Compound Interest Calculator

What is compound interest?

Compound interest is interest calculated on both your original deposit and the interest you've already earned. Think of it like a snowball rolling downhill. Your initial investment earns money. Then that money earns money too. That's the core difference from simple interest, which only grows your original amount. With compounding, interest earns interest, and your growth accelerates over time.

Here's a concrete example. Say you put $10,000 in an account earning 5% annually. In year one, you earn $500 in interest. In year two, you earn 5% not just on the original $10,000, but on the $10,500 total, which gives you $525. That extra $25 came from earning interest on your previous interest. It sounds small at first, but over decades this effect compounds dramatically.

Time is the biggest advantage

The longer your money stays invested, the more powerful compounding becomes. Imagine two investors. One starts at age 25 and invests $5,000 a year at 8% annual return. The other waits until age 35 to start the same plan. The first investor contributes for 40 years. The second contributes for 30 years. By retirement at 65, the first investor will have roughly twice as much money, even though they only invested one-third more in total contributions. That difference is almost entirely compounding. This is why starting early with tax-advantaged accounts like a Roth IRA matters so much.

This is why financial advisors stress starting early. You can't replicate 10 extra years of compounding by simply adding more money later. Time in the market matters far more than timing the market. Even modest returns compounded over decades can turn into substantial wealth.

How monthly contributions accelerate your growth

Adding money regularly supercharges compound interest. When you invest monthly instead of just once upfront, each new contribution joins the compounding race from that point forward. If you add $500 a month to your account, that first month's contribution has more time to earn returns than the last month's contribution, but they're all working together.

Let's compare two scenarios. In the first, you invest $10,000 upfront and then stop. In the second, you invest $200 every month for five years (also totaling $10,000 plus the original amount). Even if both earn the same rate, the monthly contributions version will have more money because you're constantly adding new amounts that earn returns for longer. This is why the calculator lets you adjust both your starting amount and your monthly contribution. The interaction between these two is crucial for realistic planning.

What this calculator does and doesn't do

This tool projects how your money grows by compounding interest at a fixed annual return rate. You enter your starting balance, monthly contribution, expected annual return (as a percentage), and time horizon. It then calculates year-by-year growth and shows you the total interest earned. The tool is useful for rough planning, seeing how different contribution amounts or return rates affect outcomes, and understanding the math behind compounding.

Important limitations: This calculator assumes your return stays constant every year, which doesn't happen in real life. Markets fluctuate. Some years are up, some are down. The calculator also ignores taxes (investment gains are often taxed), inflation (money's purchasing power changes), and fees (if you use a broker, investment fund, or advisor, you'll pay fees). These factors will lower your real returns. Think of this as a starting point, not a guarantee. Use it to understand relationships between variables, not as a substitute for professional financial advice.

Choosing a realistic return rate

The return rate you choose is the biggest driver of your projections. Long-term stock market returns average around 10% annually, though this varies by year and by what you invest in. Bonds typically return 3-5%. Savings accounts might offer 4-5% currently, but that changes. If you're conservative and want to be safe, use a lower number. If you're younger and can tolerate volatility, a higher rate might make sense. The point is that small changes in this rate produce very different outcomes over time. Consider diversifying across index funds and bonds for a balanced approach.

This calculator is a projection tool using fixed assumptions. It does not account for taxes, inflation, investment fees, or market variability. Actual results will differ.

Related reading: What is compound interest?, APR vs APY, and Compound Interest (glossary).