Compound Interest Calculator
Watch Your Money Multiply.
See exactly how much your investments will grow over time. Adjust your rate, contributions, and compounding frequency to find the most powerful path to wealth.
Investment Details
Final Balance After 30 Years
$556,465
Your money grows 4.72x — interest earns $438,465 on top of your $118,000 in contributions
Final Balance
$556,465
Total Contributed
$118,000
Interest Earned
$438,465
Rule of 72 Doubling
9.0 yrs
Growth Over Time
- Total Balance
- Total Contributed
Why Compound Interest Is Called the Eighth Wonder of the World
Compound interest is the process by which interest earns interest. When you invest $10,000 at 8% annually, you earn $800 in year one. In year two, you earn 8% on $10,800 — not just the original $10,000. That extra $64 seems trivial, but over 30 years, this snowball effect transforms $10,000 into over $100,000 without adding a single additional dollar.
The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 8%, your money doubles every 9 years. At 6%, every 12 years. At 12%, every 6 years. This is why even a 1–2% difference in investment returns compounds into enormous differences over decades.
Compounding frequency matters, but less than most people think. The difference between annual and daily compounding at 8% over 30 years is meaningful but not dramatic — the rate and time horizon are far more impactful. What matters most is starting early and contributing consistently.
The most powerful lever in this calculator is time. A 25-year-old who invests $300/month at 8% until age 65 ends up with approximately $1,000,000. A 35-year-old doing the same ends up with approximately $440,000. Those 10 extra years of compounding are worth $560,000 — more than all the contributions the 35-year-old will ever make.
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Frequently Asked Questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest. On a $10,000 investment at 8% for 10 years: simple interest yields $18,000; compound interest (annual) yields $21,589. The difference grows dramatically over longer periods.
What is a realistic interest rate to use in this calculator?
For stock market investments, the S&P 500 has historically returned approximately 10% annually before inflation (7% after inflation) over long periods. For a diversified portfolio of stocks and bonds, 6–8% is a commonly used planning assumption. For high-yield savings accounts, current rates are 4–5% APY. For CDs, 4–5.5%. Always use a conservative estimate for long-term planning.
Does compounding frequency really matter?
Yes, but less than most people expect. The difference between annual and daily compounding at 8% over 30 years on $10,000 is about $1,800 — meaningful, but dwarfed by the impact of rate and time. Most brokerage accounts and savings accounts compound daily or monthly, so this is rarely a decision you need to optimize for.
How does inflation affect compound interest?
Inflation erodes purchasing power over time. If your investment earns 8% annually but inflation is 3%, your real return is approximately 5%. For long-term planning, it's wise to use a real (inflation-adjusted) return rate. The historical real return of the S&P 500 is approximately 7% annually. This calculator shows nominal (pre-inflation) growth.
What is the Rule of 72?
The Rule of 72 is a mental shortcut to estimate how long it takes to double your money. Divide 72 by your annual interest rate. At 8%, your money doubles every 9 years (72 ÷ 8 = 9). At 6%, every 12 years. At 12%, every 6 years. It's not perfectly precise but is accurate enough for quick mental calculations.