The difference between the two determines whether your money grows linearly or exponentially. Over decades, it's the difference between comfort and wealth.
Albert Einstein allegedly called compound interest the "eighth wonder of the world." Whether or not he actually said that, the math backs it up. The gap between compound and simple interest starts small and grows to an almost unbelievable difference over long time horizons.
Here's everything you need to know — with the numbers laid out clearly so you can see exactly what you're gaining (or losing) depending on which type of interest applies to your money.
Simple interest calculates on the original principal only. Compound interest calculates on the original principal plus all previously earned interest.
Interest is always calculated on the starting amount. $10,000 at 5% always earns $500/year, no matter how long you hold it.
Interest earns interest. Your balance grows faster each year because last year's interest is now part of the base earning this year's interest.
| Year | Simple Interest Balance | Compound Interest Balance | Difference |
|---|---|---|---|
| 1 | $10,600 | $10,617 | $17 |
| 5 | $13,000 | $13,469 | $469 |
| 10 | $16,000 | $18,194 | $2,194 |
| 20 | $22,000 | $33,102 | $11,102 |
| 30 | $28,000 | $60,226 | $32,226 |
Same starting amount, same rate, same 30 years. Compound interest produces more than double the final balance. The extra $32,226 came entirely from interest earning interest — you did nothing extra.
💡 The Rule of 72: Divide 72 by your interest rate to estimate how long it takes to double your money. At 6%: 72 ÷ 6 = 12 years to double with compound interest. With simple interest, it takes 16.7 years to double at the same rate.
Compound interest can compound daily, monthly, quarterly, or annually. The more frequently it compounds, the more you earn — because shorter intervals mean interest starts earning faster.
| $10,000 at 6% for 10 Years | Final Balance | Interest Earned |
|---|---|---|
| Simple Interest | $16,000 | $6,000 |
| Compound — Annually | $17,908 | $7,908 |
| Compound — Quarterly | $18,140 | $8,140 |
| Compound — Monthly | $18,194 | $8,194 |
| Compound — Daily | $18,221 | $8,221 |
See exactly how compounding frequency affects your money with real numbers using our compound interest calculator — plug in any rate and compounding interval.
📈 Try the Compound Interest CalculatorSimple interest isn't just a teaching tool — it's used in real financial products:
For these products, the simple interest formula I = P × r × t gives you a useful estimate, though actual amortization schedules can be more complex.
⚠️ Warning: Compound interest works against you on debt exactly as powerfully as it works for you on savings. A $5,000 credit card balance at 24% APR compounded daily grows to $6,272 in just 12 months if unpaid — not $6,200 as simple interest would suggest.
There are two scenarios where simple interest is the better option:
Adding regular contributions dramatically widens the gap. Here's what happens when you contribute $1,000/month starting from $0 at 7% for 30 years:
| Method | Total Invested | Final Balance | Extra from Growth |
|---|---|---|---|
| Simple Interest | $360,000 | $738,000 | $378,000 |
| Compound Interest (monthly) | $360,000 | $1,219,988 | $859,988 |
With compound interest, you invest the same $360,000 but end up with $481,988 more — that's money created purely by the compounding effect.
Run these numbers with your own principal and monthly contribution. Try our compound interest calculator for a full year-by-year breakdown.
📈 Calculate Your Compound GrowthYes — for any fixed rate over any time period longer than one compounding cycle, compound interest generates more growth than simple interest. The longer the period, the larger the advantage.
Even at a lower nominal rate, compound interest can still outperform simple interest if the time period is long enough. Always compare using APY (which accounts for compounding) rather than APR for an apples-to-apples comparison.
Multiply your principal by the rate by the number of years: I = P × r × t. Or use our simple interest calculator to get an instant answer.
For investors and savers: compound interest is always the better outcome, and the advantage grows dramatically over time. Start early, reinvest earnings, and let compounding do the heavy lifting.
For borrowers: compound interest is the enemy. Pay down high-interest compound debt as fast as possible — it's growing faster than simple math suggests.
Understanding this distinction is one of the highest-leverage financial concepts you can learn.