What Is Compound Interest? The Eighth Wonder of the World

Simple Interest vs. Compound Interest

The gap between the two looks small in year one and enormous in year thirty. That's the part most people don't see until they sit down and run the numbers themselves.

Simple interest: you earn interest only on the original principal. Put $10,000 in at 10% simple, and you earn $1,000 every year. After 30 years you have $10,000 + (30 × $1,000) = $40,000.

Compound interest: you earn interest on the principal and on all the interest already earned. Same $10,000 at 10% earns $1,000 in year one. Year two, 10% on $11,000 = $1,100. Year three, 10% on $12,100 = $1,210. After 30 years, $174,494.

Same starting amount. Same rate. Simple interest gave you $40,000. Compounding gave you $174,494 — more than four times as much. The whole difference is earning returns on the returns. Boring is good.

The Rule of 72

The Rule of 72 is the back-of-the-envelope shortcut for figuring out how long it takes your money to double at a given rate of return:

Years to double = 72 ÷ Annual return (%)

At the S&P 500's historical average of 10%, your money roughly doubles every seven years. Put $10,000 in at age 25 and let it sit there: $20,000 by 32, $40,000 by 39, $80,000 by 46, $160,000 by 53, $320,000 by 60. Look at that last jump. The final doubling, from $160,000 to $320,000, produces more wealth than every previous doubling combined. That's what compounding really is. The big numbers come at the end. Which is exactly why starting early matters more than throwing in larger amounts later, and why I tell every twenty-something who will sit still long enough to listen that the first $1,000 they invest is worth more than the next $20,000 they'll save in their forties.

Time: The Variable That Does the Heavy Lifting

I've watched this play out with clients more times than I can count. Two friends, both well-meaning, both putting money away, both with the same household income. One started saving at 25 and got bored after a decade. The other waited until 35 and tried to make up for it by saving harder for thirty years straight. Here is what happened, in numbers I had to walk them through twice because they didn't believe it the first time:

• Investor A starts at age 25, puts in $5,000 a year for 10 years (total contributed: $50,000), then stops at 35 and lets it ride untouched until 65.
• Investor B starts at age 35, puts in $5,000 a year for 30 years (total contributed: $150,000), all the way to 65.

At 10% annual returns, this is what shakes out:

• Investor A (started early, invested less): roughly $1,400,000 at age 65.
• Investor B (started late, invested more): roughly $900,000 at age 65.

Investor A put in $50,000 and finished with $1.4 million. Investor B put in $150,000 — three times as much — and finished $500,000 behind. The ten-year head start was worth more than thirty years of catching up. In compounding, time beats money. Almost every time.

I think of compounding the way I think of the maple tree my husband and I planted in the backyard the year our oldest was born. For the first five years it looked like a stick with leaves. You water it, you mulch it, you check on it, and nothing seems to happen. Then somewhere around year fifteen you walk out one morning and realise the thing is taller than the house and shading the whole north side of the yard. The growth was happening the whole time. You just couldn't see it until late.

Compounding in the Stock Market

When people say stock returns “compound,” what they actually mean is two things working together inside the same account — dividend reinvestment and capital gains rolling forward inside the same position:

• Dividend reinvestment. Instead of pocketing the dividend in cash, you let it buy more shares. Those new shares pay their own dividends, which buy still more shares. Over decades, reinvested dividends account for roughly 40% of the S&P 500's total return. That's not a footnote. That's nearly half the engine.
• Capital gains compounding. If your portfolio gains 10% in year one, you keep the appreciated shares rather than selling. Year two, you earn 10% on the bigger base. The only thing required of you is the discipline of not selling.

A diversified index portfolio, left alone for decades, quietly does this on autopilot. Active trading gets in the way of it. Every time you sell to lock in a gain, you trigger a taxable event and shrink the base your next year's return is calculated on. Long-term holders preserve the base; active traders chip away at it. That's a real cost, and it shows up over thirty years the way a poorly cut joint shows up in a piece of furniture — not on the day you build it, but eventually, and you can't really fix it after the fact.

The Other Side: Compound Interest on Debt

Compounding doesn't care which direction it runs. When you owe money, it runs against you, and credit card debt at 20% interest compounds just as relentlessly as a stock portfolio at 10% — only faster, and against your future paycheck:

• $5,000 of credit card debt at 20% interest, paying only the minimum, takes roughly 30 years to pay off and costs around $12,000 in total interest. You pay back more than triple what you borrowed.
• This is why paying off high-interest debt comes before investing. A guaranteed 20% “return” from eliminating a 20% debt beats anything you can reasonably expect from the stock market.

No. I won't soften this. If you are carrying a credit card balance at 18% or 22% and you are also funding a brokerage account hoping to outrun it, you are losing ground every single month and you don't see it because the two accounts live on different statements. Pay the cards. Then invest. I've sat across the table from clients in their fifties carrying $30,000 in credit card balances and a small 401(k), and the ones who attacked the cards first slept better within a year. The ones who didn't were still there five years later, still juggling.

Practical Applications

1. Start as early as you possibly can. Even small amounts. $100 a month starting at age 20 at 10% grows to about $630,000 by age 60. Starting at 30, the same $100 a month gets you about $227,000. Ten years of delay cost roughly $400,000.
2. Reinvest every dividend. Turn on automatic dividend reinvestment (DRIP) in your brokerage account. Every dividend put back to work feeds the cycle.
3. Watch the fees. Fees compound against you the same way returns compound for you. A 1% annual fee over 30 years turns what would have been a $500,000 portfolio into about $410,000. A 0.1% fee leaves you with around $485,000. Bogle spent a whole career hammering on this single point: costs are the enemy of compounding.
4. Manage taxes. Every time you sell a stock and realise a capital gain, you hand a slice to the IRS and shrink your compounding base. Tax-deferred accounts — 401(k), Traditional IRA, Roth IRA — let compounding run uninterrupted until you withdraw.
5. Stay patient. Compounding rewards patience in a way that feels almost unfair to anyone watching from outside. The thirtieth year produces more wealth than the first ten combined. The biggest threat to your long-term number is not a market crash. It's the temptation to pull money out for something that felt urgent at the time and didn't turn out to be.

Start with what you can do this week. Open the account. Set up the automatic contribution — even $50 a paycheck, if that's what fits. Turn on dividend reinvestment. The size of the contribution matters less than the date you started. Future-you will not remember the exact amount. Future-you will remember that you started.

See also: What Is an Emergency Fund? · What Are Dividends? · What Is a Dividend Yield? · What Is Inflation? · John Bogle