Compound Interest Guide: The Eighth Wonder of the World
Compound interest is the engine behind nearly every long-term fortune. Unlike simple interest, which pays only on your original principal, compound interest pays interest on your interest — creating an exponential growth curve that rewards patience above all else.
The Compound Interest Formula
- A — final amount
- P — principal (initial investment)
- r — annual interest rate (decimal)
- n — times compounded per year
- t — number of years
With regular contributions, add the future value of a series to this amount.
Example: $10,000 invested at 7% compounded monthly for 30 years grows to 10,000 × (1 + 0.07/12)360 ≈ $81,150 — more than 8× your initial money.
The Rule of 72
A quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money.
72 ÷ 7% ≈ 10.3 years to double
At 7%, money doubles roughly every 10 years. Over 40 years, $10,000 becomes $10K → $20K → $40K → $80K → $160K — purely from compounding.
How Contributions Accelerate Growth
Adding even a small monthly contribution dramatically increases the final amount, because each contribution has decades to compound. $100/month on top of the example above adds another $122,000 over 30 years — your total contributions ($36K) earn back more than 3× in growth.
Starting early beats saving more. $100/month from age 25 to 35 (then nothing) often beats $100/month from 35 to 65 — the extra 10 years of compounding at the start outweighs 30 years of contributions later.
Compounding Frequency Matters
The more often interest is compounded, the faster your money grows. Daily beats monthly beats quarterly beats annual. The difference is small at low rates but adds up over decades.
Put It Into Practice
Enter your principal, expected return, time horizon, and optional monthly contributions to see your future balance and a growth chart.
复利是几乎所有长期财富增长的引擎。与仅对原始本金支付利息的单利不同，复利会对已产生的利息再支付利息——形成一条指数增长曲线，最终回报的是耐心。
复利计算公式
- A — 最终金额
- P — 本金（初始投资）
- r — 年利率（小数）
- n — 每年复利次数
- t — 投资年限
如有定期定投，需将定投系列的终值加到该金额上。
示例：以7%的年利率每月复利投资$10,000，30年后增长为 10,000 × (1 + 0.07/12)360 ≈ $81,150 — 超过初始本金的8倍。
72法则
一个快速心算技巧：用72除以年利率，即可估算出资金翻倍所需的年数。
72 ÷ 7% ≈ 10.3年即可翻倍
在7%的利率下，资金大约每10年翻一倍。40年后，$10,000会变成 $10K → $20K → $40K → $80K → $160K — 这完全来自复利效应。
定投如何加速增长
即使每月只增加少量定投，也会大幅提升最终金额，因为每笔定投都有几十年的时间进行复利。在上述示例基础上每月定投$100，30年后会额外增加 $122,000 — 你的总定投金额（$36K）获得了超过3倍的增长回报。
早开始胜过多存钱。从25岁到35岁每月定投$100（之后不再投入），往往胜过从35岁到65岁每月定投$100 — 起步阶段多出的10年复利效应，远超后期30年的定投贡献。
复利频率的重要性
利息复利的频率越高，资金增长越快。每日复利胜过每月复利，每月复利胜过每季复利，每季复利胜过每年复利。利率较低时差异很小，但经过数十年后差异会非常显著。
付诸实践
输入您的本金、预期收益、投资期限及可选的每月定投金额，即可查看您的未来余额和增长图表。
Try the Compound Interest Calculator
See your money grow, with an interactive chart.
📈 Open Compound Interest Calculator →