What Is Compound Interest?
Compound interest is the process by which the interest you earn on an investment itself begins to earn interest. Unlike simple interest, which is calculated only on the original principal, compound interest is calculated on the principal plus all previously accumulated interest. Learn more in our deep-dive on how compound interest works or read the compound interest formula explained.
This snowball effect means that the longer your money is invested, the faster it grows. Albert Einstein reportedly called compound interest "the eighth wonder of the world" — and for good reason. A modest investment growing at 7% per year doubles roughly every 10 years.
How to Use This Calculator
- Initial investment — Enter the lump sum you're starting with (or 0 if starting from scratch).
- Monthly contribution — How much you plan to add every month.
- Annual interest rate — Your expected average yearly return (7% is a common stock market estimate).
- Investment duration — How many years you plan to keep investing.
- Compounding frequency — How often interest is calculated and added (monthly is standard).
The calculator updates instantly. Hover the chart to see your balance at any year.
Compound Interest Examples
Here are three real-world scenarios showing how monthly investing builds wealth at a 7% average annual return with monthly compounding.
Example 1 — $500/month for 30 years
A steady investor contributing $500/month grows their portfolio to $613,544 — with $433,544 from compound interest alone.
| Year | Total Contributions | Interest Earned | Total Value |
|---|---|---|---|
| 5 | $30,000 | $6,005 | $36,005 |
| 10 | $60,000 | $27,047 | $87,047 |
| 15 | $90,000 | $69,406 | $159,406 |
| 20 | $120,000 | $141,983 | $261,983 |
| 25 | $150,000 | $257,399 | $407,399 |
| 30 | $180,000 | $433,544 | $613,544 |
Example 2 — $100/month for 30 years (small but consistent)
Even a modest $100/month grows to $122,709 after 30 years. You contribute just $36,000 — compound interest adds $86,709 on top. Proof that starting early matters more than starting big.
Example 3 — $1,000/month for 15 years (starting late)
A late starter doubling down with $1,000/month for 15 years reaches $318,811 — only $138,811 from interest. This shows why time in the market beats large contributions late.
Why Compound Interest Matters
- Time is your biggest asset. Starting 10 years earlier can double your final balance.
- Small amounts add up. Even $100/month can grow into six figures over decades.
- Consistency beats timing. Regular contributions outperform trying to time the market.
- It works against you with debt. Credit card interest compounds too — pay it off fast.
Frequently Asked Questions
How much will I have if I invest $500 a month?
Investing $500/month at a 7% annual return for 30 years grows to about $612,000. Of that, roughly $432,000 is compound interest — your contributions total only $180,000.
How long does it take to double my money?
Use the Rule of 72: divide 72 by your expected annual return. At 7%, money doubles in ~10.3 years. At 10%, in ~7.2 years. At 4%, in 18 years.
What happens if I start late?
Starting late means you have less time for compounding to work. To compensate, you'll need to contribute significantly more each month — sometimes 3–4× as much as someone who started 15 years earlier.
What's a realistic interest rate to use?
Historically, the S&P 500 has returned ~10% annually before inflation, or ~7% after inflation. For conservative planning, 5–7% is a reasonable assumption. High-yield savings accounts return 4–5%, while bonds typically yield 3–5%.
Does this account for taxes or inflation?
No — this calculator shows nominal growth. To estimate real (inflation-adjusted) returns, use a rate that's 2–3% lower than your nominal expected return. Tax-advantaged accounts like 401(k)s and IRAs help shield growth from taxes.
Monthly vs annual compounding — which is better?
More frequent compounding produces slightly higher returns. Monthly compounding at 7% effectively yields 7.23% annually. The difference is small but compounds itself over decades.