Compound Interest Calculator
See exactly how your money grows over time. Enter a starting amount, an optional monthly contribution, an expected annual return, and a time frame — the future value updates instantly.
Compound Interest Calculator
Future value
$40,387
4.0× what you put in
You invest
$10,000
Interest earned
$30,387
Assumes the annual return is compounded monthly. Returns are illustrative and not guaranteed.
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How compound interest works
Compound interest means you earn returns on your past returns, not just on your original deposit. The formula is FV = P × (1 + r)n. A quick shortcut is the Rule of 72: divide 72 by your annual return to estimate how many years it takes your money to double (at 7%, about 10.3 years).
Want the full breakdown with year-by-year tables? Read How Much Will $10,000 Grow in 20 Years? or What Is Compound Interest?
Quick reference: how $10,000 grows
A one-time $10,000 investment with no extra contributions, at different average annual returns (compounded annually):
| Annual return | After 10 yrs | After 20 yrs | After 30 yrs |
|---|---|---|---|
| 5% | $16,289 | $26,533 | $43,219 |
| 7% | $19,672 | $38,697 | $76,123 |
| 10% | $25,937 | $67,275 | $174,494 |
Frequently asked questions
How is compound interest calculated?
Compound interest uses the formula FV = P × (1 + r)^n, where P is your starting amount, r is the periodic rate, and n is the number of periods. When you add monthly contributions, each deposit also compounds, so the calculator adds the future value of that stream of deposits on top of your starting balance.
What is a realistic rate of return to use?
High-yield savings and CDs have recently paid about 4–5%. The U.S. stock market (S&P 500) has averaged roughly 10% per year over the long term, though with volatility. Many people model a diversified long-term portfolio at 6–7% to stay conservative.
Does this calculator account for taxes and inflation?
No — it shows nominal growth before taxes and inflation. To estimate real (inflation-adjusted) growth, subtract your expected inflation rate (often ~2–3%) from the return you enter.
What's the difference between APR and APY?
APY (annual percentage yield) includes the effect of compounding within the year, so it's the figure that matters for how savings actually grow. APR does not account for intra-year compounding. This calculator treats your input as an annual return compounded monthly.
How much does the monthly contribution matter?
Usually a lot — often more than the starting amount over long periods. For example, $10,000 left alone at 7% for 30 years becomes about $76,000, but adding $300/month over the same period pushes it well past $400,000. Regular contributions are the single biggest lever most people control.
Is a 7% return realistic?
7% is a common, deliberately conservative long-term planning assumption. The S&P 500 has averaged closer to 10% per year over the long run, but with significant volatility and down years. Using 6–7% builds in a margin of safety for fees, taxes, and sequence risk.