How Money Grows
Compounding, the Rule of 72, and the different ways an investment pays you — capital gains, dividends and interest.
Growth on top of growth
Compounding is what happens when your returns start earning returns of their own. In year one you earn a return on the money you put in. In year two you earn a return on your original money plus year one’s gain. Each year the base gets a little bigger, so the same percentage return produces a bigger dollar gain.
Here is a simple example. Put $1,000 into something that grows 10% a year. After year one you have $1,100. In year two, 10% of $1,100 is $110, so you end at $1,210. Year three adds $121, and so on. The percentage never changes, but the dollars keep growing.
Simple vs. compound growth
With simple interest you only ever earn on the original amount, so growth is a straight line. With compounding the line curves upward, slowly at first and then faster. Over short periods the difference looks small. Over decades it becomes enormous.
Illustration: $1,000 at 8% a year for 30 years. Simple growth reaches $3,400; compounded growth reaches about $10,063.
The Rule of 72
You do not need a calculator to estimate compounding. Divide 72 by the yearly growth rate, and the result is roughly how many years it takes money to double. At 8% a year, 72 ÷ 8 = 9 years. At 6%, about 12 years. At 12%, about 6 years.
The rule also works in reverse for things that shrink your money. If inflation runs at 3%, prices roughly double in about 24 years, which means cash loses about half its buying power over that stretch.
| Yearly growth | Years to double (≈ 72 ÷ rate) |
|---|---|
| 3% | ≈ 24 years |
| 6% | ≈ 12 years |
| 8% | ≈ 9 years |
| 12% | ≈ 6 years |
The Rule of 72 is an estimate, and it is most accurate for rates between about 6% and 10%.
Where returns come from
Investments can pay you in three main ways, and together they make up your total return.
- Capital gains: selling an asset for more than you paid. Buy at $40, sell at $55, and you have a $15 per share capital gain.
- Dividends: cash some companies pay to shareholders out of profits, often every quarter.
- Interest: payments you receive for lending money, such as from bonds or a savings account.
Realized vs. unrealized
A gain is unrealized while you still hold the investment. It exists on paper and can grow, shrink or disappear as the price moves. It becomes realized when you sell and lock in the result. The same goes for losses: a stock that is down is an unrealized loss until you sell.
This distinction matters for taxes in many countries, because gains are generally taxed when realized. It also matters for your mindset. An unrealized loss is not permanent unless the business is broken, but it is also not "not a loss" just because you have not sold.
Reinvesting: the compounding engine
Compounding only works if returns stay invested. Many brokers let you automatically reinvest dividends into more shares. Those extra shares then earn their own dividends and price gains. Taking every dividend out as cash slows the snowball.
Costs work against compounding too. A fee of 1% a year does not sound like much, but it is taken every year from a growing balance, so over decades it can consume a large share of your total gains. Low-cost funds let more of the growth stay yours.
The three levers
You can control three things that drive compounding: how much you invest, how long you stay invested, and how much you lose to fees and poor decisions. The return rate itself is the one lever nobody controls, which is why starting early and staying consistent carry so much weight.
Putting numbers to it
Let’s see compounding with a monthly habit. Suppose you invest $200 every month and your investments grow at an average of 7% a year. After ten years, you have contributed $24,000, and growth adds a meaningful amount on top. After thirty years, you have contributed $72,000, but compounding means the balance can be several times larger than what you put in, because the later years are driven mostly by growth on earlier growth. Real returns vary year to year, and some years are negative, so treat any projection as an illustration rather than a promise.
The takeaway is visible in the chart earlier in this lesson: the curve is flat for a long time and then bends sharply upward. Many people quit during the flat part. Knowing the shape ahead of time makes it easier to stay patient.
Key takeaways
- Compounding means earning returns on your earlier returns, so growth accelerates over time.
- Rule of 72: divide 72 by the yearly rate to estimate years to double.
- Total return = capital gains + dividends + interest.
- Gains are unrealized until you sell; then they become realized.
- Reinvesting and keeping fees low let compounding do more of the work.