How Compound Growth Builds Wealth

Most people think of building wealth as a question of how much they put away. Over a working life, a second question matters at least as much: how long that money has to grow. This chapter explains how compounding works, why the years you give it beat almost any other lever you control, how to estimate doubling times in your head, and the four things that quietly undo it.

Interest that earns interest

Money can grow in two ways. With simple interest, you earn a return only on what you put in. With compound growth, each year's return is added to the balance, and next year's return is earned on the larger total. The return starts earning its own return.

The difference is small in the first few years and enormous later. Take one deposit left alone for three decades at a steady 7% a year.

The deposit of $10,000 grows to about $76,123. Of that, $66,123 is growth that nobody added. Simple interest at the same 7% would add 7% of the original deposit each year, so after 30 years you would have the deposit plus 210% of it, a little over three times what you started with. Compounding ends at more than seven times. The rate is the same in both cases; the only difference is that compounding lets each year's gain join the base.

The formula behind it is short: a balance after some number of years equals the starting amount multiplied by one plus the yearly return, raised to the power of the number of years. The years sit in the exponent, which is why they matter so much.

The 7% used here is an assumption for illustration, not a forecast. For context, US large-company stocks returned an average of 10.02% a year, compounded, from 1928 to 2025 with dividends reinvested, or 6.78% after inflation. Those averages hide years of large losses, which chapter 5 covers, and nothing guarantees the next few decades will look the same.

Time is the lever you control

You cannot choose the market's return. You can choose when you start and how steadily you add. The two examples below compare a saver who starts early with modest amounts against one who starts ten years later and puts in twice as much each month.

The early saver puts in $144,000 and ends with about $741,463. The later saver puts in $216,000, more money in total, and ends with about $701,672. Doubling the monthly amount did not make up for the ten missing years, because those first years are the ones that compound the longest. Money invested in your twenties has four decades to grow; money invested in your fifties has one.

This is not a reason to despair if you are starting late. It means the most valuable thing you can do is begin with whatever amount you can manage now, rather than waiting until you can afford a larger amount. A small sum started today often beats a larger sum started in five years.

The rule of 72

A quick way to feel compounding is to ask how long money takes to double. Divide 72 by the yearly return in percent, and the answer is roughly the number of years.

• At 4% a year, money doubles in about 18 years.
• At 6%, about 12 years.
• At 8%, about 9 years.
• At 10%, about 7 years.

The rule also works in reverse. Inflation of 3% a year halves the buying power of cash in about 24 years. A yearly fee of 1% takes about 72 years to cost you half your money if nothing else happened, but against a portfolio that is also growing, it removes a much larger share of the final balance than that suggests, as chapter 7 shows with real numbers.

The rule is an approximation that works best for rates between about 4% and 12%. It is a mental shortcut, not a substitute for a calculation.

Nominal returns and real returns

A balance that grows to a large number in thirty years does not buy what that number buys today. Prices rise, so the useful measure is the real return: the growth left after inflation. The real return is roughly the nominal return minus inflation; precisely, it is one plus the nominal return divided by one plus inflation, minus one. A 7% return with 3% inflation is a real return of about 3.9%.

Here is the early saver again, measured in today's dollars.

In today's buying power the early saver ends with about $338,442, not $741,463. That is still far more than the $144,000 put in, but it is the figure to plan with. Many calculators, including ours, let you choose whether results are shown before or after inflation; check which one you are reading. The real return calculator converts any nominal return into a real one.

The same logic explains why long-term money held entirely in cash tends to lose ground. From 1928 to 2025, 3-month Treasury bills returned an average of 3.37% a year while inflation averaged 3.04%. Cash is the right home for money you need soon. Over decades, it barely kept pace with prices.

What breaks the chain

Compounding works only while the money stays invested and keeps most of its return. Four things erode it.

Costs. Fund expenses and advisory fees are taken from your balance every year, so they compound against you just as returns compound for you. A difference of one percentage point a year can cost a large share of a final balance over thirty years.

Taxes. Interest, dividends and gains that are taxed each year shrink the base that compounds. Tax-advantaged accounts such as a 401(k), an IRA or an HSA let the full amount keep growing, and in a taxable account, funds that trade rarely create fewer taxable gains.

Selling after a fall. Markets drop sharply from time to time. Someone who sells after a fall locks in the loss and often misses the recovery, which tends to arrive without warning. The years you are out of the market are years that do not compound.

Inflation. As above, growth that only matches inflation leaves you where you started in buying power.

None of these is dramatic in a single year. All of them are large over a working life, and all of them are partly within your control.