The Rule of 72 is a shortcut. Divide 72 by the yearly growth rate and you get the rough number of years it takes money to double.
How it works
At 6% a year, 72 divided by 6 is 12. So money growing 6% a year takes about 12 years to double. At 9%, it takes about 8 years. At 3%, about 24 years.
Why it is useful
It gives you a fast feel for growth and for cost. It also works for inflation. If prices rise 3% a year, what you can buy with a dollar is cut in half in about 24 years.
What it is not
It is a rough guide, not a promise. Real investments go up and down, and fees and taxes take a bite. Use it to build intuition, not to plan your life.
Practice
Our Rule of 72 Race game turns this into a quick challenge. A few rounds and the numbers start to feel natural.
Play: Try the Rule of 72 Race.
Try a few numbers
At 4 percent, money doubles in about 18 years. At 6 percent, about 12. At 8 percent, about 9. At 12 percent, about 6. Notice that doubling the rate roughly halves the wait.
Why it is only an estimate
The rule is a shortcut that works best for rates between about 6 and 10 percent. For very high or very low rates the answer drifts a little. It also assumes the money stays invested and earnings are reinvested.
Use it to plan
Ask how many doublings you could see between now and a goal date. Two doublings turn $1,000 into $4,000 on paper. Real returns vary, so treat the answer as a rough guide, not a promise.
Keep reading
- Crypto Volatility: How to Think About the Swings
- How Compound Interest Works (With a Simple Example)
- Closest Call: Guess the Number
Education only. This is not investment advice, and investing involves risk, including loss of money.
How to use it
Divide 72 by the yearly return you expect. The answer is roughly how many years it takes money to double. At 6 percent, that is about 12 years. At 9 percent, about 8 years.
Why it helps
It turns a hard calculation into one you can do in your head. It also shows how much a small change in return matters over time.
The same rule works for costs
Fees and inflation work the same way. If prices rise 3 percent a year, what you can buy with a dollar is cut roughly in half in 24 years.
Its limits
It is a shortcut, not a promise. Real returns go up and down, and investments can lose money. Use it to build intuition, not to predict your future.
Try it
Pick three return rates and work out the doubling time for each. Notice how time does much of the work.