How compound interest works — and why starting early matters so much
Compound interest is often called the eighth wonder of the world — and for good reason. Once you understand how it actually works, the urgency to start saving immediately becomes very real.
Simple interest vs compound interest
With simple interest, you earn interest only on your original principal. With compound interest, you earn interest on your principal AND on the interest you've already earned. Over time, this difference becomes enormous.
| Simple interest | Compound interest (annual) | Compound interest (monthly) | |
|---|---|---|---|
| Year 10 | $17,000 | $19,672 | $20,097 |
| Year 20 | $24,000 | $38,697 | $40,388 |
| Year 30 | $31,000 | $76,123 | $81,165 |
Why starting early is more powerful than saving more
This is the part that genuinely surprises most people. Consider two investors:
- Maya invests $200/month from age 25 to 35 (10 years), then stops. Total invested: $24,000.
- James invests $200/month from age 35 to 65 (30 years). Total invested: $72,000.
At age 65 with a 7% annual return, Maya has approximately $263,000. James has approximately $243,000. Maya invested for only 10 years and James invested for 30 — but Maya started 10 years earlier and still ends up with more money.
The compounding frequency effect
Interest can compound annually, quarterly, monthly, or daily. More frequent compounding means slightly more growth. Most savings accounts compound daily or monthly. The difference between monthly and annual compounding on $10,000 at 5% over 20 years is about $1,300 — meaningful but secondary to the rate and time horizon.
What "Rule of 72" tells you
Divide 72 by your interest rate to find how many years it takes to double your money. At 7%, your money doubles every 10.3 years. At 10%, every 7.2 years. At 4%, every 18 years. This is a quick mental math shortcut for evaluating any investment.
Time is the most powerful variable in compound interest — more powerful than the amount you save or the rate you earn. Starting today with a small amount beats starting later with a large amount almost every time.