How Compound Interest Works (And Why Starting Early Matters)

Simple vs. compound interest

With simple interest, you earn interest only on your original deposit. Put $1,000 at 5% simple interest and you earn $50 every year — $500 after ten years.

With compound interest, you earn interest on your interest. That same $1,000 at 5% compounded yearly earns $50 the first year, then $52.50 the second (5% of $1,050), then $55.13, and so on. After ten years you have about $1,629 — an extra $129 that came from nowhere but compounding.

The formula

The compound interest formula is A = P(1 + r/n)^(nt), where P is your 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.

You don't need to memorize it — that's what the calculator is for — but knowing the pieces helps. The two levers with the biggest impact are the rate (r) and the time (t). Because time is an exponent, it dominates everything else over long periods.

Why starting early beats saving more

Imagine two savers, both earning 7% a year. Ana saves $200 a month from age 25 to 35, then stops — total contributed: $24,000. Ben waits until 35 and saves $200 a month until 65 — total contributed: $72,000.

At 65, Ana has roughly $340,000. Ben has roughly $244,000. Ana contributed a third as much money but ends up with more, because her money had decades longer to compound. Time in the market beats timing the market.

Compounding frequency

Interest can compound yearly, quarterly, monthly, or daily. More frequent compounding means slightly more money, because each interest payment starts earning interest sooner. The difference between monthly and yearly compounding at 5% on $10,000 over 10 years is about $70 — real, but small compared to the effect of the rate and the time.

The dark side: compounding debt

Compounding works against you with debt. A credit card balance at 24% APR compounded daily grows frighteningly fast. A $5,000 balance with minimum payments can take over a decade to clear and cost thousands in interest. The same math that builds wealth can dig a hole — which is why paying down high-interest debt usually beats investing.