Compound interest wins. Not because it's a fancier formula, but because it pays you on money you never deposited — the interest your interest already earned. With simple interest, only your original principal ever earns.
Here's the honest comparison, using $10,000 at a 7% annual return, with numbers you can reproduce on our Compound Interest calculator.
The difference in plain numbers
Same starting amount, same 7% rate, different math:
| Time | Simple interest | Compound (annual) | Compound (monthly) | Extra vs simple |
|---|---|---|---|---|
| 10 years | $17,000 | $19,672 | $20,097 | +$3,097 |
| 20 years | $24,000 | $38,697 | $40,387 | +$16,387 |
| 30 years | $31,000 | $76,123 | $81,165 | +$50,165 |
After 10 years the difference is meaningful. After 30 years, compounding has produced more than 2.6× the simple-interest balance — $81,165 vs $31,000 — even though both started with the same $10,000 at the same rate.
Why the gap explodes over time
The formulas look similar, but behave very differently:
- Simple interest:
FV = P × (1 + r × t). Growth is linear — you earn 7% of the original $10,000 every year, forever. - Compound interest:
FV = P × (1 + r/n)^(n×t). Each year you earn 7% of a larger balance, so the base keeps growing. This is exponential growth.
That's why the gap is small at year 5 and huge at year 30. Exponential curves start slow — then bend sharply upward. Time is the variable that decides whether compounding matters for you at all.
Monthly vs annual compounding
Compounding frequency matters, but less than you might think. In the table above, monthly compounding beats annual by a small margin (~$81,165 vs $76,123 over 30 years). The bigger lever by far is time in the market, not how often interest is credited.
The takeaway
If someone offers you "interest," ask which kind. For savings and investing, compound interest is the engine of long-term wealth — and the earlier you start, the more years the curve has to bend upward. You can model your own numbers — starting amount, contributions, rate and frequency — in the Compound Interest calculator.