What Compound Interest Actually Is
Simple interest pays you interest only on your original deposit. Compound interest pays you interest on your deposit PLUS on all the interest you've already earned. That feedback loop is what makes it so powerful.
| Year | Simple (5% on $10k) | Compound (5% on $10k) | Difference |
|---|---|---|---|
| 1 | $10,500 | $10,500 | $0 |
| 5 | $12,500 | $12,763 | $263 |
| 10 | $15,000 | $16,289 | $1,289 |
| 20 | $20,000 | $26,533 | $6,533 |
| 30 | $25,000 | $43,219 | $18,219 |
After 30 years, compound interest at 5% produces $43,219 from a $10,000 investment. Simple interest produces only $25,000. The difference of $18,219 cost you exactly nothing — it came from letting interest compound.
The Rule of 72: Mental Math That Actually Works
Divide 72 by your annual return rate to get the years to double your money. • 6% return → 72÷6 = 12 years to double • 8% return → 72÷8 = 9 years to double • 10% return → 72÷10 = 7.2 years to double • 24% credit card APR → 72÷24 = 3 years for debt to double
$5,000 in credit card debt at 22% APR grows to $10,000 in 3.3 years if you only make minimum payments. The same math that builds wealth destroys it when rates are high and you're on the wrong side.
How Compounding Frequency Changes Your Return
The more often interest compounds, the more you earn. On $10,000 at 5% for 10 years:
| Compounding frequency | Final balance | vs Annual |
|---|---|---|
| Annual | $16,289 | — |
| Quarterly | $16,436 | +$147 |
| Monthly | $16,470 | +$181 |
| Daily | $16,487 | +$198 |
The gap between monthly and daily compounding on $10,000 over 10 years is just $17. Compounding frequency matters far less than the rate and how long you stay invested.
The Single Biggest Factor: Time
Starting 10 years earlier is worth more than doubling your contributions. Here's the proof:
| Investor | Starts at | Monthly contribution | Stops at | Balance at 65 |
|---|---|---|---|---|
| Early starter | 25 | $300/month | 65 (40 years) | $792,000 |
| Late starter | 35 | $600/month | 65 (30 years) | $679,000 |
The late starter invests TWICE as much money per month but still ends up with $113,000 less at retirement. Those 10 extra years of compounding the early starter had are worth more than all the extra contributions.
Can only afford $50/month right now? Start anyway. $50/month at 7% for 40 years = $131,000. Waiting 5 years to afford $100/month = $127,000. The early small amount beats the later larger amount.