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The Power of Compound Interest

Money growth mechanism

Multiply returns by interest on interest for faster growth.

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Introduction to Compound Interest 

Compound interest is the interest earned on both the initial principal and the accumulated interest from previous periods. 

Unlike simple interest, which grows in a straight line, compound interest creates an accelerating curve. The longer you invest, the faster the growth.

It’s the concept of earning "interest on interest." 

Throughout this lesson, we’ll follow lifelong friends Susan and Mike as their different investment choices demonstrate the impact of compound interest on their financial future.

Simple vs. Compound Interest 

Simple interest is calculated only on the principal. Compound interest, by contrast, multiplies faster because it includes past interest in the calculation.

The difference can be dramatic.

Imagine investing $10,000 at an annual rate of 10%:

  • After 5 years: You'll have $16,105 with compound interest vs. $15,000 with simple interest.
  • After 20 years: You'll have $67,275 with compound interest vs. $30,000 with simple interest.

With compound interest, time can turn steady savings into exponential growth.

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Susan Starts Investing Early 

At age 25, Susan began investing $5,000 annually in a retirement account with a 6% annual interest rate, compounded annually. 

By age 35, after 10 years, her total contributions amounted to $50,000. 

Utilizing the compound interest formula, her investment grew to $65,909. 

Susan's early and consistent investing showcases how compound interest significantly boosts wealth over time, underscoring its importance in long-term financial planning.

The Power of Time and Compounding Frequency 

Investment growth depends on compounding frequency and time. 

More frequent compounding — whether annually, monthly, or daily — accelerates returns by building on previously earned interest. 

Time further amplifies this effect: the longer money stays invested, the greater the exponential growth. 

Even small early investments can grow significantly over decades. 

Starting early and maximizing compounding frequency are key to long-term wealth accumulation.

Mike Starts Investing Later 

At age 30, Mike started investing $5,000 annually in a similar retirement account with a 6% annual interest rate, compounded annually. 

By age 35, after 5 years, his total contributions were $25,000. 

Applying the compound interest formula, his investment grew to approximately $28,185. 

Compared to Susan's $65,909, Mike's later start highlights how time is a crucial factor in compound interest, resulting in a significantly smaller balance despite identical annual contributions.

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