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.
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