Guide
Compound interest explained
The same force that makes loans expensive makes savings powerful. Here's how it works — and why starting early wins.
Compound interest is often called the most powerful force in personal finance, and the maths backs up the hype. The idea is simple: you earn interest not just on your original money, but on the interest you've already earned. That interest-on-interest is what turns steady saving into serious money — given enough time.
Simple vs compound
With simple interest, you earn the same amount every year, calculated only on your original deposit. With compound interest, each year's earnings are added to the balance, so next year you earn interest on a bigger number. The gap between the two starts small and then explodes.
| Year | Simple (5% on $10,000) | Compound (5% on $10,000) |
|---|---|---|
| 1 | $10,500 | $10,500 |
| 10 | $15,000 | $16,289 |
| 30 | $25,000 | $43,219 |
Same rate, same deposit — but after 30 years compounding has produced almost $18,000 more, and the two lines are still spreading apart. That widening gap is the whole point.
Why time beats amount
Because compounding feeds on itself, the years at the end do the most work — but you only get those years by starting early. This leads to a famously counter-intuitive result: someone who saves for ten years and then stops can end up ahead of someone who starts ten years later and saves for the rest of their life. The early saver's money simply had more time to compound.
The rule of 72
Want a quick estimate of how long money takes to double? Divide 72 by the annual interest rate. At 6%, money doubles in about 72 ÷ 6 = 12 years. At 8%, about 9 years. It's not exact, but it's close enough to do in your head and it makes the power of higher rates obvious.
Compounding frequency matters too
Interest can compound yearly, monthly, or daily. The more often it compounds, the more interest-on-interest you collect. The effect is smaller than people expect, but it's real: $10,000 at 5% for 20 years grows to about $26,533 compounded annually, versus about $27,126 compounded daily. That's why savings accounts advertise an APY (which bakes in compounding frequency) rather than the plain rate — it lets you compare like with like.
See it for your own numbers
Plug in a starting amount, a monthly contribution, a rate and a time span with the compound interest calculator and watch the curve bend upward. The same maths runs in reverse on money you borrow — which is exactly why understanding how loan payments work is the other half of this story.