Compound Interest Calculator UK — Savings & Investment Growth
See how your savings or investments could grow over time with our free UK compound interest calculator. Enter an initial deposit, monthly contributions, interest rate and time period to see your total balance, total interest earned and growth year by year. Compare monthly, quarterly and annual compounding to see how frequency affects your returns.
Compound interest is often described as the most powerful force in personal finance. Unlike simple interest, which is calculated only on your initial deposit, compound interest earns interest on your interest, creating exponential growth over time. Even modest regular savings can grow into substantial sums given enough time. A £200 monthly contribution at 5% interest grows to over £31,000 in 10 years and over £82,000 in 20 years. This calculator helps you visualise exactly how compounding works for your specific savings goals.
How Compound Interest Works
Compound interest works by adding earned interest back to your principal, so future interest is calculated on an ever-growing balance. The more frequently interest is compounded, the faster your money grows. Monthly compounding earns slightly more than annual compounding at the same nominal rate because interest is added and starts earning sooner. Over long periods, this effect becomes dramatic: £10,000 invested at 7% annually grows to £19,672 after 10 years but £38,697 after 20 years, nearly doubling in the second decade despite no increase in contributions.
The Rule of 72 provides a quick shortcut to estimate how long it takes your money to double. Simply divide 72 by the annual interest rate. At 6%, your money doubles in roughly 12 years. For more on savings and investments, the HM Treasury section on GOV.UK provides guidance on government-backed savings products.
Best Accounts for Compound Interest in the UK
Cash ISAs offer tax-free compound interest with an annual allowance of £20,000. Fixed-rate savings bonds typically offer the highest cash interest rates but lock your money away for 1 to 5 years. Regular saver accounts often headline the best rates (sometimes 5% to 7%) but limit monthly deposits and usually revert to a lower rate after 12 months. Stocks and Shares ISAs offer compounding through reinvested dividends and capital growth, though returns are not guaranteed and your capital is at risk.
For tax-free savings projections, try our ISA calculator. If you are saving for retirement, our pension calculator models compound growth with employer contributions and tax relief built in.
The Impact of Inflation on Compound Growth
While compound interest grows your nominal balance, inflation erodes purchasing power. If your savings earn 4% but inflation is 3%, your real return is only 1%. Over 20 years, £10,000 growing at 4% nominally becomes £21,911, but in real terms (after adjusting for 3% inflation) it is worth just £12,124 in today's money. Always consider real returns when setting savings targets, and use higher-growth investments for long-term goals where you can tolerate short-term volatility.