The Complete Guide to
Compound Interest
Everything you need to understand how compound interest works, why it's so powerful, and how to use it to build wealth over time.
Getting Started
Learn the fundamentals of compound interest.
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Simple vs. Compound Interest: Key Differences
Simple interest pays only on your principal; compound interest pays on principal plus past interest. See the gap grow over 10, 20, and 30 years.
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What Is Compound Interest?
Compound interest is interest earned on both your original money and its past earnings. Learn how it works with a simple worked example.
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Compounding Frequency
See how often interest compounds can impact your results.
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Daily Compounding: Why Banks Advertise It
Daily compounding adds interest 365 times a year — it's what turns a 7% APR into a 7.25% APY. See what it's actually worth on a real savings balance.
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How Compounding Frequency Impacts Growth
More frequent compounding earns more, but with sharply diminishing returns. See annual vs monthly vs daily side by side, and the continuous ceiling.
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Key Factors That Matter
The main variables that drive your results.
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The Power of Starting Early: Why Investing at 25 Beats 35
Starting at 25 instead of 35 costs $24,000 more in contributions but ends with roughly $281,000 more at 65. See the age 25/35/45 comparison, computed.
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The Rule of 72: Estimate Doubling Time in Your Head
The Rule of 72 estimates how long money takes to double: divide 72 by your annual return. See where 72 comes from and how accurate it really is.
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Advanced Concepts
Take your understanding to the next level.
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Inflation and Real Returns
A real return is your growth after inflation. The Fisher equation: why 7% nominal at 3% inflation is really 3.88%, and what it does to buying power.
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Taxes and Compound Interest: Understanding Tax Drag
Annual taxes on gains cut the rate your money compounds at — a 7% return taxed hypothetically at 25% compounds like 5.25%. See the 30-year gap, computed.
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Put Your Knowledge into Action
See how these concepts work in real life — model different scenarios with our free calculator.
Start Calculating Now