The Rule of 72 is a quick mental-math shortcut. Divide 72 by an annual interest rate or return to estimate how many years it takes for money to double through compounding.

Years to double = 72 / annual return percentage

Example: at 6% per year, 72 divided by 6 equals 12. The shortcut estimates that money doubles in about 12 years.

Rule of 72 table

Why it works

Compounding means returns earn returns. A dollar of interest or investment growth gets added to the base, then the next period's growth applies to the larger amount. Over long periods, this becomes powerful.

GetSmarterAboutMoney says the Rule of 72 is a quick estimate and generally works when the interest rate is under 20%. That is useful for learning, but not enough for planning a retirement portfolio.

How to use it for investing

Use the Rule of 72 for rough scenarios:

• At 4%, doubling takes about 18 years.
• At 6%, doubling takes about 12 years.
• At 8%, doubling takes about 9 years.

Do not assume your ETF or portfolio will earn the same return every year. Markets can have negative years, flat years, and unusually strong years. Fees, taxes, inflation, and currency movement can also change real results.

Why contributions matter more than the shortcut

The Rule of 72 assumes one lump sum growing at a steady rate. Real investors usually contribute over time. A 25-year-old putting $300 per month into a TFSA is not just waiting for one deposit to double. They are building a habit, buying in different market conditions, and increasing the account through both contributions and growth.

This is why a person with a modest return and steady contributions can beat a person who waits years for the "perfect" entry point. The shortcut teaches compounding, but the habit does the heavy lifting.

Nominal return is not real-life purchasing power

If money doubles over twelve years but prices also rise, your purchasing power has not doubled. Inflation matters. So do fees and taxes. A taxable account with interest income can look good before tax and weaker after tax. A registered account can change the after-tax result. A fund with a higher MER needs to earn more before the investor keeps the same return.

For planning, it is often better to run conservative cases: a lower-return case, a middle case, and a bad-sequence case where markets disappoint early. The Rule of 72 is the napkin sketch, not the blueprint.

How to use it for debt

The same shortcut can make high-interest debt feel real. At 18%, unpaid debt doubles in about four years by the Rule of 72. At 24%, the estimate is three years. This is why paying down high-interest debt can be more urgent than investing.

Debt example: why percentages feel small until they compound

A credit-card rate can feel abstract until you flip the Rule of 72 around. At 18%, unpaid debt roughly doubles in four years. That does not mean every real card balance behaves exactly that way, because payments and fees change the math. It does show why carrying high-interest debt while trying to earn a stock-market return is usually a losing race.

Paying down debt is not as exciting as buying an ETF, but a guaranteed reduction in 18% interest is hard for a risky portfolio to beat.

What the Rule of 72 does not include

• Monthly contributions or withdrawals.
• Changing returns from year to year.
• Taxes in non-registered accounts.
• ETF MERs or advisory fees.
• Inflation-adjusted purchasing power.
• Currency conversion for foreign investments.

How to explain it to yourself

A useful way to remember the rule is this: small differences in return look small for one year and large over decades. Four percent roughly doubles in eighteen years. Eight percent roughly doubles in nine. That does not mean you should chase 8% blindly. It means time, cost, risk, and consistency all matter.

If a product advertises a high return, use the rule in reverse. Ask how many years it would take to double. If the answer sounds too good for the risk being described, slow down and verify the claim.

Three ways Canadians can use the shortcut

For TFSA planning: use the Rule of 72 to understand why time matters, then use a full TFSA calculator for actual contribution room and monthly deposits. The shortcut does not know your CRA room.

For RRSP planning: use it to compare return assumptions, but remember that RRSP withdrawals are generally taxable. Doubling inside the account is not the same as doubling after tax.

For debt decisions: use it as a warning label. If unpaid debt can double faster than your investments reasonably can, the debt likely deserves priority.

Do not reverse-engineer unrealistic goals

Some people start with the desired answer: "I want my money to double in three years." The Rule of 72 says that requires roughly 24% per year. That number should make you more cautious, not more aggressive. High required returns usually mean high risk, leverage, speculation, or a scam pitch.

A healthier use is to ask what saving rate, time horizon, and reasonable return range can get you close to the goal without betting the plan on one unrealistic assumption.

Why the shortcut is still worth learning

The Rule of 72 is not precise, but it changes how people see time. A small fee, a small return difference, or a few years of delay can matter more than expected. It also makes high-interest debt easier to understand without a spreadsheet.

Use it as a conversation starter with yourself: What return am I assuming? What risk comes with that return? What happens if the actual return is half as high? Those questions are more valuable than the shortcut itself.

Fees through the Rule of 72 lens

A fee difference can look tiny for one year and meaningful over long periods. If one portfolio earns 6% before fees and another keeps only 5% after higher product and advice costs, the estimated doubling time changes from about 12 years to about 14.4 years. That is not an argument against all advice. Good advice can be worth paying for. It is an argument for understanding what you pay and what you receive.

Use ranges, not one magic return

For long-term planning, run the shortcut at 3%, 5%, and 7% instead of one optimistic number. If the plan only works at the highest return, it may need a higher saving rate, more time, or a more modest goal.

Review the assumption when life changes

A return assumption that made sense at 25 may not fit at 55. As the goal gets closer, the question shifts from "how fast can this grow?" to "how much loss can I still recover from?" Revisit the math when income, timeline, debt, or risk tolerance changes.

Better calculators to use after the shortcut

Once the shortcut gives you intuition, use a full calculator that includes deposits, time, return assumptions, fees, and taxes where relevant.

• Compound interest calculator
• Savings goal calculator
• TFSA calculator
• RRSP calculator

Sources

• GetSmarterAboutMoney.ca - compound interest and Rule of 72
• FCAC - compound interest in savings accounts
• Investor.gov - compound interest calculator