What it means
Compound growth is hard to picture, and the Rule of 72 turns it into a number you can work out in your head. At 6% a year, money doubles in about 72 / 6 = 12 years, and it works in reverse too: to double your money in 8 years, you need about 72 / 8 = 9% a year.
The rule applies to anything that grows by a steady percentage, so a debt growing at an effective 24% annually would roughly double in three years if nothing were paid, although real card terms, payments and charges complicate that comparison. Costs rising with 4% inflation double in about 18 years.
It is equally useful for showing the cost of standing still, since at a steady 3% inflation rate a fixed amount of cash would lose about half its buying power in roughly 24 years, though actual inflation changes. It is an estimate, not an exact answer, and its accuracy depends on the rate and how compounding works.
At high rates the shortcut can miss the exact answer materially, so use a calculator for plans and commitments. For business owners, the rule makes growth targets tangible.
A company growing revenue by 18% a year doubles in about four years, and that scenario can frame capacity, hiring and funding plans, but 18% must remain an assumption, not a forecast. Keep nominal and real growth separate, because a 7% nominal return with 3% inflation does not mean buying power grows by 7%.
If you compare assets with rising costs, estimate after-fee, after-tax growth and inflation on consistent assumptions. The reverse calculation gives a required rate, not a promised return, so to test a target properly, calculate compound growth over the full period and model a range of possible outcomes.
In practice
Real-world examples.
Example
A founder wants revenue to double from $1 million to $2 million in three years. The Rule of 72 says she needs growth of about 24% a year.
Example
A supplier raises prices by 8% a year. At that pace, the cost of materials doubles in about nine years, which the owner builds into long-term pricing.
Example
A business owner carries a $20,000 balance on a card charging 18% interest. Left unpaid, the balance would reach about $40,000 in four years.
Formula
Calculation
Years to double = 72 / Annual rate (in %)
Rate needed = 72 / Years to double
Worked example. Compare three uses of $100,000:
- Savings account at 3%: 72 / 3 = 24 years to double
- Balanced fund averaging 7%: 72 / 7 = about 10.3 years
- Reinvesting in a business returning 15%: 72 / 15 = about 4.8 years
Check against exact compounding at 7%: $100,000 x 1.07 ^ 10.3 is about $200,700, so the estimate is very close.
The shortcut weakens at high rates. At 24%, the rule gives 72 / 24 = 3 years, but exact compounding gives 1.24 ^ 3 = 1.907, so $100,000 grows to about $190,700 in three years and needs a little over three years (about 3.2) to reach $200,000. That small gap is why plans and commitments should use a calculator.Case study
Seen in the real world.
This illustrative and entirely fictional example follows Oakridge Dental, an invented two-surgery practice. The owner kept $400,000 of surplus cash on deposit earning 1% while local costs rose around 4% a year. His adviser showed him that at 1% the cash would take 72 years to double, while costs would double in about 18. The owner moved half the cash into a higher-yielding deposit and used the rest to fund a third surgery chair modelled at a hypothetical 20% annual return.
The rule framed the comparison, but the owner still assessed liquidity, risk and the uncertainty of the chair projection. A second scenario tests slower customer demand and a higher installation cost. In that case, the chair no longer meets its target return. The owner keeps a cash reserve rather than treating the shortcut as a reason to invest every spare dollar.
Watch out
Common mistakes.
- Treating the result as exact. It is an estimate that works best between about 6% and 10%.
- Using it for returns that are not steady. It assumes a constant rate, which real investments rarely deliver year by year.
- Forgetting fees and taxes. The rate you plug in should be the return you actually keep.
Questions
People also ask.
Why 72?
72 is close to the precise mathematical figure for doubling, and it divides neatly by many common rates such as 2, 3, 4, 6, 8, 9 and 12.
Is there a Rule of 69 or 70?
Yes. Some use 69 or 70 for more accuracy at low rates or with continuous compounding, but 72 is easier for mental maths.
Can I use it to estimate how fast debt grows?
Yes, as a rough illustration if the effective rate and balance remain steady and no payments are made. Use the agreement and a repayment calculation for real debt; minimum payments, fees and changing rates alter the path.
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