Compound interest is often called the most powerful idea in personal finance, and it works in two directions. It grows savings and investments, and it makes debt grow faster too. Understanding how it works helps you make better choices with both.
Simple versus compound interest
With simple interest, you earn interest only on the original amount. $1,000 at 5% simple interest earns $50 every year, forever.
With compound interest, each year's interest is added to the balance, and the next year's interest is calculated on the larger total.
| Year | Simple (5%) | Compound (5%) |
|---|---|---|
| 0 | $1,000 | $1,000 |
| 1 | $1,050 | $1,050.00 |
| 5 | $1,250 | $1,276.28 |
| 10 | $1,500 | $1,628.89 |
| 30 | $2,500 | $4,321.94 |
The gap starts small and keeps widening, because the compounding version earns interest on a growing base.
The formula
Here A is the final amount, P is the starting amount, r is the annual rate as a decimal, n is how many times per year interest compounds, and t is the number of years.
The Rule of 72
To estimate how long it takes money to double, divide 72 by the annual rate. At 6% it takes about 12 years; at 9%, about 8 years. It is a mental shortcut rather than an exact figure, but it is accurate enough for planning.
The three levers
- Time. The longer the money compounds, the faster it grows. This is the biggest lever and the one you cannot recover once it is gone.
- Rate. A higher return speeds things up, but returns on investments are uncertain and higher returns usually come with higher risk.
- Regular contributions. Adding money every month gives you more to compound. Automating deposits is the simplest way to keep doing it.
Compounding works against you on debt
Credit cards and some other loans compound too, often monthly or daily, and at much higher rates than savings accounts. A balance that is not paid off grows on its own interest. This is why paying down high-interest debt is often a strong financial move: it is the same compounding effect, working in your favor.
Realistic expectations
Projections assume a steady rate, but real returns vary year to year, and fees, taxes and inflation take a share. Use conservative assumptions, and run several scenarios instead of relying on one number. This article is for education and is not financial advice.
To see your own numbers, use the compound interest calculator.