WealthThe Rule of 72: Double Your Money With One Simple Calculation
How the Rule of 72 Works
The formula is elegantly simple:
Years to double ≈ 72 ÷ annual return rate (%)
For example, if your portfolio earns 6% per year, your money doubles in roughly 12 years (72 ÷ 6 = 12). At 8%, you double in 9 years. At 9%, in 8 years. It’s a practical shortcut rooted in compound interest — where every dollar of return begins earning its own return, and the pace accelerates over time.
Table: Annual Return Rate → Years to Double
| Annual Rate | Years to Double (Rule of 72) | Exact Value |
|---|---|---|
| 3% | 24 years | 23.4 years |
| 4% | 18 years | 17.7 years |
| 5% | 14.4 years | 14.2 years |
| 6% | 12 years | 11.9 years |
| 7% | 10.3 years | 10.2 years |
| 8% | 9 years | 9.0 years |
| 9% | 8 years | 8.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6 years | 6.1 years |
The Rule of 72 is most accurate between 5% and 12% — within that range, the margin of error stays under half a year.
Lean: double in 10 years
- Requires an annual return of roughly 7.2% (72 ÷ 10)
- Sets a higher, more demanding return target
- Useful if your time horizon is genuinely shorter
Lean: double in 15 years
- Requires an annual return of about 4.8%
- Sets a more realistic, achievable return target
- A practical way to recognize that some goals simply require more time than expected
Use It in Reverse: What Return Do You Need?
The rule works both ways. Want to double your money in 10 years? You’ll need an annual return of roughly 7.2% (72 ÷ 10 = 7.2). In 15 years? About 4.8%. This is a practical way to set realistic return targets — or to recognize that some goals simply require more time than expected.
For those pursuing FIRE in Canada, this tool helps visualize how quickly a retirement nest egg can compound depending on your asset allocation.
| Silent drag | Annual rate | Doubling time (Rule of 72) | Real-world impact |
|---|---|---|---|
| Inflation | 3% | 24 years | $100,000 today ≈ $50,000 of purchasing power by 2050 |
| Management fees (MER) | 2% | 36 years (a full doubling lost) | Over 30 years, a 0.2% ETF vs. a 2% mutual fund can differ by tens of thousands of dollars |
The Flip Side: Inflation and Fees
Here’s where the Rule of 72 becomes genuinely eye-opening — and a little unsettling. It applies to any consistent rate of growth or erosion, not just investment returns.
Inflation Erodes Your Purchasing Power
At 3% inflation — near the upper end of the Bank of Canada’s historical target range — prices double in 24 years. That means $100,000 today will only have the purchasing power of roughly $50,000 by 2050. Money sitting in a savings account earning 0.5% loses ground every decade.
Management Fees Steal Doublings
A 2% management expense ratio (MER) — common in some Canadian mutual funds — represents a 2% annual drag on your net returns. Apply the rule: that drag costs you a full doubling every 36 years (72 ÷ 2). Over 30 years, the difference between a 0.2% index ETF and a 2% mutual fund can amount to tens of thousands of dollars on the same starting capital.
Lean: use the Rule of 72
- Good for quick conversations and first-pass decisions
- Works well when the rate is roughly within the 5–12% range
- Assumes a constant, hypothetical average annual return
Lean: use the exact formula or a calculator
- Needed outside the 5–12% range, where the gap with the true value widens
- Needed to account for taxes on capital gains and dividends in a taxable account
- Better for precise projections since returns are not guaranteed and can fluctuate year to year
Limitations of the Rule of 72
The rule is a shortcut, not an exact formula. Keep these boundaries in mind:
- Less accurate outside the 5–12% range: at very low rates (1–2%) or very high rates (20%+), the gap with the true value widens. For precision, use the exact formula
ln(2) / ln(1 + r)or a financial calculator. - Assumes a constant rate: real-world returns fluctuate year to year. The Rule of 72 applies to a hypothetical average annual return.
- Doesn’t account for taxes: in a taxable account, capital gains and dividends reduce your effective return. Holding investments in a TFSA or RRSP removes this friction.
- Returns are not guaranteed: as noted by GetSmarterAboutMoney (OSC), equity investments carry the risk of capital loss. The Rule of 72 is a planning tool, not a promise.
For precise projections, use a proper compound interest calculator or model your retirement timeline with the 4% rule.
Putting It Into Practice
The Rule of 72 shines in quick conversations and first-pass decisions:
- Compare two investment scenarios in seconds without a calculator.
- Make the case for investing early — at 25 with a 7% return, your money doubles roughly three times before age 55 (about 10 years per doubling).
- Evaluate the real cost of fees before choosing a fund.
- Explain inflation’s long-term impact in plain language to a friend or family member.
It doesn’t replace a full financial plan, but it gives you a reliable compass for quick, confident decisions.
⏳ Calculator: the Rule of 72
Enter an annual return to see how many years your money takes to double.
Educational approximation — real returns vary and aren't guaranteed.
Frequently asked questions
How accurate is the Rule of 72?
Very accurate for rates between 5% and 12% — the margin of error typically stays under half a year in that range. Outside of it, the approximation degrades. For exact figures, use the formula ln(2) / ln(1 + r) or an online calculator.
What return do I need to double my money in 10 years?
Apply the rule in reverse: 72 ÷ 10 = 7.2%. You’d need an average annual return of approximately 7.2% to double your money in a decade. Keep in mind that no return is guaranteed — this is a planning benchmark, not a prediction.
Does the Rule of 72 apply to inflation?
Yes — and this is one of its most powerful uses. At 3% inflation, prices double in 24 years (72 ÷ 3). If your savings aren’t growing at least as fast, your purchasing power is quietly shrinking.
Is it the same as compound interest?
The Rule of 72 is an approximation built on the mathematics of compound interest. It doesn’t calculate interest itself — it simply tells you how long compounding takes to double your money at a given rate.
Does it work for debt too?
Absolutely. A credit card charging 18% interest will double the balance owed in just 4 years (72 ÷ 18) if you make no payments. That’s a compelling reason to prioritize paying off high-interest debt first.
Where can I find reliable Canadian resources on investing?
GetSmarterAboutMoney.ca, run by the Ontario Securities Commission, offers free, unbiased tools and guides for Canadian investors. The Bank of Canada publishes inflation data and policy rate history at bankofcanada.ca.
Sources & references
Educational content; verify figures with official sources before acting.