ValueWhat compound interest actually is
Compound interest means your gains start earning gains of their own. With simple interest, only the original principal earns: $1,000 at 10% for 10 years becomes $1,000 + 10 × $100 = $2,000, a straight line. With compounding, each year's interest is added to the balance and earns too: $1,000 × 1.1010 = $2,593.74, and the curve bends upward. The extra $593.74 is interest earning interest, and its share of the total keeps growing the longer you stay invested.
Contributions change the story even more. The default scenario on this page, $5,000 to start plus $200 a month at 7% for 20 years, ends at about $124,400 under monthly compounding. You personally put in $53,000 ($5,000 + 240 × $200). The rest, roughly $71,400, is compounding doing the heavy lifting. Under simple interest, the same inputs reach only about $93,500, with $40,500 of interest. The compounding bonus is roughly $31,000: money you get purely from reinvesting gains instead of pocketing them.
The three levers, in order of power
- Time. Stretching the horizon from 10 to 30 years at 7% with $200/month turns roughly $33,000 into roughly $227,000. Every extra decade matters more than the last, because the base being compounded is larger.
- Contributions. Doubling the monthly contribution roughly doubles the ending balance. Contributions are also the lever you control most directly: you cannot command a 12% return, but you can usually find another $100 a month.
- Return rate. Powerful but partly out of your hands, and higher expected returns come with higher risk and volatility. Be suspicious of any plan that only works at 12%.
A useful check: the rule of 72. Divide 72 by your return to estimate doubling time: at 7%, money doubles about every 10 years; at 10%, about every 7 years. If a projection implies doubling much faster than this, an input is probably optimistic.
Reading the comparison honestly
- Nominal vs real. This calculator projects future dollars without inflation. To think in today's purchasing power, subtract roughly 2-3% from your return (a 7% nominal return is about 4-5% real). Over 20 years, inflation quietly halves the "realness" of the headline number.
- Fees and taxes. A 1% annual fee does not sound like much, but it compounds too: it can erase roughly a quarter of gains over 30 years. Run the calculator at your expected return minus fees for a fairer picture.
- Returns are not smooth. The calculator assumes a steady rate. Real markets bounce around; the average still lands near the assumption, but the path affects when money is best added (down years are, mathematically, the best buying years).
- Contribution timing. Monthly contributions here are added at month end. Investing at the start of each month instead adds a small, consistent bonus.
For the other direction of the same math, see the CAGR calculator: given a start and end value, it tells you the annualized rate that actually happened, which is the right way to sanity-check whether your assumed return is realistic. And if you are compounding business cash flows rather than savings, that growth assumption feeds directly into the DCF calculator and the dividend discount model.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest pays only on the original principal, so growth is linear. Compound interest pays on the principal plus all previously earned interest, so growth accelerates. Over long horizons the gap becomes very large: compare the two columns in the table above.
How often should I assume compounding happens?
This calculator compounds monthly, which fits monthly contributions and most investment accounts. The difference between monthly, quarterly, and annual compounding is small compared to the effect of the return rate and the time horizon.
Why do monthly contributions matter so much?
Because early contributions get the most compounding time. With a 20-year horizon, the interest earned on early contributions can exceed the contributions themselves. Consistent saving usually matters more than finding a slightly higher return.
What annual return should I assume?
Use a realistic long-run figure for your asset mix, not a recent hot year. A diversified stock portfolio is often modelled at 6-8% nominal before inflation; subtract roughly 2-3% for a real, inflation-adjusted estimate. Always run a low case and a high case to see the range.
Are these results nominal or inflation-adjusted?
Nominal. The calculator projects the future dollar amount without adjusting for inflation. To think in today's purchasing power, use a real return: roughly your expected nominal return minus expected inflation (about 2-3%).
How long does it take money to double at 7 percent?
About 10.3 years. The Rule of 72 gives a quick estimate: divide 72 by your annual return, so 72 divided by 7 is about 10.3. Monthly contributions shorten the time further because each deposit starts compounding on its own schedule.