See how your money grows over time with the power of compound interest. The 8th wonder of the world.
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Albert Einstein reportedly called it "the eighth wonder of the world," saying "he who understands it, earns it; he who doesn't, pays it." The key distinction from simple interest is that with compounding, your interest earns interest — creating exponential rather than linear growth.
The formula is: A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate, n is the number of times interest compounds per year, and t is the time in years. The more frequently interest compounds, the faster your money grows — though the difference between monthly and daily compounding is relatively small in practice.
The Rule of 72 is a quick mental shortcut for estimating how long it takes to double your money. Simply divide 72 by the annual interest rate. At 7% annual return, your money doubles in approximately 72 ÷ 7 = 10.3 years. At 10%, it doubles in 7.2 years. At 3% (a typical savings account), it takes 24 years to double.
| Return Rate | 10 Years | 20 Years | 30 Years |
|---|---|---|---|
| 2% (savings account) | $12,190 | $14,859 | $18,114 |
| 5% (bonds) | $16,289 | $26,533 | $43,219 |
| 7% (index fund avg) | $19,672 | $38,697 | $76,123 |
| 10% (S&P 500 hist.) | $25,937 | $67,275 | $174,494 |
| 15% (growth stocks) | $40,456 | $163,665 | $662,118 |
The same force that builds wealth through investing destroys it through debt. Credit card interest at 20–25% APR compounds monthly, meaning a $5,000 balance that you only make minimum payments on can take 15+ years to pay off and cost over $10,000 in interest. Understanding compound interest is equally important for avoiding debt traps as it is for building investments.
The Compound Interest Calculator is a free online tool that shows you how your savings and investments grow exponentially over time through the power of compound interest — earning interest on your interest. Albert Einstein reportedly called compound interest the eighth wonder of the world, and with good reason: a single £10,000 investment at 7% annual return grows to £76,000 over 30 years without adding a single additional pound. This calculator makes the magic of compounding visible and helps you understand why starting early is the most important financial decision you can make.
Enter your initial investment (principal) amount.
Enter your regular contribution amount and frequency (monthly, quarterly, or annually).
Enter the expected annual interest rate or investment return.
Select the compounding frequency: daily, monthly, quarterly, or annually.
Enter the investment period in years.
The calculator displays your final balance, total contributions, and total interest/returns earned.
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which is calculated only on the principal), compound interest grows exponentially — each period's interest becomes part of the principal for the next period.
The formula for compound interest is: A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is the time in years. More frequent compounding (daily vs annually) produces slightly higher returns, though the difference is modest compared to the interest rate and time period.
The Rule of 72 is a quick mental calculation for estimating how long it takes to double your money: divide 72 by the annual interest rate. At 7%, your money doubles every 72/7 ≈ 10.3 years. At 10%, every 7.2 years. At 4%, every 18 years. This rule illustrates why even small differences in return rates have enormous long-term consequences.
The most important variable in compound interest is time. The difference between starting to invest at 25 versus 35 is enormous. Consider two investors who both invest £500/month at 7% annual return: Investor A starts at 25 and stops at 35 (10 years of contributions, then lets it grow). Investor B starts at 35 and invests until 65 (30 years of contributions). Despite investing for 3x longer, Investor B ends up with less money than Investor A — because Investor A's money had 30 more years to compound.
This counterintuitive result illustrates the exponential nature of compounding. The early years of investment are the most valuable because they have the most time to compound. Every year of delay costs significantly more than the contributions made in that year.
For young people, the most important financial action is to start investing as early as possible, even with small amounts. £100/month invested at 20 is worth more than £500/month invested at 40. The specific investment vehicle matters less than starting immediately — a simple global index fund in an ISA or pension is sufficient.
Every year of delay costs more than the contributions you miss. The best time to start investing was yesterday; the second best time is today. Even small amounts compound significantly over decades.
Reinvesting dividends rather than taking them as cash is essential for maximising compound growth. Dividend reinvestment is the primary driver of long-term stock market returns.
Investment fees compound just like returns — but in reverse. A 1% annual fee on a £100,000 portfolio costs approximately £30,000 over 20 years compared to a 0.1% fee. Choose low-cost index funds (under 0.2% annual charge).
ISAs (UK) and 401k/IRAs (US) allow compound growth without annual tax drag. Maximise these accounts before investing in taxable accounts to keep more of your returns.
When you receive a salary increase, immediately increase your investment contributions by the same amount. This prevents lifestyle inflation and accelerates wealth building.
Compound interest works over decades, not months. Short-term market volatility is noise. Staying invested through market downturns is essential — selling during crashes locks in losses and misses the recovery.