Compounding: The Most Important Rule in Finance
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Insights from The Mind Money Spectrum Podcast Episode #56
This article is based on Episode 56 of the Mind Money Spectrum podcast, originally published on January 5, 2021. You can listen to the original episode here.
The most important rule in finance may also be one of the simplest: give compounding as much time as possible.
Compounding means that your money can earn a return, and then future returns can build on both your original investment and its accumulated growth. It sounds unremarkable at first. Over a year or two, the effect may be modest. Over several decades, however, it can become the central force behind financial independence.
This principle has practical implications for professionals who want financial security and freedom. Starting early matters. Consistency matters. Avoiding costly mistakes matters. And while investment returns receive much of the attention, your savings rate and spending decisions may be the more powerful levers because they are largely within your control.
Compounding Is Growth on Top of Growth
Consider a simple hypothetical investment of $100 earning 10 percent annually. After one year, it grows to $110. If it earns another 10 percent, the second year does not add only another $10. It adds $11 because the return applies to the full $110. The balance becomes $121.
After a third year, it grows to $133.10. Without compounding, three years of $10 gains would produce $130. Compounding creates the additional $3.10 by generating returns on prior returns.
Three dollars may not seem meaningful, but the effect becomes increasingly powerful as the balance and time horizon grow. The longer the money remains invested, the greater the portion of the ending value that can come from growth rather than contributions.
This is why time is such a valuable financial resource. You can earn more money later, but you cannot go back and give an investment another decade to compound.
Why Starting Early Can Matter More Than Contributing Longer
Imagine two hypothetical investors. The first begins at age 25 and contributes $3,000 annually for 10 years. After age 34, this investor stops contributing but leaves the money invested until age 65.
The second investor waits until age 35 and then contributes $3,000 annually for 30 years. Assuming a hypothetical 10 percent annual return, the first investor would reach age 65 with approximately $917,000. The second would have approximately $540,000.
The first investor contributed only $30,000, while the second contributed $90,000. Yet the person who started earlier finished with substantially more because those first contributions had an additional decade to grow.
This example is an illustration, not a forecast. A 10 percent return is not guaranteed, actual returns vary, and taxes and fees can change the result. The underlying lesson remains important: early contributions have disproportionate value.
For a young professional, this means that a contribution made before income reaches its peak can be especially powerful. Starting with a manageable amount is often better than waiting for the perfect salary, the perfect market, or the perfect financial plan.
It may require tradeoffs. A professional might keep a car longer, buy a used vehicle, delay a technology upgrade, or direct part of each raise toward investments. These choices may feel small in the present, but money invested early has the longest runway.
If You Started Late, Start Now
The lesson about starting early should motivate action, not create regret. Past decisions cannot be changed, and feeling guilty about them does not improve a financial plan.
It is also rarely too late to benefit from compounding. Money saved in your forties or fifties may remain invested for decades. Retirement is not a single day when an entire portfolio suddenly gets spent. Assets may need to support spending through a long retirement, which gives a portion of the portfolio additional time to grow.
Many professionals also reach their peak earning years in their fifties. At the same time, expenses associated with raising children or funding education may begin to decline. That combination can create an opportunity for aggressive saving.
Someone who starts later will usually need to save more than someone who began in their twenties. That is the honest tradeoff. Higher income can help compensate for lost time, but only if part of that income is converted into savings rather than permanently higher spending.
The more dangerous situation is waiting until a year or two before retirement to determine whether the numbers work. If the portfolio cannot support the desired lifestyle, the remaining choices may be limited to spending less or working longer. Planning 10 or 15 years in advance leaves much more room to adjust.
The Rule of 72 Makes Compounding Easier to Understand
The Rule of 72 is a useful mental shortcut for estimating how long an investment will take to double. Divide 72 by the expected annual rate of return:
Estimated years to double = 72 ÷ annual return
At a hypothetical 6 percent annual return, money would double in approximately 12 years. At 8 percent, it would take about nine years. At 10 percent, it would take a little more than seven years.
The Rule of 72 works especially well for return assumptions in the general range investors often consider. It is an approximation, not a promise. Actual investments do not earn the same return every year, and a higher expected return usually requires accepting greater risk.
Still, the rule provides useful intuition. A small difference in annual return can create a large difference over several doubling periods. It also shows why fees deserve attention. Every dollar paid in unnecessary costs is a dollar that no longer remains available to compound.
As a fee-only fiduciary advisor, I believe investment costs should be transparent and justified by the value being provided. Keeping costs reasonable does not guarantee success, but it helps reduce a known drag on the portfolio.
Losses and Gains Are Not Symmetrical
Compounding also explains why investment losses require careful attention. If a $100 investment declines by 50 percent, it falls to $50. A subsequent 50 percent gain adds only $25, leaving the investor with $75. Returning from $50 to $100 requires a 100 percent gain.
The same principle applies to smaller declines. If $100 falls by 25 percent to $75, it needs a gain of approximately 33 percent to return to $100.
This is one reason simple averages can be misleading. A negative 50 percent return followed by a positive 50 percent return has an arithmetic average of zero, but the investor still lost 25 percent. The geometric result reflects what happened to the actual dollars.
For financial planning, the practical lesson is not that every decline must be avoided. Avoiding all volatility would generally mean giving up much of the growth potential needed to outpace inflation. Instead, the portfolio should take an appropriate level of risk for the goal, time horizon, and investor.
Money needed in the near term should not depend on a stock market recovery arriving on schedule. Money intended for goals many years away may have more capacity to accept equity risk. A diversified mix of stocks and bonds can help align the portfolio with those different time horizons without relying on complicated alternative investments.
Inflation Compounds Too
Compounding is not always beneficial. Inflation compounds against purchasing power.
At a hypothetical inflation rate of 2 percent, money that earns nothing would lose approximately half of its purchasing power over 35 years. The account balance may look unchanged, but what that money can buy would be meaningfully lower.
This helps explain the role of stocks in a long-term portfolio. Even near retirement, investors may still need some equity exposure because retirement itself can last for decades. Cash and high-quality bonds can provide stability and support near-term spending, while stocks offer greater long-term growth potential and a better opportunity to outpace inflation.
Gold provides a useful historical illustration. An ounce of gold in ancient Rome may have purchased a full set of clothing, and an ounce of gold in more modern times could also buy a suit. The anecdote suggests that gold may preserve purchasing power over extremely long periods. But preserving purchasing power is not the same as creating real growth.
For financial planning, my preference is to use understandable investments with clear roles. Stocks can provide growth, bonds can provide stability and income, and cash can cover short-term needs. Investors do not need exotic products merely to make a portfolio appear sophisticated.
A Thought Experiment About Manhattan
A familiar story says that Dutch settlers acquired Manhattan in 1626 for goods valued at roughly $24. The historical details are uncertain, and an investable stock market like the one available now did not exist for the people involved. As a thought experiment, however, the story demonstrates the scale of long-term compounding.
If $24 could have compounded at 7 percent for roughly 400 years, it would have grown into trillions of dollars. At 3 percent, the result would have been only a few million dollars. Both outcomes are much larger than the starting amount, but the difference between the two ending values is enormous.
No individual has a 400-year investment horizon. The example is useful because it makes two ideas visible. First, time can turn a small amount into a large amount. Second, a modest difference in the annual rate of growth can become significant when compounded for long enough.
Benjamin Franklin understood this concept. He left funds in trust for Boston and Philadelphia with instructions that the money remain invested for 200 years. What began as a comparatively modest amount eventually grew into millions. His experiment showed that patient capital can have consequences far beyond one lifetime.
Do Not Confuse Investment Returns With Investor Returns
A portfolio can earn one return while the person who owns it receives another. The gap often comes from behavior.
Investors may hold cash while waiting for the ideal entry point, chase an investment after a period of strong performance, or abandon a strategy following disappointing results. These choices can lead them to miss part of the return generated by the investments themselves.
A strong recent result may be interpreted as evidence of skill, while poor recent performance may be treated as proof that a strategy is broken. Yet returns frequently move back toward longer-term averages. Moving money toward whatever has recently performed best can therefore mean buying after gains and selling after declines.
For most investors, a simpler approach is more durable:
- Define what the money is intended to accomplish.
- Match the portfolio to the goal and time horizon.
- Use a diversified allocation of stocks and bonds.
- Keep costs reasonable.
- Invest available long-term capital rather than waiting indefinitely.
- Rebalance according to a disciplined process.
- Avoid changing strategies in response to headlines or recent performance.
The objective does not have to be beating the market. Capturing an appropriate share of market returns while avoiding major behavioral mistakes can be a more realistic and productive goal.
Focus on the Variables You Can Control
Professionals often devote considerable energy to finding an extra percentage point of return. Over long periods, an additional percentage point can matter. But future market returns are outside your control.
Your savings rate, spending, investment costs, asset allocation, and behavior are much more controllable. In financial projections, modest changes to savings and spending can sometimes improve the outcome more than assuming a higher return.
This is valuable because it redirects attention from prediction to action. You do not need to know what the market will do next year to improve your financial plan. You can increase an automatic contribution, invest idle long-term cash, reduce an unnecessary recurring expense, or direct part of a bonus toward a major goal.
Consider the following practical process:
- Identify the purpose of each pool of money. Separate near-term spending needs from long-term goals.
- Protect near-term obligations. Do not rely on stocks for money that must be available on a specific date in the near future.
- Automate long-term investing. Regular contributions reduce the temptation to wait for the perfect moment.
- Increase savings with income. When compensation rises, direct part of the increase toward financial independence before lifestyle expenses absorb it.
- Review fees and complexity. Understand what you own, what it costs, and why it belongs in the portfolio.
- Measure progress against your plan. The relevant question is whether you are on track for your goals, not whether another investor recently earned more.
The Most Important Rule Is Also the Most Actionable
Compounding rewards patience, but patience alone is not enough. Money must first be saved and invested. It must then remain invested through periods when markets are uncomfortable and predictions sound persuasive.
If you are early in your career, begin before the contribution feels impressive. If you are in your peak earning years, use that income intentionally. If retirement is approaching, coordinate your stock and bond allocation with your expected spending rather than abandoning growth entirely.
The best time to start may have been earlier. The best available time is now.
A fiduciary financial plan should connect investment decisions to the life those investments are meant to support. The goal is not mathematical elegance for its own sake. It is to use time, savings, and disciplined investing to create greater security and more freedom over the decisions that matter most.
This material is for general informational purposes and is not intended as individualized investment, tax, or financial advice. All examples are hypothetical or historical illustrations. Past performance does not guarantee future results, and indices cannot be invested in directly.
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