.
How UK Investors Can Benefit from Compound Interest
For UK investors, the most tax-efficient way to benefit from compound interest is through a Stocks and Shares ISA. Here is why this matters so much:
Outside an ISA, investment returns are subject to Capital Gains Tax (above the annual exempt amount) and dividend income is taxed above the dividend allowance. Inside an ISA, all growth — capital gains, dividends, and interest — is completely tax free, forever. This means compound interest works at its full rate rather than being eroded by tax year after year.
The annual ISA allowance is £20,000 per person (2024/25 tax year). Couples can shelter £40,000 per year between them. Over a working lifetime, this can protect hundreds of thousands of pounds of investment growth from tax entirely.
Within your ISA, a globally diversified index fund is the most straightforward way to access long-run compound growth. The Vanguard FTSE Global All Cap Index Fund, for example, gives you exposure to over 7,000 companies across 50+ countries for a combined annual cost of approximately 0.22%. You set up a monthly direct debit, dividends are reinvested automatically, and the power of compound interest does its work over decades.
The Danger of Fees: Compound Interest Working Against You
Just as compound interest can work powerfully in your favour, fees can work powerfully against you. Even a seemingly small difference in annual charges has a dramatic impact over decades.
On a £200/month investment over 30 years at 7% gross return:
- 0.22% annual fee (index fund): final portfolio approximately £225,000
- 1.50% annual fee (typical actively managed fund): final portfolio approximately £163,000
That is a difference of £62,000 — lost entirely to fees compounding against you. The actively managed fund would need to consistently outperform the index by more than 1.28% per year just to break even, which the evidence shows most actively managed funds fail to do over long periods.
Always check the Total Expense Ratio (TER) of any fund before investing. For passive index funds, a TER below 0.25% is achievable. Add your platform fee (Vanguard: 0.15%, InvestEngine: 0%), and your total annual cost should be well under 0.40%.
How to Get Started with Compound Investing in the UK
The process is simpler than most people think. Here is a straightforward path for a UK investor starting from scratch:
- Build an emergency fund first. Three to six months of expenses in an easy-access savings account. Do not invest money you might need at short notice.
- Open a Stocks and Shares ISA. Vanguard, InvestEngine, and Moneybox are popular options for beginners. All are regulated by the FCA and offer ISAs.
- Choose a global index fund. The Vanguard FTSE Global All Cap or the iShares MSCI World ETF are both solid, low-cost choices that give you global diversification instantly.
- Set up a monthly direct debit. Automate your investment so it happens on payday, before you have a chance to spend the money elsewhere.
- Leave it alone. Do not check it every day. Do not panic sell during market downturns. The data shows that time in the market consistently beats timing the market.
Use our free compound interest calculator to model your specific situation — enter your starting amount, monthly contribution, expected return rate, and time horizon to see exactly how your money could grow.