The Rule of 72 in Investing
The Fastest Way to Guess How Long Your Money Takes to Double. No calculator, no spreadsheet, no finance degree required. Just one number, one division, and a little curiosity.
FINANCIAL PLANNING
8/2/20262 min read
The Rule of 72: The Fastest Way to Guess How Long Your Money Takes to Double
No calculator, no spreadsheet, no finance or accounting degree required. Just one number, one division, and a little curiosity.
Picture a snowball rolling downhill
Imagine you're standing at the top of a snowy hill with a small snowball. You give it a push. As it rolls, it picks up more snow, gets bigger, and because it's bigger, it now picks up snow even faster than before. That's compound interest in a nutshell — your money doesn't just grow, it grows on what it already grew.
The only question left is: how steep is the hill? A gentle slope (a low interest rate) means your snowball takes a long time to double in size. A steep slope (a high interest rate) means it doubles much faster. The Rule of 72 is simply a way to guess how many "rolls down the hill" — how many years — it will take for your snowball to double, without needing a physics equation to work it out.
The formula
Take the number 72. Divide it by the annual rate of return you expect from your investment (fixed term deposit, treasury bill, certificate of deposit, real estate, business etc). The number it returns is roughly the number of years it will take your money to double.
The Rule of 72
Years to Double = 72 ÷ Annual Rate of Return (%)
That's it. No compound interest tables, no scientific calculator. Just 72, divided by whatever percentage return you're working with.
Let's roll the snowball at different speeds
Here's what the Rule of 72 looks like in practice, at a few common rates of return:
So if you invest GHS 10,000 at a steady 9% a year, you can roughly expect it to become GHS 20,000 in about 8 years — and roughly GHS 40,000 about 8 years after that.
How Fast Does the Snowball Double?Years needed to double your money at each annual rate (72 ÷ rate)
3% - Savings account will take 24 years to double
6% - Conservative will take 12 years
9% - Stock market will double in 8 years
12% - Aggressive growth will achieve twice the number in 6 years.
The higher the rate, the shorter the bar — and the faster the snowball doubles.
Use it in three steps
Pick the annual rate of return you expect (or are being offered) — say, 8% on a proposed investment.
Divide 72 by that number: 72 ÷ 8 = 9.
That's roughly how many years it takes your money to double, all else being equal: about 9 years.
One important catch: the hill works both ways
The Rule of 72 doesn't just apply to investments — it applies to debt too. If your credit card charges 24% interest a year, that debt can double in about 72 ÷ 24 = 3 years if left unpaid. The same snowball effect that grows your wealth can just as easily grow what you owe. It's a useful reminder to point the hill in the right direction.
Good to know
The Rule of 72 is an estimate, not an exact calculation. It's most accurate for rates between roughly 6% and 10%; at very high or very low rates, the real doubling time can drift a bit from the shortcut. It also assumes your rate of return stays fixed every year, which real investments rarely do. Think of it as a quick gut-check, not a substitute for thorough assessment of an investment.
References: Investopedia, Corporate Finance Institute
Disclaimer: This content is for educational purposes only and does not constitute financial advice. Please consult a licensed financial advisor before making investment decisions.
