Compound Interest Calculator
中文简介：输入本金、每月定投金额、年化收益率和投资年限，即可算出最终金额、总投入和总利息，并附逐年明细表。Free and no sign-up required.
What Is Compound Interest?
Compound interest is interest earned on both your original money and on the interest that money has already earned. In the first year, your savings grow only on the amount you put in. After that, each new round of growth applies to a slightly bigger pile, so the balance does not rise in a straight line — it curves upward. This is why two people who save the same total amount can end up with very different results: the person who starts earlier gives compounding more time to work, and time is the most powerful ingredient in the whole formula. Small, regular contributions combined with a long horizon usually beat large, late contributions.
The Formula Behind This Calculator
This calculator compounds monthly. Your annual return is divided by 12 to get a monthly rate, and your monthly contribution is added every month. The balance after n months is: FV = P × (1 + r)n + C × [((1 + r)n − 1) ÷ r], where P is your starting principal, C is your monthly contribution, r is the monthly rate, and n is the number of months. The first part of the formula shows how your initial lump sum grows on its own. The second part shows how each monthly deposit grows for the months that remain after it is made — early deposits compound for longer, which is why consistency matters more than deposit size at the beginning.
A Worked Example
Suppose you start with $10,000, add $500 every month, and earn an average annual return of 8%, compounded monthly, for 10 years. Your total contributions are $70,000: the $10,000 you started with plus $60,000 of monthly deposits. The calculator shows a final balance of about $113,669. That means roughly $43,669 of the ending value is pure interest — money your money earned by staying invested. Stretch the same plan to 20 years and the interest portion grows far larger than in the first decade, even though your monthly deposit never changes. That acceleration is the snowball effect of compounding, and it is the main reason long-term investors focus on starting early and staying consistent rather than chasing perfect timing.
How to Use These Results
Treat the output as a planning estimate, not a promise. Real investments do not grow at a perfectly steady rate: markets rise and fall, fees and taxes reduce returns, and inflation shrinks what your final balance can actually buy. Try running the calculator with a return one or two points lower than you hope for, and see how the result changes. If the plan still looks acceptable under the more cautious assumption, it is probably a robust plan. You can also experiment with the monthly contribution — for many households, raising the monthly amount slightly or extending the horizon by a few years has a bigger effect than finding an investment with a slightly higher return.
Frequently Asked Questions
How often is interest compounded in this calculator?
Monthly. The annual return you enter is divided by 12, interest is applied every month, and your monthly contribution is added each month as well. Monthly compounding produces a slightly higher result than annual compounding at the same stated rate.
Does this calculator account for taxes, fees, or inflation?
No. It shows a simple before-tax, before-fee estimate using a constant rate of return. Real results will differ. If you want a more conservative picture, lower the annual return by one or two percentage points to roughly allow for fees and taxes.
What is a realistic annual return to enter?
It depends on what you invest in. Savings accounts pay far less than diversified stock portfolios, which historically have averaged higher returns over long periods but with large swings along the way. Enter a range of values and compare the outcomes instead of relying on a single optimistic number.
Why does starting earlier make such a big difference?
Because interest itself starts earning interest. A deposit made today may compound for hundreds of months, while the same deposit made ten years later compounds for far fewer. The gap between the two outcomes grows with time, which is why the early years of a savings plan are disproportionately valuable.
Disclaimer: This tool is for education and planning only and is not financial advice. Figures are estimates based on a constant rate of return; actual investment results will vary.