How Compound Interest Works (And Why Time Matters More Than Rate)

Compound vs simple interest

Simple interest is always calculated on the original amount. Compound interest is calculated on the original amount plus all interest added so far. On 10,000 at 5% for 3 years, simple interest pays 500 each year (1,500 total). Compound interest pays 500, then 525, then 551.25 - because each year starts from a bigger balance.

The compound interest formula

P is the starting amount (principal), r is the annual rate as a decimal, n is how many times a year it compounds, and t is the number of years. A is the final balance. Example: 10,000 at 7% compounded monthly for 10 years. A = 10,000 x (1 + 0.07/12) ^ (12 x 10) = 10,000 x 1.00583 ^ 120 = about 20,097. The money roughly doubles.

Why compounding frequency matters

The more often interest compounds, the more you earn, because interest starts earning interest sooner. The same 10,000 at 7% for 10 years grows to about 19,672 compounded annually, but about 20,097 compounded monthly. The gap widens with higher rates and longer periods, but the effect of frequency is smaller than most people expect - rate and time dominate.

The rule of 72

To estimate how long money takes to double, divide 72 by the interest rate. At 6%, money doubles in roughly 72 / 6 = 12 years. At 9%, about 8 years. It is an approximation, but a useful one for fast mental math.

Why starting early beats a higher rate

Time is the most powerful input because the exponent grows. Investing 5,000 a year from age 25 to 35 (ten years, then stopping) often ends up larger by retirement than investing the same amount from 35 to 65 (thirty years), because the early money compounds for decades longer. The lesson: contributions made earlier do more work than a slightly better rate later.

Regular contributions

Most real savings add money each month, not just once. Regular contributions compound too, each one growing from the date you add it. A modest monthly top-up often outgrows the starting balance over long periods, which is why automatic monthly saving is so effective.

Use the Compound Interest Calculator

The Compound Interest Calculator does the full formula for you, including monthly or yearly contributions, your chosen compounding frequency, and an optional inflation adjustment so you can see the real (purchasing-power) value. Enter your numbers and it shows the final balance, total contributed, and interest earned, plus a year-by-year breakdown.

Frequently asked questions

Is compound interest good or bad?

It depends which side you are on. On savings and investments it works for you. On credit card debt it works against you, because the same snowball applies to what you owe. Paying off compounding debt early saves a surprising amount of interest.

What is a realistic rate to assume?

For long-term stock market returns, many people model 6-8% before inflation as a rough planning figure, though actual returns vary year to year and are never guaranteed. Cash savings typically pay much less. Use a conservative number and treat the result as an estimate, not a promise.