How Compound Interest Works (With Real Examples)

Compound interest is interest earned on your original money plus the interest it has already earned. Over time, that snowball effect becomes the single biggest driver of long-term wealth.

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Simple vs. compound interest

With simple interest, you earn interest only on the original amount. $1,000 at 5% simple interest earns $50 every year, so after 10 years you have $1,500.

With compound interest, each year’s interest is added to the balance, and next year’s interest is calculated on the bigger number. $1,000 at 5% compounded annually grows to about $1,629 after 10 years.

The compound interest formula

The standard formula is:

A = P × (1 + r/n)^(n × t)

• A = final amount
• P = starting principal
• r = annual interest rate (as a decimal)
• n = number of times interest compounds per year
• t = number of years

A worked example

Suppose you invest $10,000 and add $500 every month for 20 years at an average 7% annual return, compounded monthly.

What	Amount
Total you contribute	$130,000
Approximate final balance	~$300,000
Growth from compounding	~$170,000

More than half the final balance comes from growth, not from your contributions. Try your own numbers with our compound interest calculator.

Why starting early matters

Time is the most powerful input. Someone who invests for 30 years usually ends up with far more than someone who invests the same total amount over 15 years, because the early money compounds for longer.

Key takeaways

• Compound interest is interest on interest.
• The longer your time horizon, the stronger the effect.
• Regular contributions plus time beat trying to time the market.
• Returns are never guaranteed, and real investments go up and down.