What Time Does: Compounding, the Rule of 72, and Monthly Investing
Time is an important factor in long-term saving and investing. When investment returns are reinvested, they may generate additional returns over time. This is known as compounding.
Understanding how compounding works, why starting earlier can matter, and how regular investing affects investment purchases can help with long-term financial decisions.
The following examples are mathematical illustrations, not investment forecasts. Unless otherwise stated, they assume constant returns and exclude fees, taxes, and inflation.
1. The Power of Compounding
Compounding occurs when returns are reinvested and begin generating additional returns.
For example, suppose $6,000 earns a hypothetical annual return of 7% over 40 years.
If the returns are reinvested annually, the future value would be approximately $89,847.
$6,000 × (1.07)^40 ≈ $89,847
By comparison, if the same $6,000 generates a simple annual return of 7% without reinvestment, the annual return would be $420.
Over 40 years, the accumulated simple interest would total $16,800, producing a combined amount of $22,800, including the original principal.
The difference illustrates how reinvesting returns can affect long-term growth.
However, actual investment returns fluctuate, and losses are possible. The assumed 7% annual return is not guaranteed, and the example does not account for investment fees, taxes, or inflation.
2. Why Starting Earlier Matters
Consider three individuals who invest $200 per month under different contribution schedules.
For this illustration, assume a constant nominal annual return of 6%, compounded monthly, with contributions made at the end of each month.
| Saver | Contribution Period | Total Contributions | Illustrative Value at Age 65 |
|---|---|---|---|
| A | $200/month, ages 25–65 | $96,000 | ~$398,300 |
| B | $200/month, ages 35–65 | $72,000 | ~$200,900 |
| C | $200/month, ages 25–35, then no further contributions | $24,000 | ~$197,400 |
Saver A contributes over 40 years and accumulates the largest illustrative balance.
Saver B starts ten years later and contributes for 30 years.
Saver C contributes for only ten years, then leaves the investment untouched until age 65.
Although Saver C contributes considerably less than Saver B, the illustrative ending balances are similar because Saver C's money remains invested for longer.
The example demonstrates the potential value of time, not a guaranteed investment outcome.
Actual returns vary, and market losses, investment costs, inflation, and taxes can materially change the results.
3. Understanding the Rule of 72
The Rule of 72 is a simple calculation used to estimate how long it may take an amount to double at a constant annual compound growth rate.
Divide 72 by the assumed annual growth rate, expressed as a percentage.
For example:
- At 6% annual growth: 72 ÷ 6 ≈ 12 years.
- At 3% annual growth: 72 ÷ 3 ≈ 24 years.
The Rule of 72 can also help illustrate the effect of inflation.
If prices increase by a constant 3% per year, the general price level would approximately double in 24 years.
This helps explain why long-term financial planning should consider not only how money may grow, but also how its purchasing power may change.
The Rule of 72 is an approximation. Actual doubling times depend on the precise growth rate and compounding assumptions.
4. Monthly Investing and Dollar-Cost Averaging
Dollar-cost averaging is the practice of investing a fixed amount of money at regular intervals, regardless of market prices.
When prices are lower, a fixed contribution purchases more units. When prices are higher, the same contribution purchases fewer units.
Consider the following hypothetical example:
| Investment | Unit Price | Amount Invested | Units Purchased |
|---|---|---|---|
| 1 | $10 | $200 | 20 |
| 2 | $8 | $200 | 25 |
| 3 | $5 | $200 | 40 |
| 4 | $8 | $200 | 25 |
| 5 | $10 | $200 | 20 |
| Total | $1,000 | 130 |
The average cost per unit is approximately $7.69, calculated by dividing the total $1,000 invested by the 130 units purchased.
By comparison, investing the full $1,000 at the initial price of $10 would purchase 100 units.
In this particular example, investing gradually results in more units because prices decline and later recover.
However, if prices rise steadily, investing the full amount earlier may produce a better outcome.
Dollar-cost averaging does not guarantee higher returns, prevent losses, or eliminate investment risk.
Regular investing may be a practical approach when savings become available through monthly income. For money already available to invest, the decision between investing immediately and spreading contributions over time should consider investment risk, market exposure, liquidity needs, and personal comfort with volatility.
The Bigger Picture
Compounding, starting earlier, and regular investing are useful concepts, but none guarantees financial success.
Investment outcomes depend on several factors, including the amount contributed, time invested, actual returns, fees, taxes, inflation, and the level of risk taken.
The purpose of understanding these concepts is not to predict investment returns, but to make more informed decisions about saving, investing, and long-term financial goals.
References
- Financial Consumer Agency of Canada — Savings and Investments
- Canadian Securities Administrators — Investor Education