Compound interest pays interest on your interest; simple interest never does. That one difference is small in year one and enormous by year thirty. On $10,000 at 5%, the gap after 30 years is $18,219 — more than the original deposit.
The two formulas, in plain English
- Simple interest: every year you earn 5% of your original $10,000 — a flat $500/year, forever.
- Compound interest: every year you earn 5% of your current balance — $500 in year one, $525 in year two, $551 in year three, and accelerating from there.
$10,000 at 5% — the divergence, year by year
| After | Simple interest | Compounded annually | Compounded monthly | Compound advantage |
|---|---|---|---|---|
| 10 years | $15,000 | $16,289 | $16,470 | +$1,470 |
| 20 years | $20,000 | $26,533 | $27,126 | +$7,126 |
| 30 years | $25,000 | $43,219 | $44,677 | +$19,677 |
Notice the shape: simple interest adds the same $5,000 every decade, while the compound column adds $6,289, then $10,244, then $16,686. That acceleration is the entire case for starting early — the last decade of a 30-year run earns more than the first two combined.
Where you actually meet each one
- Compound (working for you): savings accounts and HYSAs, CDs, reinvested dividends, retirement accounts, index funds.
- Compound (working against you): credit card balances — the most expensive compounding most people ever experience.
- Simple (mostly loans): most auto loans and many personal loans accrue simple interest on the remaining balance — one reason extra payments against principal are so effective.
The frequency footnote
The monthly-compounding column above beats annual compounding, but not by much — $44,677 vs $43,219 over 30 years. How often interest compounds matters far less than the rate and the time. We break that down with exact numbers in daily vs monthly vs annual compounding.