What Is Compound Interest?
The most powerful force in finance. Your money earns money, then that money earns money — forever. Here is how it actually works.
$1.1M
$500/mo for 30 yrs
Rule of 72
Doubling Shortcut
Exponential
Growth Pattern
What Is Compound Interest?
Compound interest is interest earned on your principal AND on all previously earned interest. Unlike simple interest (which only earns on the original amount), compound interest creates a snowball effect — each period's interest becomes part of the base that earns the next period's interest.
Einstein reportedly called compound interest the "eighth wonder of the world," adding: "He who understands it, earns it; he who doesn't, pays it." Whether or not Einstein actually said this, the math is undeniable. Compound interest is the engine behind every successful long-term investment.
Simple vs Compound — $10,000 at 10% for 30 years:
Simple Interest:
$40,000
$10,000 + ($1,000/yr x 30)
Compound Interest:
$174,494
4.4x more than simple interest
The Rule of 72
The Rule of 72 is a simple formula to estimate how long it takes your money to double: divide 72 by the annual return rate.
| Annual Return | Years to Double | Context |
|---|---|---|
| 1% | 72 years | Traditional savings account |
| 4% | 18 years | High-yield savings / bonds |
| 7% | 10.3 years | Conservative stock portfolio |
| 10% | 7.2 years | S&P 500 historical average |
| 12% | 6 years | Aggressive growth portfolio |
At 10% returns, $10,000 doubles to $20,000 in 7.2 years, then to $40,000 in 14.4 years, $80,000 in 21.6 years, and $160,000 in 28.8 years. The same $10,000 in a savings account at 1% would only be $13,400 after 30 years.
The Power of Starting Early
Compound interest rewards time above all else. Starting 10 years earlier is worth more than doubling your monthly contribution.
Key insight:
Alice invested only $60,000 more than Bob but ended up with $1.5 million more. Carol invested DOUBLE what Bob invested ($360K vs $180K) but still did not catch Alice. The extra 10 years of compounding was worth more than doubling the contribution. Time beats money.
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Compound Interest Works Against You Too
Credit cards, student loans, and mortgages charge compound interest on what you owe. A credit card at 24% APR compounds your debt just as ruthlessly as the stock market compounds your investments.
Compound interest FOR you:
$10,000 invested at 10% = $174,494 in 30 years
Compound interest AGAINST you:
$10,000 credit card debt at 24% (min payments only) = $42,000+ in interest paid
Priority order: Pay off high-interest debt first (anything above ~7%). Then invest. Earning 10% in the stock market while paying 24% on credit cards is losing 14% per year on that money.
Glen's Take
Compound interest is not complicated. It is just slow — and most people quit before it gets interesting.
The first 10 years of investing feel frustratingly slow. You contribute $60,000 and your portfolio is worth maybe $100,000. The magic happens in years 20-40, when compound interest does more work than you ever could. Warren Buffett made 99% of his wealth after age 50. Not because he was a better investor at 50 — because compound interest needed time to compound.
Open a Roth IRA. Buy a low-cost index fund. Set up automatic monthly contributions. Then wait 30 years. Compound interest will do the rest.
Frequently Asked Questions
What is compound interest in simple terms?
Compound interest is interest earned on both your original investment AND on the interest you have already earned. It is money making money on money. With simple interest, you earn interest only on your original amount. With compound interest, each year's interest becomes part of the base that earns next year's interest — creating exponential growth.
What is the Rule of 72?
The Rule of 72 is a shortcut to estimate how long it takes your money to double. Divide 72 by the annual interest rate. At 10% returns (stock market average), your money doubles every 7.2 years. At 4% (savings account), it doubles every 18 years. At 1% (traditional savings), it takes 72 years to double.
How does compound interest build wealth?
Compound interest creates exponential growth. $500/month invested at 10% returns grows to $113,000 in 10 years, $380,000 in 20 years, and $1.1 million in 30 years. You only contributed $180,000 over 30 years — the other $920,000 is compound interest. The longer the time period, the more dramatic the growth.
What is the difference between compound and simple interest?
Simple interest is calculated only on the original principal. $10,000 at 5% simple interest earns $500/year forever — always $500, never more. Compound interest is calculated on the principal plus all accumulated interest. $10,000 at 5% compound interest earns $500 in year 1, $525 in year 2, $551 in year 3, and so on — each year's interest is larger than the last.
How often should interest compound?
The more frequently interest compounds, the faster your money grows — but the difference is small. Daily compounding earns slightly more than monthly, which earns slightly more than annual. For practical purposes, the compounding frequency matters far less than the interest rate and the time horizon. Focus on earning a higher rate for a longer period.
Recommended Resources
Tools & books I actually use and recommend
TradingView
Best charting platform out there. Real-time data, screeners, and a community of millions of traders.
Try TradingViewThe Psychology of Money
Morgan Housel on why managing money is about behavior, not intelligence. Short, brilliant chapters you'll re-read.
View on AmazonThe Intelligent Investor
Ben Graham's timeless guide to value investing. The book Warren Buffett calls "the best investing book ever written."
View on AmazonSome links above are affiliate links. I only recommend products I personally use. See my full disclosures.
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