What Is Compound Interest?
Compound interest is the process of earning interest on both your original deposit and on the interest that has already been added. Unlike simple interest — which is only calculated on the initial principal — compound interest causes your money to grow at an accelerating rate over time. Albert Einstein is often (perhaps apochryphally) credited with calling it the “eighth wonder of the world.”
In practical terms, compound interest is the engine behind long-term savings growth in the UK, whether you hold a savings account, a Cash ISA, or a Stocks & Shares ISA.
The Compound Interest Formula
The standard compound interest formula is:
A = P × (1 + r/n)^(n×t)
Where:
- A = the final amount
- P = the initial principal (your starting deposit)
- r = the annual interest rate (as a decimal)
- n = the number of times interest compounds per year
- t = the number of years
For example, £10,000 invested at 5% compounded annually for 10 years gives:
A = £10,000 × (1 + 0.05)^10 = £16,289
That is £6,289 in growth — compared to just £5,000 you would earn with simple interest over the same period.
Simple vs Compound Interest Comparison
| Year | Simple Interest (5%) | Compound Interest (5%) |
|---|---|---|
| 0 | £10,000 | £10,000 |
| 1 | £10,500 | £10,500 |
| 5 | £12,500 | £12,763 |
| 10 | £15,000 | £16,289 |
| 20 | £20,000 | £26,533 |
| 30 | £25,000 | £43,219 |
Over 30 years, the compound interest example produces £18,219 more than simple interest on the same £10,000 deposit. The longer your time horizon, the more dramatic the difference becomes.
How the Calculator Works
Our compound interest calculator lets you model how your money could grow over time. Enter:
- Starting amount — your initial lump-sum deposit
- Monthly contribution — any regular amount you plan to add
- Annual interest rate — the rate you expect to earn
- Compounding frequency — how often interest is calculated (monthly, quarterly, or annually)
- Time period — how many years you plan to save or invest
The calculator shows your projected balance year by year, breaking down how much comes from contributions and how much from compounding growth.
The Impact of Regular Contributions
Compound interest becomes even more powerful when combined with regular monthly contributions. Consider the difference between a lump sum alone and adding £200 per month:
| Scenario | 10 Years | 20 Years | 30 Years |
|---|---|---|---|
| £10,000 lump sum only (5%) | £16,289 | £26,533 | £43,219 |
| £10,000 + £200/month (5%) | £47,374 | £108,687 | £209,916 |
Regular saving turns compounding from impressive to transformative. Even modest monthly amounts can build substantial wealth over decades.
How Compounding Frequency Affects Growth
The more frequently interest compounds, the faster your balance grows — though the differences become smaller as frequency increases:
| Frequency | £10,000 at 5% After 20 Years |
|---|---|
| Annually | £26,533 |
| Quarterly | £26,851 |
| Monthly | £27,126 |
| Daily | £27,181 |
The jump from annual to monthly compounding is meaningful; from monthly to daily, much less so. Most UK savings accounts compound either daily or monthly, which works in your favour.
Tips to Maximise Compound Interest
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Start as early as possible. Time is the single most powerful ingredient in compounding. Starting five years earlier can make a bigger difference than saving a higher amount later.
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Be consistent with contributions. Setting up a standing order for regular monthly savings removes the temptation to skip months and keeps compounding working at full effect.
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Use tax-efficient wrappers. Holding savings in a Cash ISA or Stocks & Shares ISA means compound growth is not eroded by tax — you keep the full benefit.
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Reinvest returns. If you hold investments, choose accumulation units or reinvest dividends so that returns generate further returns.
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Avoid unnecessary withdrawals. Every withdrawal resets part of the compounding process. Where possible, leave your savings untouched and let time do the work.
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Shop around for better rates. Even a small increase in interest rate has a significant effect over long periods. Review your savings provider at least once a year.
Compound Interest and Debt
The power of compounding cuts both ways. Credit cards, personal loans, and mortgages also use compound interest — but against you. Paying off high-interest debt early can save you thousands in compounding charges. If you hold debt alongside savings, check whether the interest rate on your debt is higher than the return on your savings. If so, paying down the debt first is usually the better financial move.
Frequently Asked Questions
Answers to common compound interest questions are provided in the FAQ section above. For personalised financial advice, consider consulting an independent financial adviser regulated by the FCA.